Transcript: The Soberg Principle: Cold, Heat, and Metabolic Resilience

Full transcript with clickable timestamps linking back to the source video.

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Welcome to the Huberman Lab podcast where we discuss science and science-based tools for everyday life. I'm Andrew Huberman and I'm a professor of neurobiology and opthalmology at Stamford School of Medicine. Today my guest is Dr. Susanna Soberg. Dr. Susanna Soberg completed her doctoral thesis work at the center of inflammation and metabolism and the center for physical activity research at the University of Copenhagen in Denmark. Her research has focused on how deliberate cold exposure and deliberate heat exposure can be used to enhance human metabolism. She is the first author of a seinal study which discovered the minimum thresholds for deliberate heat and deliberate cold exposure for increasing brown fat thermogenesis which is essentially a mode of increasing heat production and metabolism in the body and for establishing actionable protocols that can be used outside of the laboratory to improve metabolism and human health. Dr. Sober's research was published in the journal Cell Reports Metabolism in 2021, adding to a long and important history of research focusing on the role of cold and the role of heat in altering various aspects of the body's physiology, including hormone health, metabolism, and changes in neurotransmitters such as dopamine and epinephrine. In fact, today's discussion with Dr. Soberg focuses on the role of deliberate heat and deliberate cold exposure on metabolism. But it also includes discussion of the effects of cold and heat on things like neurotransmitter production, namely dopamine and epinephrine and norepinephrine, the so-called catakolamines, which strongly impact mood and metabolism. In addition, Dr. Soberg answers many common questions about deliberate cold and deliberate heat exposure, including, for instance, the difference between cold showers versus cold immersion up to the neck versus total body cold immersion, including whether or not going back and forth between heat and cold changes fundamentally the way that heat and cold impact the metabolism, hormones, and neurotransmitter production. And we talk

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neurotransmitter production. And we talk about almost every single nuance and variation on deliberate cold and deliberate heat exposure protocols as it relates to the underlying science. In particular, how cold receptors at the level of the skin are impacted versus cold reception and perception at the level of the brain and how all of that impacts systems of the brain and body relating to mental health, physical health, and performance. Based on her scientific research and academic training, as well as her understanding and use of deliberate heat and deliberate cold exposure protocols, Dr. Soberg is considered one of the world's leading experts on these topics. In fact, she is the author of a recent book entitled Winter Swimming, which is, I have to say, a terrific book because it breaks down chapter by chapter the different aspects of deliberate heat and deliberate cold into its various constituent parts, including cold acclimization, the cold shock response, dangers and safeties of cold water, the impact of cold and impact of heat on various aspects of human health, as well as specifics relating to sauna versus ice versus cold swimming. showers, etc. It's a very thorough read and a very easy and accessible read that if you are interested in deliberate cold or deliberate heat exposure or both will allow you to embrace those protocols with the greatest degree of confidence that you're going to obtain the specific endpoints that you're interested in and to do so safely. Before we begin, I'd like to emphasize that this podcast is separate from my teaching and research roles at Stanford. It is however part of my desire and effort to bring zerocosted consumer information about science and science related tools to the general public. In keeping with that theme, I'd like to thank the sponsors of today's podcast. And now for my discussion with Dr. Susanna Soberg. welcome. Thank you. So great to have you here. I feel like I should give a little bit of the backstory of how we got connected, which was that for many years I've been interested in cold thermogenesis. It was the topic of my senior's thesis in college and I've of course followed the

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college and I've of course followed the uh popularity of Wimhof and we've had Dr. Craig Heler my colleague from biology department at Stanford who works on cold and its impact on physiology and sports performance. So, for a long time, I've been interested in this area, but there's been a real uh lack of new, let's say, high-profile quality scientific information in terms of how, for instance, cold plunges and sauna, how that impacts human physiology. I know there's been some information out there, but it's been sort of scattered. And then a little over a year ago, I see this paper in Cell Reports Medicine and was immediately struck the first of all the fact that it was in Cell Reports Medicine. I've been on the Cell Press editorial board for a long time now. Cell Press journals are of course phenomenal journals and the title and the content of the paper was directly in line with the sorts of practices that people are very curious about and then are starting to emerge. things like sauna cold plunges and there was your name first on the author list and I reached out to you through social media and we've done a little bit of live content there together and I've been tracking what you've been doing in the world in terms of your book and talking about the results in your manuscript and talking about the science and impact of deliberate cold exposure and sona and I have to say that it's been a wonderful and remarkable thing to see and you're bringing so much quality information about this area that for a long time I think was kind of niche and is now becoming more and more mainstream. So I'm going to start off with a thank you for being here and a thank you for the work that you've done and I'm looking forward to talking to you about it today. So my first question um to get things started is what is happening when we get into an uncomfortably cold environment? So, for instance, if I'm really hot on a hot day, jumping into a

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really hot on a hot day, jumping into a cold pool feels really good. But if I'm already kind of uh at room temperature or I'm a little bit chilly, getting into that same temperature of water doesn't feel so good, right? There's a shock there. So, if you could just walk us through what happens when we get into uncomfortably cold water, whether or not it's by way of shower or cold plunge at the level of our physiology, and if you'd like, our psychology, I think that's a good place for us to start because I think it will orient people to their own experience if they do that. Yeah. And for those that haven't done it, um, might start to peel back some of the the layers as to what the underlying mechanisms of cold are. Yeah, thank you for that question. It's really good to just address what actually happens in our physiology when we get cold. And you can get cold in many ways. So you can just head out for the one that gives you the most potent stressor which is submerging into cold water. And but you could also go in outside in the cold wind. That's also going to activate your um your sympathetic nervous system. So get all these neurotransmitter going in your body and so your your catacolamines. Um let's just address that we are taking a cold plunge for example. So if you are very hot for example um before you go into the cold water it's going to feel less stressful but the the temperature uh difference from your skin to the cold is definitely going to give you a a shock but your core temperature is warmer and that's going to feel a little bit better. So that's why when people go into a sauna for example and go out than in into the cold water they can do it easily easier than if they were cold beforehand. Could I just um ask you a few questions? So you you mentioned the sympathetic nervous system which for um people listening who aren't familiar with that is the the branch of our nervous system that's responsible for creating accelerations in heart rate um feelings of alertness. It's accompanied with stress and the stress response, but it's accompanied with waking up in the morning for that matter. So, it's not

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morning for that matter. So, it's not always about stress. And then you mentioned the catacolamines, which um are dopamine, epinephrine, and norepinephrine. So, maybe a little bit later we'll talk about those individual neurotransmitters. But you rais a really important point, which is something I get asked about a lot for people that are curious about using deliberate cold exposure, which is how cold should the water be? And I know it's very hard to give a straight prescription for that because I think it boils down to what you just said, which is it's really the difference between your current temperature and really the temperature of the surface of your skin and the temperature of the water. So if you're very warm, getting into cold feels good. If you're already cold, getting into more cold feels stressful. Um, is there any way that we can start to gauge what is the best way to approach a deliberate cold exposure protocol? I mean, should it feel uncomfortable? And that leads into the question of how do we balance the discomfort with the amount of time that we spend in? So, for instance, if it's just a little bit uncomfortable, will spending more time in the cold get us the same benefit as getting into very uncomfortably cold water for a very short period of time? Yeah, that is really good question and I definitely think that this could be um future studies on this as well to really unravel uh what kind of protocols are the best way or also for which outcomes of course. So if the temperature is uh very cold and you feel that and you also feel very cold then you you should stay in the water a little bit longer. So I think it's just you should get uncomfortable cold. So as long as you get uncomfortable cold, it's cold enough and you get this what we call the cold shock. So the cold shock is activation of your sympathetic nervous system and these activation of the the catacolamines which you just mentioned before. Does the shock mean that I'm having trouble controlling my breathing? Is that a good gauge? Uh yeah, you can say so because that's kind of like how we define it. So you hyperventilate so you have a faster um uh breathing rate. Um so that increases

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uh breathing rate. Um so that increases also because you activate your gasping reflex. If you are new to this um but if you are adapted it it kind of subsides with time with adaptation. So what you can do is that you can train this cold exposure and you can kind of like get adapted to it so you don't have this hyperventilating response every time you go out in the cold water. So this is like building up your resilience, building building up your adaptation is going to make this shock like subside a bit. So it's it's always harder in the beginning, but you should do hard things, right? It's not something that we you shouldn't think about cold water and cold water immersion as something that is comfortable. It should be hard because that's the point of it, right? If you enjoy it, then yeah, then I'm I'm thinking something is wrong. It's not right. You should not enjoy it. Well, this is an important point that you're making because I think that many people shy away from deliberate cold exposure because it's uncomfortable in a way that at least from my experience is very different than the discomfort of exercise. Because with exercise, for instance, um if running hard, you know, running fast and breathing hard is uncomfortable, you can slow down or walk. If um you know lifting weights is uncomfortable, you can remove some weight or reduce the number of repetitions or stop. With deliberate cold exposure, I suppose you can be sort of halfway in, halfway out of the water or partially underneath the cold shower, but it's very hard to titrate and adjust the level to it's kind of all or none. And I've seen um I should just I can tell this by anecdote. I've done some work with military special operations. Um, I won't say which country. This was outside the US. Um, and these are very tough individuals. They're used to going without sleep and doing hard, high consequence, higher risk kind of work. And they were asked to do some cold water exposure training. And I was there that day and it was remarkable. About a third of them just

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remarkable. About a third of them just went straight in and just kind of grinded through it, you know, like they looked stoic anyway to me. Um, there were a few whimpers, no cries. about a third um talked a lot and got really you could tell that they were agitated and anxious, but they made it through. And then about a third of them just simply would not get in past their knees or thighs, were just it seemed like they were just dreading the whole experience. Some actually didn't actually go in completely. Um which was really surprising to me and I you couldn't tell based on their physical appearance or anything else about them. They're all high performers as to who would have this response. So it seems like people vary tremendously in terms of their ability to embrace the discomfort of the cold. Is that from your studies? Is that your experience as well or or are there these weird mutants who seem to just love going into the cold for the first time? So some people just feel better in the cold and some people uh dread the cold even more. And you can say the more people are pushing the cold away they might feel the cold pain even more. So they would definitely people who are maybe the soldiers you just talked about they some of them might be already adapted to the go cold. So if they are not scared of the cold they go out and they embrace the cold in a better way. It could also be that some people have a more sensitive nervous system and when you are a bit sensitive to the cold, you will of course try to get away from it, right? And you will also have the cold pain more um feel the cold pain more if there if if you avoid it. So the more you avoid the cold, the the more painful it will feel when you go into it. So yeah, you mentioned being outside in a t-shirt versus um cold immersion up to the neck versus shower. I think um this is something a lot of people wonder about. What are the differences in terms of impact short-term and perhaps even long term between cold showers, cold plunge to the neck? So that could be in ice water or just very cold water. Um

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water or just very cold water. Um immersion with dunking one's head and then coming up because obviously people have to come up for air at some point and then simply being outside on a cold day in shorts and a t-shirt or something of that sort. So there are different outcomes um because there they are very different uh exposures of the cold to your cold receptors in your skin. So the more you can say you cover your body in the cold which you would do in cold water because they're of course covered totally and and the molecules are closer to your skin. You will have a more potent um activation of all your code uh receptors in the skin. So that one will definitely activate your autonomous nervous system more and rapid h compared to going out in a t-shirt in the cold wind just go for a walk. Um but that is also something that's going to activate your uh sympathetic nervous system meaning then that you will have an increase in norepinephrine uh and you will activate something called uh the the brown fat. So, this is a healthy kind of uh fat tissue that we have in our body and when you activate that, that's going to um increase your metabolism. Before we talk about brown fat, and I'm so glad you brought it up um because so much to talk about there. Uh what about cold shower? I mean, obviously cold shower somewhere in between. Yeah. Um being out outside in the air, cold air versus uh being immersed up to the neck. If we had more studies on on cold showers, we would learn more about how does that activate our metabolism, how does that increase our neurotransmitters in the brain, which could also have an impact our on our mental uh balance. So I think that would be interesting for the future. Um but what we do know is from from uh from activating brown fat and uh both from rodent studies but also in humans is that as soon as we get cold on our skin we will activate our brown

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on our skin we will activate our brown fat. So it is kind of like our first responder in in the body to keep uh our um temperature up. So our muscles is like the second tissue in our body. We have two tissues which can increase our uh thermogenesis. So the brown fat which is like always like temperature regulating our body and then we have uh the the muscles which will secondarily uh start to shiver and that's going to increase our um temperature in the body. But as soon as you go into a cold shower you will activate your brown fat also immediately. So it could be good also for increasing metabolism in theory because we haven't really any studies showing how much does it actually activate the brown fat. So, if someone out there wants to do a study on that, great. I've thought about why there are fewer studies of cold showers than cold immersion. And I think the answer to my mind is that from a methodological standpoint, it's just harder to do because if people are getting into cold water up to the neck, they're getting into cold water up to the neck. Whereas, if people are getting into a cold shower, some people are larger or smaller, some people are going to stand under the shower with it hitting their head, some people the back of the neck. You could direct people to do it, but it's a little bit um more difficult. Also, I think uh for you and I are both research scientists. There's a little bit of a um methological challenge that might seem silly to people, but it's a real one, which is if people are in a cold shower, also the water is going to be I'm kind of pushing their clothing against their skin. There's a certain vulnerability in uh for most people um coming to a laboratory in the first place, let alone being observed while they shower. immersion under the water and you know some people might roll their eyes and say okay is that the barrier but you know science exists in these worlds and this will vary by culture and things of that sort but we run human subjects in my lab and I'll tell you just um the process of getting people to the laboratory and having them park and find the lab and you know it's a whole new environment with people in lab coats and people moving around and where's the restroom I

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moving around and where's the restroom I mean it's a there's a certain amount of stress just associated with taking part in a study for most human subject. So, um I uh totally agree. However, we need more studies of of cold showers. It's just a harder environment to control in my mind. So, it sounds like any form of cold to the skin that people register as what you called a cold shock or uncomfortable like h like this is kind of jarring activates the brown fat. Do we know what the pathway is from cold receptors on the skin to the brown fat? I mean, how does the brown fat know that we're cold? Yeah, really good question. And it seems that I I think that of course in the future we will know much more about these pathways. But what we do know is that uh the co - receptors will send a signal to our temperature regulating center in in the brain. So hypothalamus um and that's going to be um taking in this message and we have so many co - receptors in the skin. So it's going to be very fast as you can say if you immerse the body into cold water. this is going to be so rapid uh so it will have a rapid increase in uh neurotransmitters in the brain. So no adrenaline adrenaline cortisol and uh which is not that much but it's still there. So you have this increase in uh no adrenaline which will then immediately activate the brown fat um because the you can say the activator is the most potent one cold and no adrenaline that's going to activate the brown fat. But there's also a direct pathway from the co receptors in the skin to the brown fat which really shows that if because of these different pathways it shows that that it could be that this tissue to keep us warm was was developed in in our ev evolvement as humans to keep us warm and to save us whenever the temperature on our skin varies it just a little bit to keep us in that right homeostatic balance so we don't get hypothermic

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don't get hypothermic um but also so we don't get hyperothermic but because it seems that the brown fat is also activated when we get warmer in our skin so it's also um maybe a temperature regulator in our body but the pathways is different. I think it's also a third pathway from directly from the muscles. So the brown fat is also activated um even though the the muscles are starting to shiver. So there's a an extra pathway that way to keep our uh our temperature up. So muscles and brown fat are working together to to keep us warm so we don't uh suffer too much in the cold water. That's super interesting. And what I hear you pointing to is the existence of three parallel pathways. And this notion of parallel pathways comes up over and over again in biology as you and I know and I think it's important for people to know about because um as you uh said so so eloquently the when something is very important to our survival or andor evolution evolution the brain and body uh install multiple mechanisms for it not just one. And um and so it sounds like it's cold skin, cold on the skin triggers a response in the hypothalamus which then activates brown fat. Cold receptors in the skin directly to the brown fat and then shivering in the muscle to the brown fat. Um I want to talk about brown fat in depth and learn from you more about brown fat. Um before that however I want to ask about shiver. Um, I've heard that shiver causes the release of succinate, um, which then activates the brown fat. Is it known whether or not inducing shiver is important? And when should pe people shiver? I mean, I've gotten into cold plunges and shivered while I was in there, and then I've also had the experience experience of getting into a cold plunge or a cold shower, then getting out or even standing outside on a warm day after swimming in a pool and then starting to shiver. So the shiver comes later. So

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shiver. So the shiver comes later. So how important is shiver and does it matter when shiver happens? Yeah. Well shivering is is good because that increases your metabolism and that's going to burn some calories in your body. You shouldn't be so afraid of shivering I think because the shivering as long as you don't get too hypothermic. So if you don't sit in the cold water for too long um and what you just said by shivering after you get up that is because of the after drop. something called the after drop is when your core temperature uh decreases even after you get out of the cold water and it always does that um your body because it as soon as you get into the cold water uh all the your blood vessels is going to constrict because you need to keep your blood in your core and and keep your vital organs warm. So as soon as you get up that those blood vessel will open again and the warm blood would flow out and get colder and then flow back again into the core and that's going to decrease the temperature in your core of course. So that's the drop. Got it. Oh, I'm so glad you explained that. I've heard um years ago Wimhof I heard him talk about the drop and I've heard colleagues of mine talk about the drop but that's the first time I've ever heard it explained clearly. Let me make sure I understand this. So um I get into cold water. Obviously, I'm cold. Vessels constrict to keep blood near the center of my body, keep me alive. I get out. The The warming up of my body allows those vessels and capillaries to dilate again. The blood goes out to the surface, but the surface is still cold and so that blood is cooled and then my core body temperature drops. And that's what you're referring to as the drop and that's what induces shiver. Exactly. Great. And then am I right in thinking that then the shiver activates brown fat which then warms me up again? Yes. Got it. That's why you should end on the cold. But we can get back to that. Yeah, let's talk about Yes. Ending on cold is um you know it's what I refer to as and what has now become known as the soberg principle which is um a really

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soberg principle which is um a really important principle about the importance of ending on cold um and not doing what I do which is to get into a hot shower or or back in the song. But we'll we'll get back to that in a few minutes. So, um, that's wonderful, um, that you can explain that so clearly because I think that shiver is something that a lot of people do avoid. People think, "Oh, I don't want that, you know, the chattering of the teeth." And, um, and it it feels like a loss of bodily control, which really it is. It's it's an autonomic response. Yeah. But I don't think that people should should avoid it that much. It's just like seeing shivering as a way of your body in a like it's training. It's training for your for all your cells. is training for your muscles. It's training of your metabolism and that's going to increase your what's called the insulin sensitivity. So if you can like in your mind get used to the thought of shivering is just like when you go exercising in the training center and get that feeling of like oh this is tough now it hurts a little bit. Yeah it's going to hurt because that's what shivering also does but it's just a different way of training your cells in your body. It's going to create what is healthy stress. It's called homesis in the cells. And the more you expose your your muscle cells or your brown fat cells to these kind of like healthy stresses, exercise, cold and heat exposure, it's going to make them uh better at like activating and also um at keeping you healthy. So as long as the the cells get exposed to this, it's going to keep them on its toes you can say because it becomes more robust um increasing these heat shock proteins and cold shock proteins in the cells to um make you more robust for the next time and that is also what happens when you go to the training center and I keep like do drawing that parallel because people today know more about we know more about exercise and what that's is going to do to your muscle. cells. Um, and but the same kind of like training is also what you do when you go out and and into the cold water and submerge

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and into the cold water and submerge into cold water because that is just your cold training center. You can say that and and also your heat training center going into the sauna because the cells are getting stronger with hormetic stress. So, it's the same process just different practice. I'm so glad that you brought up the fact that the discomfort or the embarrassment or both of shiver is still crucial to uh actually to reach for and try and experience the same way that with exercise um I think a lot of people don't realize this but when we did our series with Dr. Randy Andy Galpin, it became clear to me what should have already been clear to me and I think that most people don't realize which is that if we were to measure um heart rate, blood pressure, um stress hormones and inflammation in a human being during exercise, it would look as if they were ready to die. Blood pressure would be high. Inflammation is through the roof. But all of that is setting in motion an adaptation or set of adaptations that allow blood pressure to be lower at rest that allow um inflammation markers to be lower at rest. All the things that everybody is seeking with exercise in in addition to of course the aesthetic changes that people are seeking with exercise. Sounds like the exact same things are happening with the cold. So um the redundant message here seems to be that the more discomfort provided it's done safely just like with exercise, the more shivering the um the more cold shock provided it's not to the extreme and stop somebody's heart, right? We can talk about thresholds for that a little bit later. It sounds like all of that is going to set in motion some long-term changes that will make people feel better and will improve health. Could you just touch on a few of the longer term changes that are known to occur? I mean, I'm well aware of the study showing that uh I think it was European Journal of Physiology. It was uh the European Journal of Physiology showing long lasting increases in catakolamines, dopamine, norepinephrine and um epinephrine for many hours after deliberate cold exposure. What are some of the other things that happen at the

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of the other things that happen at the level of metabolism and brown fat in let's say the hours and um day after deliberate cold exposure as soon as you go in of course there's an activation but it seems like no you're asking for the the later outcomes like blood pressure and stuff like that is that what you mean yeah blood pressure but also in terms of metabolism I know that you know in your study you should and we'll talk about brown fat in depth here in a moment but that there were changes to the brown fat that equate to changes in, for instance, people's ability to be comfortable in colder environments when they're not doing deliberate cold exposure. Or in the same way that I can um exercise on an exercise bike or go for a hard run, but then if I go hiking uh with the family on Sunday and it's a steep climb, I could do that steep climb more easily because I'm quote unquote fit as a consequence of the exercise. What are so what are some of the fitness adaptations of deliberate cold exposure? Yeah. So what happens is that you you get adapted a little bit every time you go. So you will like exercise get a little bit stronger. So every time you go into the cold water for every time you will be more exposed to it. You will feel more comfortable in the cold. So you going to you're going to build your adaptation which happens on a metabolic level which is going to be the brown fat. So you have more activation of your brown fat. the mitochondria in the the brown fat cells are going to be um you have more of those and they will be more efficient at heating you up because it expects the body expects you to to do this again. So you are prepared in a way. The capillaries in your skin is also be will also become better at like constricting. So you will have a better shield of your body to uh prepare you for the next time. So you will be become better at h going into the cold water in that way. So the the body makes these mechanism and changes your body in a way. So you can expose yourself to the next time right and uh and also you will have um also a your um stress response

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have um also a your um stress response will also be uh subside a bit. So you will have a less increase of your uh catakolamines um with time. With time also you have because of this activation of your uh brown fat or your muscles you will have an increase in um uh in in your metabolism which will then make your insulin sensitivity uh better. And this is shown in in studies. For example, um there was this interesting study I found just before I I started my PhD which was from um Gibberto um at L from 2016 where they measured um metabolism metabolism not in not on brown fat but they measured insulin sensitivity in middle-aged men and women uh during one winter swimming season. So they were not very young like they were in my study. Uh but they were middle-aged and I think this is very interesting. So they during these four five months they were winter swimming they saw that they had a lower blood pressure after the season and they had a lower heart rate and they also saw that they have a better insulin sensitivity and I think that is very interesting because if you can have a better insulin sensitivity you can prevent lifestyle diseases. So um and will lower blood pressure which is a a very strong outcome also for telling how much inflammation you have in the body. And because it didn't measure uh brown fat um I figured that it could be that was the missing link that was the one of the explanations to why we see this um less inflammation in the body. So um the longer outcomes the long-term outcomes could be that you lower your blood pressure and have a lower heart rate. um you also um have a better insulin sensitivity and a better glucose balance, but that was shown that is shown in my study. I'd like to take a quick break and acknowledge one of our sponsors, Athletic Greens, now called AG1, is a vitamin mineral probiotic drink that covers all

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mineral probiotic drink that covers all of your foundational nutritional needs. I've been taking Athletic Greens since 2012. So, I'm delighted that they're sponsoring the podcast. The reason I started taking athletic greens and the reason I still take athletic greens once or usually twice a day is that it gets me the probiotics that I need for gut health. Our gut is very important. It's populated by gut microbiota that communicate with the brain, the immune system, and basically all the biological systems of our body to strongly impact our immediate and long-term health. And those probiotics and athletic greens are optimal and vital for microbiotic health. In addition, Athletic Greens contains a number of adaptogens, vitamins, and minerals that make sure that all of my foundational nutritional needs are met, and it tastes great. If you'd like to try Athletic Greens, you can go to athleticgreens. com / huberman, and they'll give you five free travel packs that make it really easy to mix up Athletic Greens while you're on the road, in the car, on the plane, etc. And they'll give you a year supply of vitamin D3 K2. Again, that's athletic greens. com / huberman to get the five free travel packs and the year supply of vitamin D3 K2. And we'll get back to the uh insulin sensitivity and glucose balance. Um that's a an an impressive list of benefits. Um you know, blood pressure, of course, most people are aware of blood pressure and what it is. It's what they measure when we go to the doctor. And um it's not very sexy nowadays. you know, blood pressure. People, oh, you know, blood pressure, it's not, you know, people want to hear about the inflammator and the microbiome and all of that stuff is really interesting, but um I think that blood pressure doesn't get enough attention. Um, and we have spoken to on this podcast to Dr. Peter Aia, who um is an expert in longevity and health span and things of that sort. And and I was surprised to learn again I shouldn't have been surprised that the number one reason people die worldwide is cerebral vascular disease and cardiovascular disease. And there basically three things on the list of things to address. One is not smoking

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things to address. One is not smoking um or vaping by the way not so there are a few other things uh related to blood markers APO and things of that sort. But then the big one is blood pressure. And so it's it's interesting because we don't think about blood pressure that much anymore um as a as the kind of people interested in health optimization and health, but blood pressure is so vital to control. So it's wonderful to hear that deliberate cold exposure is one way to control blood pressure. I'm guessing in concert with other forms of exercise. Yeah. Um Um let's talk about brown fat. And um if you if you're willing, I'd love to drill into brown fat at a deep level. Um, again, my understanding of this is is far more elementary than yours. Obviously, you're the expert. My understanding about brown fat is that it's located in specific areas of our body. Uh, maybe more widespread than when I learned in school. I thought it was I was taught it was just at the clavicles and the back of the neck and upper back, but who knows? I learned that there's more of it when we're children, maybe more distributed throughout our body, and that it's rich in mitochondria, but what is so special about the brown fat? Like, if we could just go into the biology of brown fat a little bit, what does it look like? U you've measured it in human subjects, where is it distributed really? Can it expand its distribution? Can we activate and expand the amount of brown fat as adults? And um for those of you that are cringing already thinking we're talking about getting fatter, it's quite the opposite. We're talking about not subcutaneous fat, but fat located around the the organs. But please educate me. Um tell me where I'm wrong and um expand my knowledge on brown fat. Okay. Yeah, you're not wrong. But um it's defin it's true that there are more locations of the brown fat than we previously thought. Um there's this very nice study from 2017 by lighter at where they had made these pensites uh overlays

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they had made these pensites uh overlays of um their subjects where they can see where in the body do we have brown fat and where can we grow more brown fat um so to say um so the brown fat is can is very plastic. So it means that it can grow and they can decrease. And this is proven in in studies where we have seen um um people with fail cryiocyto is like a very specific cancer type where pe where from the 70s where we can see that if they have this specific kind of cancer type um they have a t they have this tumor on the adrenal gland. So they have like a huge increase in norah adrenaline and because of that they have this continuous activation of the brown fat and they have grown a lot of brown fat in the whole body or abdomen or where it's located in these six different places but it is like ve very much compared to like normal people. Um, and what they then see, uh, what we learned from this study is that brown fat can apparently grow if you have, um, an increase in no adrenaline in the body. It's not like you want that because when that happens, you have a high blood pressure. You don't want it chronically, right? You you just want it on like a short amount of time and then it can grow for a bit, but you don't want it chronically, of course not, but because it it activates also your sympathetic nervous system. showed they have also showed they had high blood pressure. They had uh they lost a lot of weight of course because this is activating your metabolism. So they they found luckily that when they uh removed this benign uh tumor uh that uh the brown fat um decreases again to normal size and they gain weight again and they had normal blood pressure. So the story ends well but it's kind of like proof of concept of that brown fat can actually grow. So it's plastic in its way of like it can grow and it could decrease again. So that's very good good studies to to see what it what the body

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studies to to see what it what the body is capable of. But we don't of course want all that brown fat. We just want it to be um we just want to keep it actually and and keep it activated because what we see in studies is also that after the age of 40 um people um studies have shown that there is an association with having less brown fat but increased obesity. So of course we we don't know yet whether brown fat decreases with age and therefore we get obese or we get obese and therefore we have less brown fat. But as brown fat is an insulin sensitive organ in our body and we get obese uh just like the muscles get less sensitive um insulin sensitive the brown fat does as well and therefore it maybe decreases. It could be a theory that I think could be one of the reasons why we we don't see that much brownfed in in elderly uh people. Some have a lot especially people working outside that there are studies showing this who people who um work outside do physical work outside farmers and um interesting interesting they expose themselves to it so they'll just keep it in that way. Mhm. It's um and I suppose we should um clarify for people in case they don't know that insulin sensitivity is a very good thing. You want that. You want your cells to be sensitive to insulin. Um insulin insensitivity is type 2 diabetes. Um and is associated with obesity. Um so just a point of clarification there. Uh yeah, it's interesting to me. I usually work out at home, but I I go to a gym once or twice a week if I can because it's good if I see uh the outside world. Um and there are a few individuals at the the gym who are they're not particularly large or muscular um but they are incredibly incredibly um lean and their posture is great presumably from the muscularkeeletal work um and they they're in their 70s

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work um and they they're in their 70s and 80s. I mean, it's remarkable, right? And um and I know all the telltale signs of hormone augmentation. I'm very good at spotting that. There are a few telltale signs. I've talked about this on other podcasts. And they're not that's not why they're um they're fit. They're they they're clearly of that look. And you see this outside the gym, too, of course, for people that look like they've done a lot of physical labor their whole life. Yeah. They're just moving a lot. They they have strong hands and features and they're um and they're not necessarily excessively lean, but you can tell that they've been using their muscular skeleletal system. And I like to talk to these people and ask them like not what are you doing now for your workout, but what what did you grow up doing? you know, and I would say, and obviously I haven't run statistics on this, but more than 75% of them respond that they grew up on a farm or that they did some sort of manual labor or were a postman or a postwoman or doing something where they moved a lot for their early years and throughout middle age. And most of them are now in retirement, but some of them are still working and they all still moving a lot. So the relationship between shiver and brown fat makes sense to me. But is it the case that as we're just moving around I've heard of neat non exercise induced thermogenesis. So if we're just moving around um that we are activating brown fat or does there need to be this stressor? Does there need to be shiver and a cold stimulus or a heat stimulus to activate the brown fat? In other words, um is just staying active enough or do we need to do some sort of temperature uh shock type thing like deliberate cold exposure? Yeah, I think that is really good question because how how also why do we have this tissue then? If it's if it has to be extreme then you can question what what do we need this tissue for? But it seems that you can activate the brown fat with just a little bit of exposure to to cold. So cold is the the most potent stressor activator of our brown

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potent stressor activator of our brown fat because it's our temperature regulating organ in our body is a first responder to that. So the muscles will be a little bit too late uh and therefore we have maybe these two kind of tissues. So actually just exposing yourself or hand actually just to cold water. So studies have shown um that if you just put your hand in cold water, not that you're going to do that all day or or every day or anything, it's not it's it's not something you have to do, but it just shows that you can activate your brown fat just by getting a temperature change on your skin. So you can go outside in t-shirt. That's why also we were just talking about well people who works outside or move a lot or get out in and out of it like changing the temperature of their body all the time they will have more brown fat and uh activating that is going to keep your metabolism higher and your insulin sensitivity studies have also shown this. So the brown fat can be activated as soon as you just change your temperature in the skin. So going outside in a t-shirt, wearing cooling vests, also studies have shown this for 10 days, it's going to also uh grow your uh your brown fat. So you can get more brown fat if you expose yourself to the cold. You don't have to start in a cold uh shower. You don't have to start in a cold uh plunge if you're not really ready for that yet. But just exposing yourself to the cold wind has also shown to activate your brown fat. Or if you don't want to be like in this uh awake state um then you can also just sleep in the cold and you won't notice it that much maybe. But studies have shown that if you sleep in 19 ° C um then you will activate your brown fat and you will grow your brown fat so you have more of it. So this very nice studies um from Hansen ital from 2017 showed that uh a group of subjects who slept in a room at 24 ° and then they made this PET CT scannings to see how

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made this PET CT scannings to see how much brown fat do they have from the beginning. So what we call baseline then they measured again after um a month of sleeping in 19 ° and they saw I think it's remarkable just one month at 19 ° sleeping there. They had a duvet on and they were still had clothes on when they're sleeping. So they're under a cover under a duvet. Under under a duvet. Yeah. The subjects were sleeping at 19 degrees for one month had increased his insulin sensitivity. The next month they slept at 24 °. They measured this again and then they had decreased actually a little bit and then they slept at 27 °. So quite warm room actually for for the fourth month. um and they saw even less activation of the brown fat and also insulin sensitivity. So it seems that you can expose yourself and pretty rapidly the brown fat will respond to this because it's so sensitive to noradrenaline, right? So if you keep exposing yourself to a little bit of cold, you will also get a little bit adapted to it. But that's because the brown fat um has grown these more mitochondria in the cells. So these small energy fabricates that's going to activate the cells and that's going to take up glucose and fat from the fatty acids from the bloodstream to keep uh the thermogenesis up and that's going to clear up some sugar and it's going to clear so in the bloodstream and some some fat as well. So the brown fat can in that way decrease uh our unhealthy fat which is the white fat. Um and the white fat is what we don't want too much of but we still need some of course. Um, and it's our energy storage. So, it's very important that it's there. We just don't need a lot of it. So, on our thighs and also around our inner organs, that's where it's it's located. So, if we can have activation of the brown fat just by going out in the cold and just by sleeping in a cold room or if you are have courage for it, you can go out and expose yourself in a cold plunge. Um, cold showers is also going to do the trick. So, you can do

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to do the trick. So, you can do different variations of this. Just exposing yourself to various temperatures is going to activate the brown fat because it was involved to keep us in a perfect homeostatic balance regarding temperature. So to keep us alive. Incredible. Uh I want to just get a clarification around this 19 ° C um room that they're sleeping in. So they're under a comforter, a duvet, and um and you mentioned they had clothes on. uh the room is 19 degrees C, but the temperature underneath their blanket might not be 19 degrees Celsius. So, presumably it's the cold on their face that's activating uh the the increase in uh brown fat that was observed. Is that a reasonable uh expectation? I I think so. Yeah, because it's you have so many co-ceptors in your face. So it's actually it's enough and it I think it corresponds very well with the studies showing that you can activate the brown fat just by putting a hand into a a bucket of cold water. Uh and I did this experiment myself in in my studies just to see how well did they uh respond to cold water. So it was 4 ° Celsius cold water for 4 minutes and then I just measured blood pressure and heart rate to see do they have like an activation of this. I actually also measured the brown fat during this uh cold exposure for four minutes with an infrared thermography camera to see can I see that the brown fat is activated and just just to uh go back to the location of the brown fat. So usually you cannot really see activation of your brown fat because it's located centrally in your around your central nervous system. Um and and the biggest depo as you mentioned before is up here under the the calavicular bones. So um and very close to the skin surface and because it's so close to the skin surface I could measure it with this very expensive camera. You and it's not very feasible for people to go home and do this. Don't because it takes a lot of

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do this. Don't because it takes a lot of practice. I can tell. Um but we measured the brown fat with this um and and I could see that after a few minutes that the activation was there an increase in temperature uh arose from that um activation just four minutes. So it's very rapid and I'm also measured in my study how deep was the brown fat under your skin. So it's very close to the surface which also shows that it it needs to be there to to heat you up and heat heat your inner organs. Well, I'm delighted to hear all of this and I'll I'll tell you why. Uh, one is by way of anecdote. I mentioned a little bit earlier that as an undergraduate, I worked in a lab that studied thermogenesis and we were doing that in animals, but we had this room that was very cold. The whole room was cold. Uh, the guy who I worked for at the time, Harry Carile was a very accomplished physiologist. He came from this lineage. I don't know if this literature is still um discussed much, but it's a beautiful literature um from Rothwell and Stock. They were the ones who discovered um non-ex exercise induced thermogenesis. The fact that people bounce who bounce their legs a lot and move around a lot and have a lot of kind of um stocastic movement um burn up to 18 1, 800 calories more per day than people who sit more still. Fascinating. Incredible. Just incredible. Um I don't think that work deser gets as much attention as it deserves. Published in journals like nature, so very uh fine journals. In any event, uh, one of the things that I noticed when I started working in that laboratory was that I was cold because the room was cold. And, um, Dr. Carile, Harry, um, said, "Well, the key is to wear a t-shirt in here for about 2 or 3 days and then you will cold adapt." I thought, "Well, wouldn't I want to put on a hoodie and get warm in there so I was comfortable?" And he said, "No, actually, what you want to do is get yourself uncomfortably cold, activate your brown fat." And indeed when I did that, I think it was just two days of being in that cold environment. Then I could come back on the third day and be perfectly comfortable because the brown fat had expanded um or or added

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fat had expanded um or or added mitochondria or both. And I was perfectly comfortable in that environment. I also got very very lean in those um in those days and weeks. Now I' I've never been somebody who's very lean, nor am I somebody who carries a lot of excess atapost tissue. I'm kind of somewhere in the middle. I'm sure I could adjust that with feeding if I want to. But it was striking uh what a powerful effect it had on my entire system of thermal regulation. And one of the things that I uh also delighted in when Cell Reports um Medicine published your study is they had an accompanying um press release that went out to to those of us that receive press releases and it described a um a saying in Scandinavia which is um essentially I'm not going to attempt to uh speak uh Danish even though I have much of my family is in Denmark. Believe it or not I'm from Denmark. Um we have a lot of Danes in my family. Um, I won't embarrass myself by trying to speak Danish as I did before the the uh microphones were rolling, but um that there's a saying that I think essentially translates to in the fall when when you're approaching winter, you want to actually wear fewer layers, not bundle up when you go outside so that you can prepare yourself for the cold of winter and be able to heat yourself up using your brown fat. And that in the spring as the temperatures are warming, rather than removing layers, you want to wear more layers in order to be a little bit uncomfortably warm so that in the heat of the summer, you're better at cooling your body. Do I have that right? And maybe do you know the saying and would you be willing to share it? Only the Swedes and um uh and Danes will be able to understand. Um maybe the Norwegians do. If you don't know it, that's okay. Yeah. So I know that I know the the concept of it because we say it you should wear less before winter and uh and more before summer. So well there it is in English. So it doesn't have to be esoteric but yeah and and you're completely right and I think this is the this is just something that we know in the

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something that we know in the Scandinavian countries. I think that we intuitively know this. But if we just go back a little bit in history, I think that um around the 1950s, the the Russian government went out and said, "Well, we should do something about the tobacco tobacco um pandemic or epidemic that was at that this time." So they they wanted to have the the people um be more resilient to the cold and also increase our immune system. So in Scandinavia and actually also in Russia, we uh put our babies outside to sleep in the prom and that is like to uh also to to get more resistant to the cold but also to increase our immune system and we still do that in Denmark. So we do you really Yeah, we do. Babies are taken out in the cold in the snow in frosty rain everything. My two boys have been sleeping out in winter for at least the f their first many three, four, five years because it's like very good for them and they get a better immune system and get resilient to the cold. So they will have less colds and also they run around in a t-shirt when it's super cold because they have activated all their brown fat. I didn't understand at that time I must say but I kind of like intuitively also knew because we have inherited this uh way of doing things with our culture. So and I have heard people coming from the US saying Danes are crazy. They put their babies outside in proms and leave them there and then they go inside and drink coffee in the cafe. Well, I don't think Danes are crazy. I adore the Danes. Um they're they're amazing uh culture and people. I'm so fortunate to have family members uh from Denmark. But I did notice so when when we were in Copenhagen and I know um we we saw you there uh not long ago. That was June. Um the water in the harbor was was cold for for even though the Pacific

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was cold for for even though the Pacific is close to here, which is very cold. Uh felt pretty cold. But I it was summertimeish. Um Um so people were in summertime mode, right? T-shirts and shorts and things that that sort. But it did strike me that people in Copenhagen are dramatically fitter than they are in the United States. I mean, first of all, everyone's bicycling everywhere. Yeah. Um, not many people wearing sunglasses, so trying to extract as much photon energy from the sun as possible, which I support. Uh, as everyone knows, that's a I'm a big fan of getting sun. But also um when we did see swimmers um they were swimming in this cold water and like it was nothing and their the range in age of the swimmers was what was remarkable. You saw the kind of fit triathlete looking types but also young kids like really young kids and then people probably in their again their their 70s 80s maybe even 90s really uh remarkable uh vastly different than what you see if you go to the ocean here in uh Los Angeles or or elsewhere. So, um, yeah, you Scandinavians are on to something with this. I'd like to talk about your study. um if you could give us a little bit of the backdrop about what motivated that study and then um and then walk us through what you did, you know, who the subjects were um what you had them do, what you measured um in as much detail as you would like to share because I think it's such an important, you know, even I fair to say landmark study because it also explored not just cold but sauna and the the co-use of cold and sauna as a a to probe metabolism and brown fat and other markers as well. And as you do this, uh, I'm hoping at some point that you might, um, tell us some of the observations that you might have made that interested you that perhaps were not in the paper because that's one of the great benefits of sitting across from somebody who who did the work in

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from somebody who who did the work in detail. So, um, yeah, if you could tell us about your study um, and, uh, what you did and what you discovered. Thank you for that question, Andrew. I I love to like also explain a little bit what did we do because when people read this kind of paper they just see the numbers they don't see what what happened before that and human studies are very different from from my study my studies you can do a knockout of something and then everything is like perfectly matched and controlled doing human studies is very far different from that because people are different even in the groups. So yeah, but what we um when I started this research in 2016, I did not really know what the brown fat was. So I started reading up on all this and I was very interested in preventive medicine. Um also the studies that I did before brown fat was also like very much in in the preventive side like how can we that was about something else but the sweet tooth and how can we lower our sweet tooth and stuff like that. So, but after that I wanted to to do something new. So, I looked into brown fat, got hired in this fantastic research group where they it's a cell group. So, they mostly did cell studies and they didn't have anyone to do a human study yet. Um and um but they really wanted me to to do that. So, I read upon a lot of research about how does the brown fat get activated what have been done already and I mentioned the paper before with sleeping in the cold. I found that particular paper very fascinating and that was also where um at that time I was like okay so cold exposure as an intervention of sleeping in the cold could be a good thing to go out and say well people do this but on the other hand is first of all it was already done that was one thing but the other thing was like well I wanted to to see if we can do like some kind of activity so we can have people move also or go and do something together or whatever. And the code uh made

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whatever. And the code uh made us think about, well, what about winter swimming? And it was kind of like a bit of a joke in the beginning. It's like, winter swimming, yeah, it's going to activate the brown fat, right? But but when we read the literature, we couldn't really find anything about activation of the brown fat with cold water besides hand in a bucket of cold water that really that was already there. So we were just thinking, okay, so it should be very potent um activation of the brown fat if it's cold water, but very different from cold air. So it was kind of also a new thing we were going into and uh we knew that we were going to do like a more of a um proof of concept study um at the beginning of it because it was like winter swimmers um must in theory activate the brown fat, right? But we kind of didn't really know with was this kind of stressor too much, too little or what will happen actually. But we had this idea about well we always say that cold water and winter swimming will activate your metabolism. But do we know if it do does that? No, we don't. So um and while this idea was a little bit fun fun at the beginning um we kind of accepted it was like okay let's just try this out but because we didn't have the funding for it we was like okay let's do a proof of concept study um let's go with a small number but enough to see um a difference between the groups. So the power calculation of that study is done on uh what we know uh from PETT scannings of the brown fat. So that's the main outcome of that of course. So um and we wanted to go a little bit small on the numbers of of participants because we wanted to dig a little bit deeper into the different mechanisms and also redo some of the the days. So I really wanted to do that to see if I can replicate also the findings and that's going to take a lot of funding but it's also going to take a lot of time to do it. Um so the proof of

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time to do it. Um so the proof of concept was just going small but looking at different mechanisms. We also took um fat biopsies for example and looked at the white fat to see if there was any differences between the groups before and after and stuff like that. So that's kind of like how it started and then the first year was like a field study for me. So I was not a winter swimmer when I started this. It was just Oh really? No I wasn't um at all and I would say I was a bit afraid of the cold myself. um bit of a cold uh always cold having big socks on and sweaters and stuff like that. So I was like I am so comfortable. I'm just like everybody else very comfortable. I like being com completely temperature neutral. Um but I started like playing with this thought like well if this is so healthy in theory I should not pack myself up. I should start not doing that. Yeah. But the first year observation of winter swimmers on the jetty they kind of joked about it say come on Zo you need to try this you cannot study this unless you have tried it and I was like haha very funny of course I can do that but I couldn't I I read the literature I understood in theory what happens when you go into cold water but I completely understood it when I first tried it the first few times not so funny it felt painful it was just like running too long uh after a long break and you and your muscles hurt the day after, right? You you completely regret that you took that extra mile. What about when you say uncomfortable, you mean uncomfortable when you got in and when you were in or uncomfortable afterwards? Because I find that um on rare occasions, well, I should just full disclosure, I I do deliberate cold exposure every morning for about a minute to two minutes in a cold plunge. There are days that I miss, but when I'm at home, I do that. And when I travel, I do a cold shower. I do finish with a warm shower. So, um, and we'll talk about why that's probably not the best idea, but um, and I've been doing it for

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idea, but um, and I've been doing it for some years now, um, on and off. Uh, but so just full disclosure, I'm a devote. Um, and I have family members that uh hate the the cold, but have gotten into it and and are starting to like it, but they don't. And I don't necessarily like the experience in the cold water, but I love the way I feel when I get out. And I have I'm 100% on that statement about loving it when I get out. Occasionally, it feels good to be in there. It feels invigorating. And I think I've learned to control the gasp reflex and the hyperventilation. And I just have told myself what we know, which is that the forebrain struggles to engage for the first 20 or 30 seconds, but if you can get past that wall, it's it's far easier to to push through. Um, but when you say that it was really uncomfortable, do you mean the experience of getting in or you also felt lousy afterward? Yeah, it's and very important to clear that out. I only felt very uncomfortable doing it at the moment. But afterwards, the first time I went uh with with a group and actually my husband was as well because I I really wanted someone I knew um coming along because it's very normal if you haven't done this before. You feel a little bit anxious about it. You didn't this is shown in studies as well because blood pressure and heart rate goes up in in those who are new to this kind of activity. So um I was a little bit anxious about it. So, it was really uncomfortable just doing it, but afterwards, as soon as I got up, I felt fantastic. And we went into the sauna and I did three rounds because I just loved it. I loved the feeling afterwards because you have all these neurotransmitters going in your brain and you feel more positive. You feel I feel invigorated. I had so much energy. Uh, and that like I I could totally see why people would do this to get energy throughout the day because I definitely had that. I didn't have to do three dips to to get that. I think one would be enough. And I often do that also now today. I do one dip. Sometimes I do two

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today. I do one dip. Sometimes I do two or three uh dips um in in one round. You can say in one day, but uh often it's like just one or two times a week. For me, that is enough to to get that energy and to get that positive feeling. And and I think that that is also why I I put up my study in that way. I wanted to study the lowest dose, you can say the lowest amount that we can get away with but still see um health benefits. So what I observed there on the jetty was that some did it a long time. They were in the water for a very long time and to me it seemed maybe a little bit extreme. Could you give me an example of long time? Well, so maybe they were like really swimming and they could be 20 minutes or half an hour. That's a long time. And there was like ice and people who came up. I I mean I just didn't really feel that this is a something that I wanted to go out and recommend to people after my you didn't want any of your research subjects dying either because if you're not adapted I mean people you know people can do that also a 20 minute cold shower or 20 minute cold plunge I know people do it but it's probably not a good idea no probably not it's going to exhaust your cells and they make them age too fast so exactly that's uh when you pass that homemetic stress this the healthy stress level. That's what is happening. The quite opposite is it's a almost chronic stress actually in the cells. Well, what happened then was that um I found out if if I want to have this protocol get through ethical comedy, I I really needed to go like very like sleek with the not too long and and make sure that they were also very healthy and and to get approval of course of this study. Um, but what I did was to to recruit winter swimmers who already have been swimming for two or three seasons. And I just observed them. I said, I'm not going to do an intervention study yet. I did that after, but I I wanted to do like a proof of concept where they were already

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of concept where they were already adapted to the cold and then compare them to a match control group who were matched on um on you can say diet. So were they vegetarian or not? Um, and one of them was in each group. Uh, also they weren't all vegetarians. No. Just one in each group. Yeah. I was gonna say with all the amazing fish and meat in uh in Denmark, I'd have a hard time being a vegetarian. The breads are amazing. The fruits and vegetables, too, but Okay. So, there were a couple vegetarians in each group. Yeah. One one in each. Yeah. Okay. And they were matched vegetarian. I have family members that are vegetarian, so I'm just poking fun. The uh Yeah. But they were matched on different things. So what we usually match them on is also BMI. Um we chose one gender in this study and we would always choose both men and women normally but we do see that there are different uh brown fat levels uh depending on gender. So women have more brown fat than men. Really h interesting. Yeah I think it's interesting. Uh that deserves study. Yeah. Why actually I think it's interesting because women are also smaller so in size and mass right but they also have um a lower peripheral temperature especially on hands and ears and and is that right that's that's documented that women do run colder than men yeah and there's physiologically I didn't say psychologically psychologically no we won't go to the psychological cold cold heat that's different something else that's a different podcast another time so women are is colder physically. Uh so on hands and ears, it's measured on that. Um and feet as well. So compared to men and men have bigger hearts than women and they can pump out more blood uh peripheral uh than in a woman's body. So they that could be an explanation for the colder hands for example. Thermomfortable state is also different between genders. So men are more comfortable at 22 ° C and

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men are more comfortable at 22 ° C and women are thermomfortable at 24 ° C. And this this Okay, so the thermostat wars of home have been now validated. Yes, there's a stud. By the way, prior to um starting recording, uh I made the executive decision that we were going to go with Celsius throughout the podcast um because the majority of the world uses Celsius. So for those of you that think in Fahrenheit, um the internet is your friend in making those conversions. So we're sticking with Celsius. So men tend to be thermoc comfortable at 22 ° C, women at 24. Okay. Interesting. Explains a lot about like also some arguments in the homes where men are turning down the heater and women are turning up the heater and they cannot really So it's it's really it's I'm on both sides here. I understand the men, we understand the women, but it's there is a difference there. Which was also one of the reasons why we had we in this proof of concept study chose one gender. So it is not like only because we wanted to study men. It was just to see to eliminate uh all the confounding factors which could have an impact on on our results. So um that was one of the reasons. Um but also because we yeah so women have have more brown fat than men and otherwise we would have to like do four groups or something like that and not having funding yet we were like okay we need to do like just one group just a control group and then a and a group who already winter swimmers. So I recruited winter swimmers who have been swimming for two to three seasons um because I wanted them to be already adapted but not going too long in the water. So they told me uh I did a lot of screening here of course beforehand and interviews to see to ask them how much do you do and um how much do you how long do you stay in the water and I um monitored how long did they then stay in the water and

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did they then stay in the water and recruited based on um that they only did like two to three times per week. It seems reasonable uh for Denmark at least to do that and they stayed only in the water for one to two minutes. So the co-arch subsides very quickly and you will get this activation of your rest and digest system which is your parasympathetic nervous system. So the the other branch of your autonomous nervous system and you get that activation because you submerge into cold water and when you do that you have an activation of your diving response and uh that's going to slow down the you can say the the consumption of oxygen also in your body and that's going to slow down your heart rate. Could I pause you on this because I've heard this before that when we get into cold water shower or immersion we get this sympathetic autonomic response. So increased blood pressure, increased heart rate, release of norepinephrine from the locus ceruius in the brain, release of um adrenaline, dopamine, adrenaline from the adrenals, dopamine presumably within the brain, but that the parasympathetic response is activated when we put our face into cold water or go underwater and that's a calming relaxation response. So, this brings us back to I don't want to take us off track from you describing the study, but this brings us back to the first question, which is if I go completely underwater for a moment when I start my cold plunge, does that change the physiological outcome as compared to if I just submerge myself up to the neck? And that and actually nowadays there's seems to be a little bit of a movement online of people putting a bowl of ice water on their countertop and submerging their face into it. Did you see this? This is starting I've seen more and more posts about this. So um could you just touch on the what the dive reflex is and why it act perhaps activates the parasympathetic response this calming response. Well so the diving reflex is activated when you submerge into cool water. Um

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when you submerge into cool water. Um even just to the neck. Yeah. Or I thought you had to get your face under. I'm not arguing different. You're the expert. I just want to Yeah. I haven't really I haven't read that. I've just seen that you can activate your uh diving response as soon as you go under water with your body. Um so you don't have to do it with your face as far as I understand. I could be wrong though. Um yeah, so when when you activate your uh diving response, you will slow down your um your oxygen consumption in your body. And that is because the body tries to res preserve um oxygen so you will not get hypothermic too fast. So it's kind of like a survival system in your body. Um so this survival system is very um important for us of course. So that will be activated and uh because of that you will after maybe one minute or so I can't be precise on that because I maybe also varies a bit in in humans. So one to two minutes you will have full activation of the sympathetic nervous system but also the parasympathetic nervous system. Um and that's going to activate for examples something like serotonin in your brain which is like also good for mental balance and people feeling in mental balance afterwards they after they go up. So that is like measured on a questionnaires and also measured like on on anecdotes of course people tell all the time that they feel good afterwards. We need studies on this. So if anyone's sitting out there thinking that's interesting, then please do some studies on that to get more out on that. Yeah. So you had So you observe these winter swimmers who are done this for a few seasons. Yeah. They're coming around for a new season of winter swimming and you've decided to recruit them as subjects. They are getting into cold water. Um climbing down a ladder or jumping into the water up to their neck. Yeah. Climbing. Yeah. Okay. climbing down a ladder into because this is done outdoors. What a fun study to do. My my graduate thesis

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fun study to do. My my graduate thesis was done under fluorescent lights with no windows in a building that uh I had a ton of fun as a PhD student. I actually lived in the laboratory as a PhD student. I loved it so much. But um not something required to do a PhD by the way, but um they're climbing down the ladder. Yeah. Getting in up to their neck, staying in for one to two minutes, and then getting out. And how many times a week are they doing this? So they do this two to three times per week and for each time they go each day they go uh they take three rounds uh of so three dips and two sauna sessions. So they start in the cold and they end in the cold water. Okay. So it's get in for one to two minutes then get out and get into the sauna. Yeah. Um what is the temperature of the sauna? Uh about 80 ° C. Okay. Then how long are they in the sauna? So they stayed there for 10 to 15 minutes. So depending on if they went two times per week or three times per week. Okay. And then they get back into the cold for a few minutes. Yeah. 2 minutes. Up to two minutes. Yeah. Okay. Then back into the sauna 15 minutes or so. Yeah. Then back into the cold for a third round. Back into the sauna and then then they're ending on the cold. And then back into the cold again. And then ending on cold. And uh and we will talk about why it's important to end on cold the so-called soberg principle. Um how cold was the water in this particular give an average because I realize it's outdoor winter swimming so it's going to vary depending on wind chill and things as well. of course. So it's a very uncontrolled environment to do this kind of study in. But I wanted to do something that was also very close to something people could do for free going out in nature and use that and also have the nature like it's a very healthy impact on us. It lowers our stress uh stress level as well. So by doing so I also measured the temperature every time they went. So I have this graph and it's actually in the winter swimming book. It shows the temperature

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swimming book. It shows the temperature in Denmark going like from October to April and it's like starts at 12 degrees. I think it's around 12 12 degrees Celsius um the water and then it goes down to 2 ° in on average in January and then up again. Um so it's within the spectrum of very cold water I would say from around 15 15 ° C and and down. uh it was actually not colder than like 2 to 4 ° like in in on average when it was the coldest. So it doesn't have to be that cold to be good enough and and enough to activate our metabolism. Um and what time of day are um the participants doing this cold sauna alternation? So I think they did this uh throughout the day. So I didn't control whether they wanted to go in the morning, in the afternoon or or in the evening at the time where I set up the study. I w I was not controlling it in that way. I wanted them to go whenever they had time. And I also think that is the most important message to give out to give to people is when do it when you have time. It's not if doing it when you get home from work and it's 6: 00 in the evening and this is the time where you are where you can do it then try out if it's going to impact your sleep or not. Uh if it doesn't impact your sleep then fine but you have to try for yourself and find out what works for you. It's the same for coffee for example, right? Some people can drink coffee in the evening and go to bed and they can sleep. Uh I can't. Or exercise. Exactly. So I can't do that. And that's because the coffee exercise cold water um immersion is going to activate your sympathetic nervous system. You have an increase in stress response in your body and and that's going to make it really hard to fall asleep for some people at least. Maybe you are super exhausted anyways and then you will just crash anyways. But yeah, but um that's that's the only thing. So I just told them to do this if they can during the daytime. Um and

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they can during the daytime. Um and that's primarily what they also did. Mhm. And then uh all along you're measuring brown fat by way of this infrared uh camera, right? Um so what did you observe in terms of changes in brown fat? How quickly did that occur? And um and then I'd like to ask also about sauna a bit more. Um because earlier you mentioned that you can activate brown fat with sauna as well with heat on the surface surface of the skin. Um, how long did it take before you observed significant increases in brown fat? Um, and was it increased density of m brown fat or distribution? Was it, you know, showing expansion to different regions throughout the body? And, um, maybe you could also touch on some of the changes in in insulin sensitivity and metabolism. Yeah, a very good good question. And and I didn't mention this before, but besides measuring um temperature as an outcome for brown fat activity, we also uh did PET MRI scanning of the brown fat. So this is like the the golden standard uh for measuring brown fat. And it's not very feasible for normal people to get an an a PET CT or PET MRI scanning of the brown fat is super expensive. Um, so we had both uh to see if we could have like a continuous measure of brown fat in humans because that was already not not out there. So I wanted to see during both the experimental days but also during day and night what kind of like circadian uh rhythm do we have in our brown fat activity. So that's why I wanted to have that as well. So the PET CT scanning or the PET MRI scanning was to see upon cold activation um stimulation for some hours. Do we have activation? Can we see the brown fat in this subject? And also during thermutrality or thermomfortable state uh how is that activated in each

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state uh how is that activated in each of the group of course. Ah so you want to see how comfortable people were away from the cold water and sauna just at different um temperatureed environments. Is that right? Yeah. So I also measured that how comfortable are you? I I made this scale like visual analog scale and asked them how comfortable do you feel with this temperature? Um, and throughout the study days during cold exposure and thermal comfortable day, I had a whole day where I just kept them thermomfortable to see do they activate the brown fat if they're just completely thermal comfortable as good as we could get with that because we were asking people um on a scale from one one to 10 and five being thermomfortable, where are you on this scale? So, one would be very cold and 10 would be super burning hot. Um, yeah. And so that was a way to like try to figure out how do they actually feel also during the studies. I also measured um electromyiography so of of muscles to see do they shiver during the cooling day. Uh sometimes people shiver before they know they're really shivering. So I had interesting yeah yeah so so our conscious perception of shivering might not be the best readout of shiver. Yeah. Well, you if you also get adapted to the cold water, you will have less shivering. They will be less vigorous. They will be very small. So, you wouldn't probably know that you are shivering because this the shivering is so small and the mitochondria in the muscle cells will be so densed that it doesn't need to shiver maybe that much to get that thermogenesis going. um compared to when you're completely new uh to cold water exposure, you're not adapted, then the body needs to create these mitochondria, these energy fabrics to keep you you warm. And that's also what the exercise is in the beginning. But um when we measured this, we we did see that the winter swimmers were shivering less or uh having less

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shivering less or uh having less vigorous shivering when they said I'm cold. So even though they they perception their perception of the cope was pretty similar in in the groups um we could see that the the activation of the the muscles that we measured on um were different and more victories in the control group. Were the subjects incentivized to be in the study? Were they paid or anything of that sort? They just happen to like doing uh cold and sauna and so that's why they did the study. Um well they got paid a little bit for it but not much and um that's how we do this. Sure. I was just curious. Yeah, I was just curious. There might be some folks that wonder. So So what did you discover in terms of changes in brown fat, insulin resistance or insulin sensitivity rather and um metabolism. So what we we saw uh was we had this kind of different measures uh to see what's to try and unrell what what's actually going on when they are already adapted to the cold water compared to a control group who was matched on on various parameters. We we did see that the winter swimmers had an increased insulin sensitivity. They produced less insulin uh on all the experimental days. So besides from just cooling them and measuring the brown fat on each of these cooling days, there were two cooling days and one thermomfortable day, right? So I wanted to measure insulin when they just they were fasting, meaning that they hadn't eaten in uh 8 hours before the study day. Um and they were completely laying still, not moving, just in a bed. Um and we measured insulin uh during the experimental day just to see how what level are they on and we could see that the winter swim had lower production of insulin and they also when they had an glucose drink. So we give them that to see if they to test before we enroll them in

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they to test before we enroll them in studies to see if they have diabetes for example and not knowing for example that that wouldn't that would like ruin maybe the study. So we test for that and see if they have like a normal curve. So what we did see in that was that the winter swimmers had a faster glucose clearance in the bloodstream. So after 2 hours uh we could see that they had a lower level and it went the curve went down faster than in the control group. So despite having lower insulin release they have better blood glucose clearance which is really what you want what we all seek right? You don't excessive insulin is bad. Insulin being a um more or less a chaperone for blood glucose um um can do all sorts of other things as well of course but um and having high blood glucose obviously terrible yeah for cells especially brain cells I don't think people realize how toxic high blood glucose is having high glucose is for brain if you want to kill neurons you make their uh put them in an environment where there's too much sugar sugar ah ah um um oh yeah very neurotoxic I mean that's and their mechanism like insulin that buffer that we keep you know keeping blood glucose in a reasonable range so that um that doesn't happen I mean I think that's why people will go into insulinemmic shock um hypoglycemic shock is also possible so that range in which neurons are happy is not a tremendously large range incidentally the range in which neurons are are um happy and surviving uh is much greater as one gets colder than when you heat up I mean you you can basically destroy brain cells by getting too hot for too Oh yeah. You can definitely destroy brain cells permanently by getting too cold for too long, but you have to get really really cold for a really long time. Yeah. Very interesting. Yeah. We we're thinking about doing an episode on um uh sort of survival of the brain after death kind of things which actually happens. You know, you hear about these people who are declared dead and then come back and there is actually now a lot of crowd

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and there is actually now a lot of crowd preservation type approaches for that. This is uh anyway we risk going into the the uh the esoteric now. So we'll steer us back to our discussion about your study. But um so if I do the math, these subjects are in the cold. Let's say they're doing three rounds of cold for one to two minutes three two or three times a week. What were the thresholds that you discovered were important for getting these positive changes in um such as reduced blood sugar or clearance of blood sugar um being more efficient uh reduced insulin um improved brown fat distribution and density. Um how much cold exposure do people need? How much heat exposure do people need in order to extract these benefits? Yeah. So when we then calculated the numbers together, we can see that this was ended up being 11 minutes uh in total per week. Uh so not in one session of course, but they had two to three visits to the water and the sauna per week. So when we divide that out, it corresponds to being in cold water 1 to two minutes at a time, but also in the sauna 10 to 15 minutes at a time. And I think this is very like um also similar to what we see in other studies when we look for example to the observational studies from the Finnish uh cohort study from LA and Ital for example. They published this very amazing paper in 2015. um some results from this long cohort study where they show that um that up to 30 minutes uh in the sauna was uh healthy and they you you lower your risk of cardiovascular disease and that's like the threshold and if you go further than that then there is not more uh um healthy benefits to to gain from that. So um and before that it's like 19 minutes then you will have this dose response relationship up to 19 minutes that's really in decreasing your risk of

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that's really in decreasing your risk of cardiovascular diseases and I think with that's per week. 90 minutes per week uh 90 minutes per session now per session. Yeah. per session. If we um uh then compare that with my study which was 10 to 15 minutes per session, then I think it fits very well with what we call the homatic stress or healthy stress. So you expose the cells to this kind of like potent very stressful situation uh where they increase heatshock proteins in the cells and that will repair the cells. But if you then overdo it and you go beyond the maybe 30 minutes in the sauna, this observational study from uh Finland with more than up to 2, 000 sona bthers where they have followed these for 20 years, they see that uh 30 minutes per session is like enough and if you go above that you don't get more health benefits out of it. So I think there's a window where we can say the healthy stress corresponds to like 10 minutes and I think it's like per session and it's not much actually. So you don't need to it shows that you don't have to expose yourself very much to the heat or very much actually to the cold to get this healthy uh benefits uh from going into cold going to heat and have healthy benefits on your cardiovascular system. So I think this is very important also message to get out that you don't have to go extreme. You don't have to swim for a half an hour in the cold water. You can go in the water for 1 to two minutes um per session but go up to 11 minutes per week in total. And for the sauna my study showed uh 57 minutes in total per week. And if we also then divide it out on these two to three days and two sessions each day correspond to 10 to 15 minutes. So it's a low threshold but I think it's it's good to have that to maybe we can aim for that if if people need to have something to

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if if people need to have something to to aim for. And I think it's really good to have that because then you you don't have then you don't overdo it and if you overdo it you exhaust the cells and that will increase your risk of cardiovascular disease also. So, well, I get a lot of questions about this and I did solicit for questions for this podcast on on Twitter and one of the questions that I got was as one becomes more cold adapted, do the benefits start to wear off or can people do too much cold exposure? Of course, the answer to that is yes, you can become hypothermic. I'm sensing a different different answer now which is if I understand correctly um the threshold is 11 minutes total per week of deliberate cold exposure divided into two or three sessions of maybe 1 to 3 minutes depending on how long somebody stays in and then 57 minutes I want to be careful not to round up um to an hour but divide it into maybe three 20 minute sessions or so you know um so one doesn't have to be perfect as long as you get beyond that threshold. But I I wonder something which is is it the case that if somebody said, "Oh, you know, I'm just going to do one 11 minute session per week." That might actually not be as beneficial as dividing it up because what you told us earlier is that the hormetic response depends on having that cold shock. You actually don't want to become too cold adapted. I mean once the blood pressure response drops down so in minute four five and six you're getting very cold and you're shivering but you're one is not getting the autonomic stimulus that they want. I guess I could liken this to um if exercise worked in a way where it was only the first few minutes of exercise that really triggered the adaptation. Of course this is not how it works but um in fact probably quite the opposite. Um, but if that were the case, then it's not simply the total amount of exercise, but dividing up the sessions into little bouts where every single time it acts as a stimulus. That seems to be the key here. Um,

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be the key here. Um, this is very important. Um because having watched the landscape of this on social media but also in books and generally um I think you're the first person to really touch on this that the goal is not to get so cold adapted that you can sit in for the full 11 minutes in one session where the goal isn't to be able to do an hour of very hot sauna if you want to. I suppose people could do it for other reasons but if the goal is to improve these health metrics Yeah. then the idea is to keep the stimulus a stimulus short. Exactly. Yeah. Great. Well, this also I think there's practical feasibility as you pointed out because getting into a cold shower or cold immersion or natural body water for a couple of minutes is far less uh um you know challenging to most people than finding a full morning to go, you know, spend there. Um but I've never really heard it articulated that the longer sessions might not be beneficial and might actually be detrimental. Um Um that's very important. Uh were there any other observations that you made um that did not make it into the paper or that were kind of in the the margin notes in terms of psychological benefits um or anything of that sort. There was this recent study on soldiers that talked about weight loss. It's sort of a controversial study um for a lot of reasons. Um but one of the things they remarked uh in the paper was that there were a lot of psychological changes, improved buffering against anxiety. Um they even the men and women in that study um reported one of the significant effects was um significantly improved sexual satisfaction of they didn't tell us what that meant for these subjects but so we won't go there but but a number of subjective improvements. Was there anything that you observed or took note of in in your study that perhaps didn't make the main abstract but that we should be aware of? Uh yeah, there was some um and I'm I'm today I regret that I didn't measure on sleep for

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that I didn't measure on sleep for example. I uh I frankly didn't really think about that when I I when I designed the study. Um so we were very um much occupied with the metabolism and kind of had the thought maybe this could impact uh sleep quality. Um and I wish I just I had the thought that why don't you just ask them in a questionnaire but I asked them every morning or every morning it was not many mornings just two mornings actually we measured on um but the winter swimmers um told us before I wrote them that they had a really good sleep quality. The control group also had that, but they told me on the day where we measured brown fat on on a day and a night, so for actually two days and two nights, um they told me that they didn't they had a good night's sleep, but they also woke up. So, it's just telling me that they also had like a quick wake up and then they fell asleep again. Um and the winter swimmers uh told they have a really good sleep. So it's like and in general they also say say we we sleep very well. I sleep very well. So it's anecdotally in general it corresponds to what I heard in my study but nothing that I measured on um which would could be fun to do in the future but we didn't measure on sleep quality. That would have been a really good idea to do. They also told me that they were very comfortable uh when they were cold. They they don't mind winter swimmers. didn't mind um going out for example um in in the cold with a t-shirt. They were also less uh scared of showing their skin. That was also one observation. Interesting. Yeah. So kind of a reduced social anxiety. Yeah. They were just so comfortable in the lab. You as you just mentioned before coats on and everybody's joining around. It was very busy and um all the other um scientist out in the hallway

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other um scientist out in the hallway and also my supervisor had her office down down the hallway and and one of the winter swimmers one day just got out of bed after had been in the study for eight hours. We it was a long day, right? He jumped out of the bed and had his clothes in the bathroom and he went out completely naked. He didn't care. He just went out. It was like wait. So that's a side effect perhaps of getting too comfortable with the cold. Um we're not recommending that. Although in your book you um did a uh you dedicated some Let me start that again. Although in your book you dedicated some uh pages to um naked winter swimming or I should say naked cold water exposure as opposed to um uh with bathing suit. Yeah. Are there any data on this? I'm sorry chuckling but um I think in most places in the United States it's uh skinny dipping is not um is not legal uh most public beaches there are few in fact where I I my laboratory before moving to Stanford was in San Diego and I at the Sulk Institute for Biological Studies beautiful building incredible sciences done there the beach right below that it's called Blacks Beach okay okay and um and it's a known nude beach um and so whenever tourists were heading down the the stairway way there. Um, I would, you know, sort of let them know, especially if they had kids, I'd let them know, you know, it's and it's a nude beach of a particular um uh of a particular genre. So, I I'd give them a little warning about what what they could expect down below. It's a um in any event, th those beaches are quite rare um in the United States, maybe compared to Europe, I don't know. Um but yeah, maybe. Yeah. So, is there anything special about um clotheless versus clothed um exposure? Yeah, I think in that sense uh we are a bit more free with this kind of like but but remember we also had this winter swimming culture for so for hundreds of years in Denmark and the winter the oldest winter swimming clubs that we have especially the one we have

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that we have especially the one we have in Copenhagen where I did my next study which we haven't talked about but and it's also not published yet but um in that winter swimming club it's the oldest one we have and it's huge and they they swim naked at this facility men and women and they have sona where they can go in together and they also have the separate sonas but it's very much a a Danish thing and and I think it's good if people want that um and I had it in my book because people want to know if they have to swim with their bathing suit on or if they can take it off or what's the difference is there any difference in this and if you ask me there is no difference of if you have your little skinny bikini on it's not going to do any difference to your cold exposure or your adaptation. It's not going to do any difference for your benefits of course, but I think that it has something else. It has something to do with how you also observe yourself, how you observe your surroundings and it's some sense of freedom in skinny dipping. So I think people in Denmark who does this is they do the winter swimming because they feel free when they do this. to come home from work, they go to this club and they skinny dip and they feel like in touch with nature and they have maybe done this their whole life. So this is an old tradition in Denmark. In some of the clubs um but the newer clubs that are coming, they don't skinny dip. So everyone has bathing suit. I never skinny dip because there are people around people with phones and taking pictures all the time. So this different different nowadays. Everything's recorded. Yeah. And also this this old tradition is also fading away because of that. Yeah. I um I use sauna and cold at home, but when I travel there there's a a ba. So Russian ba has hot sauna and cold plunge. Um there's one in San Francisco called Archimedes Ba. Um and that one is

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called Archimedes Ba. Um and that one is clothing optional. So some people are clothed such as myself and then other people are not. And it's co-ed most of the time. I think they have male female separated uh evenings or something like that. And then um the other BAS is Spa 88 which is in on Wall Street in New York is an amazing BA as well. Um and these are starting to crop up in different cities or maybe they've been there for a long time and as deliberate cold exposure and sauna gets more popular more people are using them. The the one in New York that I that Spya 88 is always closed. Um and it's interesting because you know people hear uh naked or skinny dipping and they they might get certain ideas in mind. It um all these places are very well lit and they all have a tone of kind of um of health that is about the kind of health and wellness. Um I guess the point being that um there's no requirement uh to do one thing or the other. Um although in the studies that uh you did obviously um people were clothed but I I did um I did pay attention to those pages in your book. I thought it was interesting that you put some um some dedicated uh passages in your book related to this and I think my publisher wanted that. Oh, your publisher wanted that. It was not me. It was like my publisher really wanted to have a little discussion about that. So I was like okay. Well I think it you know it points to a larger theme which is I think for a lot of people who already do these practices um there's no shock there. um for people that do not do deliberate cold exposure or sauna. I think that um you know there is this idea perhaps that oh you know these are um traditions that are are kind of fringe or that they're kind and I just I want to um cue that point because there's so many things that are happening right now in biomedical research and medicine. you know, serious quality peer-reviewed studies published in excellent journals like your paper on things like deliberate cold exposure, sauna, um the use of particular supplements, natural natural herbs and supplements. I mean, there's an entire branch of the National Institutes of Health in the United States dedicated

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Health in the United States dedicated just to the study of supplements and behavioral interventions for health like meditation and breath work. Really incredible. It's really incredible. and psychedelics of course being something that for a long time was part of a certain community and feel and now is being um frankly adopted by mainstream med medicine even pharma. So the the times are changing um and so uh yes I think it's important to know that um it's perfectly acceptable and encouraged to wear clothing. Absolutely. Yeah. And and one other thing that uh I wanted to to mention going back to your questions around if there were any observations in the studies which we really maybe haven't discussed yet and maybe it's in in the back of the the paper and not mentioned that much was one of the winter swimmers didn't have any brown fat when we measured him. Zero. And uh in we do see this um in in Zero. And uh in we do see this um in previous studies as well that some humans don't have any brown fat. Was he did he carry a lot of white fat atapost tissue? Was he obese? No, he wasn't. No, he was not obese because that he would not have been in the study then. Oh, right. Yes, you mentioned this earlier. Forgive me. No, it's right. But he was um but what I did observe before I knew that he didn't have any browner was that during the cooling experiment where I cooled them for uh two hours before they go into the PET scanner uh he was not able to control his uh shivering like the winter swimmers uh could. So he got very uh cold very easily uh compared to the others. So, and without I didn't know what was different about him. But we could all all me and the three others were working on the experiment. We were like, "Okay, what's going on?" Because we turned down the the temperature, but he started like shivering and then we had to turn it up again and it was just

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had to turn it up again and it was just all over the place. The temperature is not it wasn't that controlled like the others. It was pretty similar protocol. I could just do pretty much the same because they were same size and also same gender. So it was easier uh to like foresee what was going to happen and when will they start shiver I quickly learned that but with this subject it was just with this uh volunteer was just very much different and then when we scanned him and didn't find any brown I didn't even think about it. So when we scanned him we didn't see anything. I told the PCT people to like oh you put up the wrong uh scanning blame the technology. Yeah, it the technology it was like this scanning looked like the thermonutral day, the thermomfortable day where we also scanned them to see if they have any brown fat. So you have made a mistake. I was pretty sure and they reanal anal an analyzed this um scanning and they just concluded well it the scanning was fine. The experiment went well it was just that he didn't have any brown fat. So he was like what we just in the paper called a brown fat negative. So he didn't have any and in in my studies it would be called a knockout. So he didn't have any brown fat. So what the observation with him and I think that would be that's interesting is that he both shivered very early on and didn't regulate his temperature as well. He also told me that then he was like a five on the scale of how comfortable he felt with the cold. Um so it was from 1 to 10 and five being thermal comfortable and 10 being uh very uh cold uh and and one very hot. So on this like scale up and down and he he was like more up and down on this scale than any of the others. It was an observation that I did. Um, but we did see in his blood samples also that his blood samples looked a bit more like the control group. Um, also his

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like the control group. Um, also his insulin levels were like the control group, so a little bit higher than the other winter swimmers. And he also had um a a he his glucose clearance was not as fast as um as the other winter swimmers. So he was like an outlier, what we call it. And in the analysis, we also had to take him out of the analysis because he was an outlier. Um, so the results showing that the brown fat uh is uh more efficiently activated in the winter swimmers is without him having him in that group. But it didn't ruin this the study if we I tried to put him in as well and didn't it didn't ruin the results or anything. But just to to keep it more clear, we put we took him out of the analysis. Yeah. So, he was a mutant, a knockout. Yeah. And I'm sure they're out there. Um, very interesting. So, if you shiver early, um, then perhaps you have less brown fat to begin with, although it's hard to conclude from one person. That's sort of the implication there or you haven't adapted to the code. So, you should build that up. Yeah. Right. So in addition to looking at regulation of blood sugar, brown fat, metabolism and so on, were there any markers that you examined in the deliberate cold exposure group as compared to controls um that reveal to you that deliberate cold exposure could have additional benefits um say for uh immune system function um or for any function for that matter. Yeah. So for um we looked at in inflammation of course we measure the outcome of blood pressure and so on but we also measured the IL6 in the study just to see also an inflammatory anti-inflammatory marker. So IL is 6 went up and it also follows with the IL 10. So that is like also very known in the literature. So we measured that and I think it's very important to to think about the cold exposure and the heat exposure as something that then lowers

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exposure as something that then lowers the inflammation in the body and with if we can do that we will have an open door for um uh preventing lifestyle diseases right so for um type 2 diabetes but actually also for some mental uh diseases as well so as known as depression and anxiety and also Alzheimer's disease which are all associated um in uh research also new research showing that that inflammation increases the risk of depression, anxiety and Alzheimer's neurological diseases. So if we can decrease inflammation in the body, we will decrease modern lifestyle diseases but also these increasing um mental diseases that we see in these modern lifestyle times. Um, so I think that that it's I think it's very interesting that we can go out in nature and we can use these natural stressors and it it I don't want to have it sound very romantic or anything. It's just exposure to temperature actually just a cold or to heat that is going to trick our body into a natural state again and and reset it where the the homeostasis the balance has is lost a bit. So the body is going to repair itself in that way. And I think it's beautiful that we can do that just by changing the temperature of our body. And although people are very scared of doing this because in our times we have been away from cold away from heat temperature for some for decades now um since we isolated our houses better and uh we are more sedentary. We also sit more indoor. We don't move as much. So this very um modern sedentary lifestyle has made us more temperature comfortable just neutral. So no no wonder I mean that obesity is increasing. We don't expose oursel to the natural stasis that we did earlier on um in in our evolvement but al up until maybe the 70s

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evolvement but al up until maybe the 70s the 60s where we started having more like uh comfortable lifestyles right and obesity increases in the 80s we can see that from statistics so I think that if we can take in cold and heat and you mentioned other things also before but of course exercise is very important here and also a bit of fat testing actually because it all increases the homatic stress in the body. So it's doesn't have to be uh other than natural stresses to the body which then could keep us in that natural balance again. Could we talk about what I refer to as the Soberg principle, which is to end on cold? And the reason I called it the Soberg principle is because um in reviewing Oh, by the way, I wasn't a official reviewer of your paper, but I mean in reading and um reviewing your paper for its after published contents, I noticed that you had people end on cold. And this had been a longstanding debate in the the uh deliberate cold exposure community. Should you warm up in a with a warm shower afterwards or get back in the sauna? What should you end on cold or end on heat? And the sober principle says end on cold as I understand it in order to force your body to heat itself back up and thereby increase metabolism further still. Is that right? Yes. So when you end on the cold, you you force your body to heat up by itself and that will require that you activate you keep your brown fat activated and also your muscles which is a good thing. It's a good collaboration to keep your thermogenesis up and that's like an an exercise even when you go home. So in that way you don't have to think about your cold exposure or dipping in in your plunge uh or open sea or what it is as just a an exercise that you do for one to two minutes and then it's over. If you end on the cold you have an exercise for your body going on for hours afterwards. And that's not only on your metabolism but it's also

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only on your metabolism but it's also going to keep your neurotransmitters activated as well and increase that because your body is still cold. So you need that those neurotransmitters to activate the brown fat as well. So that's going to make your brown fat cells more efficient and also your muscle cells more efficient. So increasing mitochondria in the cells which will then generate heat very fast. So if you have done this for a few times, so maybe three, four, five times um being new to this but have tried it a few times, you will notice a switch where you like feel that you get easily warmer and you can keep yourself warmer and that is also what we shown in in my study is that the the winter swimmers were physically uh warmer on the skin compared to the control group. So they when they are out of the cold when they're out of the cold just relaxing and we tested this in in on the days where they were sleeping in the lab. So we could see that they had uh a more activation of the brown fat a higher temperature. So probably because they also lose heat uh they have a higher heat loss to the body compared to the control group um because they have a more vascular skin uh because of the contrast of cold and heat. So they lose heat faster from the body during the day. But is that a bad thing? No, probably not because that's going to keep your brown fat and your muscles a little bit activated. So we you will have to it has to work to keep you warm. I and I would hypothesize that it also might lead to some of the um subjectively reported improvements in sleep because in order to fall asleep, you need your core body temperature to drop by about 1 to 3 °. Yes. So, it's not just sufficient to be sleeping in a cold room and under the blanket, you also need your body temperature to drop. Yes. And so, what you're um saying, if I understand correctly, is that by forcing by ending on cold and forcing oneself to heat up naturally um that increases the brown fat stores, which I sort of see as

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brown fat stores, which I sort of see as a kind of like the oil in the candle. Yeah. Of the furnace that is thermogenesis. And that in turn leads to increased heat loss which people might think oh I don't want to lose heat from the body but there are times when you want to lose heat from the body. Basically it sounds like what we want is to be a very efficient heating and cooling system. Yes. That it's not about being cold or being hot. It's really about keeping the system tuned well. Keeping the oil in the candle this brown fat um functioning. Yeah. What could I ask one question about um fed or fasted? Is there any rather are there any known benefits of doing deliberate cold exposure and / or sauna fasted uh versus um after a a meal say within the last hour or something of that sort? I do my deliberate cold exposure first thing in the morning. Um so in general I'm fasted because I don't eat until a little bit later in the day. Yeah. Uh but what's known about that and was that looked at in your study? I know you measured glucose, but that was as a separate um test away from the cold. Yeah. But I also tested glucose on the days on the cold. So, we measured that as well on on the cooling days. Um specifically on fasting and fat, I don't know. Uh I don't think that I've seen studies specifically on this. Okay. Um more science needed. Um, a number of people ask about the use of deliberate cold exposure to offset some of the symptoms of various diseases. Now, here we're not talking about curing disease. We're talking about offsetting symptoms. Um, one question I've seen quite often is whether or not people with Ray nodes syndrome. This is a syndrome. My high school girlfriend had the syndrome and I'll never forget uh we went were at a a school dance together and um this was when we first started dating and um she had reodess which leads to very poor blood flow to the the uh the extremities and um and she was

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uh the extremities and um and she was very cold so she left to go to the bathroom and warm up her hands in the warm water. And I was left standing there at the dance and people came up to me and asked you know why I was there and who I was there with. And I kept telling them who I was with and they didn't believe me cuz they couldn't believe that she would be with me. Made total sense if you knew me at the time. Um I was I I was way out of my league with her um at the time. I like to think eventually I caught up. But in any case um uh she was in the bathroom for about an hour. So at one point I did consider the possibility that she had just left. But indeed she hadn't. She warmed her hands back up. But people with Ray nodes suffer from this um from this thing of very very cold extremities. um their fingertips will even turn blue um you know as if they were starting to get frostbitten. It's quite dramatic. Um and that question gets asked whether or not there's any use of cold to try and increase the um elasticity the plasticity of the uh small capillaries and vessels. Um by everything you've described um up until now, it seems like that would be a logical thing to do. Um, and in addition to that, whether or not people with autoimmune conditions, people with um, any other types of conditions are known to benefit from deliberate cold exposure. I'm not aware of any studies, but I get asked about this a lot and there were a lot of questions about this for you in in the Twitter feed. Yeah, thank you for those questions and I get them as well on social media and I have to say that I I haven't seen any studies directed on this outcome and measuring reoids syndrome. Um I do know that it's it's not that rare actually a problem and I know that many women or more me women than men uh suffer from this. Um but logically it would help them if they exposed their hands to cold and also heat to make the ve it more vascular but and I have heard from people saying that it had helped them but also heard for some others saying it didn't help them. M M so studies are needed on this specific

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so studies are needed on this specific topic I think. Mhm. I hurt my hands when I go into the coine. I don't have this syndrome at all. But I keep my hands above the water. You do? Yeah. And I do that um often I take a little bit of a swim and then of course I have to have my hands in the water. Uh but it helps me when I then get back to the daddy um and then take my hands up because then I can stand there for a little bit and get my one to two minutes exposure uh and then I can go up because then otherwise that would stop me from being in the water enough time that I as long as I I would like to. So if people suffer from having this pain in the fingers and it it can be very intense. So just take the hands up a bit from the water and uh and that's going to help you. Also boots, neuroprint boots. It's going to help on the feet. Some people have the hurt um feel the pain in the feet and or in the ankles and that's going to help them also a little bit. Okay. So there is no problem with keeping hands out or feet in uh neopre booties if people feel the need to do that. If that's what the if pain of the hands or feet is a barrier for people doing uh deliberate cold exposure then it seems it would be okay to do uh to keep hands out or to keep your feet in yes because then you you do get the exposure uh but of course hands and feet are very potent uh places in your body to get a fast activation uh of your nervous system of course but if you can just you can also just dip them and then take them up it's going still going to activate that but you have your full body is covered in in the coke receptors, you'll have a full activation anyways. So, you you are providing very reassuring information to people because I know a number of people that do not like to put their hands in. I find that the more of my body I get in, the more comfortable I am psych I don't know if it's psychologically and or physiologically. I find that if where there's an

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I find that if where there's an interface between the water and the cold, it's most uncomfortable. So, I prefer to just get everything under. I keep my head out. Although I these days I've been dunking all the way in and then coming out and then dunking once more with my head under before I get out after the the plunge. Um that raises a different question. Now we're getting into kind of the practicalities of deliberate cold exposure which I think are important. Um sometimes I'll experience and I hear from a lot of people that they'll get a kind of back of the head headache at the interface of the water. Um you know when they're in doing cold immersion to the neck. Um, okay. I assume this has to do with blood flow, that there's vasoc constriction right up until the neck and in the region surrounding it, but that maybe there's still blood flow to the head, but do we know what the origin of these um headaches is? And again, this doesn't happen for everybody, but some people do experience them. Okay. Yeah, I haven't really heard about that one specifically. Um, so but I would say that there are different reasons for maybe keeping your head out of the water, but it seems like maybe for some that could be a reason for like just getting like a quick head dunk going all the way in. That's what I've started doing to eliminate I I wasn't getting headaches, but I could I noticed that interface and I wasn't in the rest of the experience it so experience of it so much. So I started dunking all the way in. I noticed in some of the photos that you've put out um and in your book that you'll sometimes wear a cap while you go in. Yeah. Okay. And well, it comes from uh different reasons. Uh so let's talk about some of the physiological reasons. So when you submerge in cold water up to the neck um studies have shown and this is from Denmark studies from Bispia hospital um that uh when you submerge into cold water up to the neck at 0 ° uh so 0 degrees Celsius very cold um you will have a decreased blood flow to the brain by around 30 to 40%. and makes sense because you activate the sympathetic nervous system uh and and therefore you will have a less blood flow to the brain

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will have a less blood flow to the brain makes you maybe a little bit dizzier and proof again that you're you need a heart more than a brain because when the sympathetic nervous system gets activated uh blood flow is maintained to the heart to keep you alive but obviously taken away from the brain to keep you from thinking. That's why it's hard to think when you're stressed. Yeah. Well, the muscles and and your vital organs need to you have to be able to run away from that tiger, right? The rationale makes total sense. Who am I to disagree with mother nature? Well, but yeah. So, one of the reasons being that you should keep your head out of the water is that you could increase that uh decrease in blood flow to the brain further if you dunk the head. So there's this very nice paper from um from a a research group in Canada where they have collectively looked at different papers where they uh compared heat loss in a group in the papers where they dunk the head and compared to heat loss uh submerging up to the neck to see how much extra heat do we lose from our core when we dunk the head. So uh and I think it's very interesting that if you submerge up to the neck you have a heat loss of 11% from the body core and when you then uh also dunk the head you will increase that heat loss rate by 36%. Wow. So that mean I'm not saying that I'm not here to say what is right and what is wrong. I just think that people should know the information so they can for themsel evaluate what is best for them. But if you increase your heat loss rate by 36% from your core, that's going to increase your after drop, which we touched upon a little bit earlier, even further. So that's meaning that you are closer to hypothermia than you are if you just submerge up to the neck. So you should really think about whether this is like something that you want to do or if it's just better for you not to get that cold in your core. The beanie is also because I have a

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The beanie is also because I have a little bit of sensitive ears. So it meaning that if there's wind and because we swim in the open sea in Denmark, we have a lot of wind. Our wind our conditions are just very rainy, very windy and when the temperature is also freezing, you could get this um what is that called? Um so very cold and lightaded just from wind. So if you also submerge into cold water and you then get up, you could you will get a brain freeze immediately. So it is enough to just go up to the neck, wear a beanie to just not get dizzy also because the the um the heat loss is increased of course, but it also the blood flow to the brain has decreased. So the beanie will keep you a little bit warmer so you can stay for one to two minutes. So it's just a way of like getting around some of the conditions also. So people can choose that if they they feel that. But it's quite normal to do in in Scandinavia wear a beanie. I love it. And um so for those of you afraid of doing a two-minute cold shower, what uh Dr. Silverberg just described uh let's uh you see how um she and others are capable of doing things far harder than that. Um when the way you describe it with the cold wind in Scandinavia and uh um is is quite striking. along the lines of covering the head. Um there's this um seemingly paradoxical paradoxical uh thing of people going into hot saunas and wearing wool caps. You know, if you go to a BA or you go to a sauna and there are people who are um well from Eastern Europe or typically or Finland or um Russia or uh Ukraine or elsewhere, what you'll see is that many of them are wearing wool caps in the sauna, which many people think is to make it hotter. That's actually not the case. It actually insulates you from the heat environment. The urg the sense of urgency to get out of the hot sauna is a brain driven mechanism. And so um the reason that people wear wool hats in the

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reason that people wear wool hats in the sauna is it actually lets you stay in the sauna longer. Um because it takes a lot of heat to the skin before you feel that you you have quote unquote have to get out. Whereas so when you insulate the brain um you don't get that signal. Um it's pretty interesting. I've tried this before just by putting a towel over my head in the sauna and you can stay in there, you know, much more easily and for for much longer. You know, as we talk about these different stimula for um the hormetic response, the adaptation to stress, you know, it occurs to me that the the big ones in our evolutionary history have been light, right? I mean, you were talking about seasonal changes. Um we know there especially as you go up to Nordic countries there are seasonal changes in the amount of light by time of year. Dramatic ones in fact less so at the equator of course um light temperature food movement and it it's sort of interesting and and at the same time perhaps it should have been obvious to us that there are stimula that our bodies have evolved to adapt to in very powerful ways. And so the idea that temperature, heat, and cold could evoke these tremendous physiological changes that are beneficial for us probably shouldn't surprise us at all. I mean this is why we I mean these are not um esot esoteric mechanisms. They're actually the the foundational mechanisms by which our our body and the bodies of other animals adapt. Um so I do have a question about the uh different ways that people could approach deliberate cold exposure. So, for instance, um children, uh I've been to BAS where their kids, you know, six or seven years old with their parents at the BA um and so they're in hot sauna. I'm not suggesting people do this if they're not, you know, adapted to it and, you know, talk to your parents' kids and talk to your kids' parents. Um talk to your doctors. Um but is remarkable. I mean, uh children doing sauna from a young age or deliberate

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sauna from a young age or deliberate cold exposure. Uh, are there any data on this and is it safe assuming that you know obviously that they can swim and or they're doing this in a tub or shower? Um, and then I'd also like to ask you about are there any additional male female differences? I know your study focused ma um on men but um other studies have focused on both and you of course um are a woman and can attest to your own experience with this. Um so children, men women um differences there um in terms of protocols. Is there anything that people should build into the structure of of their deliberate cold exposure that's unique to that? Uh so yeah, so this was on cold exposure. So um yeah, I think that um starting with the the question about children, um I think that it's important to to think about as children are smaller than adults. So we cannot really completely transfer all the information and the benefits and also protocols for how long and stuff like that to children. We cannot do that because they are just smaller in mass. And one study that actually um proves this is a study where they have compared heat loss uh in um children, boys who were 12 years old uh compared to um adults um men and looked at heat loss of the core temperature and exposed them to a one or two minutes um cold exposure. So immersion up to the neck. And what they saw was that the boys in this study could actually defend their core temperature in same way as the adults could, but they had to use their muscles way faster. So it means that they couldn't stay for as long uh and they use more energy to defend their core temperature compared to the adults. But for one minute it seems that they could actually comp but they will be colder when they then come out

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be colder when they then come out because they are smaller in their mass to their ratio. Right? So it means that if the surface is so large on children and their mass and muscles being smaller to that ratio, it means that they can be in the water uh less time before they get hypothermic. So just think about that. They are just smaller. they can't defend their temperature for a very long time. But in this study, they saw that for up to I think it it was a minute or so, they can one minute. Yeah. I'm glad you mentioned hypothermia and smallerbodied um people, children. Uh I used to do some um Pacific uh ocean swims in the morning um without wet suits and I adapted to it pretty quickly and these are fairly long swims and we brought an excellent swimmer with us um a kid that was interning with me for a while. Um he's 16 years old at the time and um very lean and um he wasn't small for his age but he was smaller than us than the it was all guys on the swim that day. Sometimes women join us. Um, and he got hypothermic. Um, and he's an excellent swimmer and he didn't report feeling overly cold, but um, fortunately we got him to shore and heated him up again. Um, so he lived. I don't think his mother's going to ever let him go swimming with us again. He's thriving in the world. He's a university student now. Um, and he recalls that swim. I mean, this is why you always want to ocean swim with a buddy, with people. Um, yeah, he he became hypothermic. his teeth turned yellow. He was kind of slurring his words. He wasn't making sense. You know, we got him on the shore and he was kind of, you know, drooling and a little bit semi - euphoric and then kind of, you know, it was um hypothermia is no joke. So, I think um yeah, so I I I'm really glad that this is coming up because the cold is a powerful stimulus and um and I and kids are at a and smallerbodied people are at a a greater risk of hypothermia. So, a good reason to approach it with caution, maybe start with cold showers, get uh then cold

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with cold showers, get uh then cold immersion and still water. Natural water and open bodies of water of course are always going to be um more dangerous for other reasons, currents and things of that sort. Yeah, exactly. Okay. And drowning. Yeah. So, important note there. Um what about any additional male female differences or similarities that um we should be aware of? And this comes up all the time on social media. Anytime I post anything about a study, it's it's what about women? because often times there are differences. Um yeah. And there are we we also just talked about the difference in in temperature in men and women. So it means that if we did if we replicated my study in women, it could be that they would have enough you can say cold exposure with just 9 minutes per week. It could be because they uh apparently are also just colder and uh and they have uh increased metabolism in their brown fat. It's just they have more brown fat. It could be. But this is just something that I frankly don't know. But women also do a cold exposure winter swimming with the 11 minutes protocol. I do it myself and feel uh good about it. So I would say that um women uh also regarding activation of the brown fed it should be the same um in theory but I don't know if women actually do need to have another protocol when it comes to this rapid cold exposure. I think that it's another question if we are talking about ice swimming when it comes to how far can you be in the cold water without getting hypothermic then there will be differences in men and female but but if you do this cold exposure for a very brief amount of time which is what I try to to talk to about talk about the what we call also micro stressing the body to to increase the homemetic stress the healthy stress then this is such a short amount of um exposure that it it's fairly the same. I think women can look at this as

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think I think women can look at this as a fairly good protocol for the for for them as well. I always say that if you really dread the cold and um and don't like the cold, then you are a perfect candidate for using deliberate cold exposure because the sympathetic aka the stress response will be greater and thereby the adaptation to that shorter one or two minutes is um is going to be much greater. Right? For people that are perfectly comfortable in the cold, it's harder to get a an adaptation response. the same way that if somebody's very strong and they can lift a very heavy weight that that very heavy weight is unlikely to evoke the same kind of same degree of of uh adaptive responses if somebody is not quite as strong. So another reason to keep these exposures relatively short and more frequent than to do longer duration exposures frequently. However, let's say somebody only had two days a week um to do deliberate cold exposure. Maybe they don't have access to a sauna. Maybe they do. Would you suggest that they get in for one or two minutes, then get out, then get back in for another couple of minutes, then get out and call that four, you know, four or five minutes um to try and get to that 11 minutes total per week. Um as opposed to getting in for a full 5 minutes um and then getting out and coming back a second time that week. I know this is getting down into the weeds, but these are the sorts of things that I think people really want to know because not a lot of people either don't live close to a body of water or don't have a cold plunge um that they can do this with. Although cold shower apparently works too. So most people live close to a shower. Yeah. As definitely I think the the changes in temperature is what is strengthening your your cells in the body. So if you can do the short amount of uh exposure and then get out and get back in that is going to you can say um um strengthen yourselves because you are challenging them to to adapt to um

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challenging them to to adapt to um changing temp temperatures. So during one session you can change this right you you can do it if you are able to go to cold water but also a sauna then you just do it that automatically you will have a change in temperature but you could also do it by variating the temperature in your cold plunge if you have a plunge or if you have the open sea or you have seasoning seasons even you have we have that in Denmark so we have four seasons and the temperature is going to vary with that so we have nature who can just change this for us and we don't have to think about it if you have a cold plunge. Well, then I would say that changing the temperature is what is going to create this homemetic stress and also keep your cells on its toes. You can say because you are try the body will still be um stressed to try to adapt to the new temperature as it's seen as something actually toxic to the body, right? It's a small small piece of toxity that you are exposing yourself to. You don't have to swallow it, but it's enough that you touch it actually. Yeah. Great way to frame it. Um that brings me back to this idea of circadian time. In your study, you didn't um control for specific time of day. And now I'm realizing that may be a great asset to the whole thing. So, we know for instance that our bodies go through pretty dramatic shifts in temperature from the time we wake up. um our body starts heating up as we wake up and um continues to heat until the afternoon and then starts to drop in the later afternoon and then assuming all things are working correctly um that body temperature drops and we sleep. So, I could imagine that doing deliberate cold exposure at different times um just by way of convenience or by way of intention could be very beneficial because my body temperature is going to be quite a bit warmer at one time of day versus another. And in that way, keeping the system tuned and that's really what I keep hearing coming through in as you explain these data and all these um beautiful studies, yours and others, is that it's not really about getting cold,

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that it's not really about getting cold, it's about going from warm to cold and from cold to warm. It's not it. And I've love this idea because I I probably said this a hundred times on my podcast and a million times in my life and I'll continue to which is that biology is not an event. It's a process. Like these these um metabolic and thermmore regulatory processes are indeed like the turning of a knob. It's a verb as opposed to a noun. And I and so I I think if people can internalize that idea that they're going to um have a lot more flexibility, a lot more fun and and get a lot more benefit as opposed to thinking, okay, I need to get into X degrees of water for X amount of time on X number of days, you know, in a very way all the time, how much and how cold and and I mean it's like it's just like well because we also don't have studies showing exactly if you just keep 5 ° in your water and you do that for a month then what happens we maybe in the future we will know much more about this and I'm I'm sure it's going to come and I really hope so but I just think by logically changing that temperature up and down and you also do that in your water it doesn't really it's not that important what temperature you you will have your water then then just keep changing it going up and down it could be all up to 12 ° C you're going to activate your brown fat anyways I mean 12 uh 19 ° uh cold air is enough to activate your brown. So maybe we don't have to go as cold as I think many people think. Um and putting ice even all the time. You don't have to. It's not I don't think it's necessary to to expose yourself to that cold uh temperature all the time. Um but vary it a bit. So keep the system off balance and it's the way to keep it tuned. Yeah. you mentioned a study that is more recent or ongoing that's not published. Um if you're willing um could you share maybe some of the uh data from that uh findings from that study um with of

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findings from that study um with of course the the cue to everybody that these are not yet published data so the the conclusions could change the data could change for that matter. Yeah. So we h we haven't analyzed all the data yet and I I know from the study that we did publish that we we would need to look more at the data. So I don't really have any results yet that I can share because we are still in very preliminary analysis of this. So I I wouldn't know yet what to exactly say about it. But what we looked at was both men and women meth. So that's that's coming. Oh that's fantastic. Um, that answer is going to please a great number of people and intrigue everybody. So, well, listen, I I want to really thank you for coming here today to talk about your work um and the incredible direction that it points to because I think that um you know, no one study is definitive, but your study really again stands as a landmark in the landscape of exploring deliberate cold exposure and heat, how it can impact and potentially impact our because frankly there just haven't been that many um high resolution uh detailed modern studies of this. There have been studies of sauna. There have been some studies of cold. There are a lot of groups in physiology that work on um hypothermia and very cold exposure. But um most of the temperatures used in those studies just aren't practical. So first of all, I just want to thank you for doing the work that you've done and for the work that you continue to do. I'm um uh waiting with baited breath as they say to um uh to hear the results of this uh study that's ongoing on both men and women. So um we'll have to have you back to to inform us about that soon. And I want to thank you for um the incredible public education efforts that you've been doing on social media um and with respect to your book. Um and we of course will put links to all of those things in the show not caption so people can learn from you and can continue to

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can learn from you and can continue to learn from you. We we certainly need more scientists who are both experienced with doing hardcore research as it's called and who also do the practices. I think that's a wonderful additional asset. You know, you're not just behind a lab coat or bundled up in a down uh in a down feather jacket as everyone else is getting in the cold. who do these things and that you are so open and generous in the way that you share uh knowledge which includes coming here today to to share knowledge with me and uh our audience. So thank you ever so much. You're very welcome. I am so pleased to be here and thank you so much for inviting me and I could explain my study and I can share some of my insights from from doing that. So I'm very grateful for being here. Delighted and we'll have to have you back again. Thank you for joining me for today's discussion all about deliberate cold and deliberate heat exposure science and protocols with Dr. Susanna Soberg. If you'd like to learn more about Dr. Soberg's research or you would like to learn about the research of her institute, the Soberg Institute, please see the links in the show not caption. Also in the show not caption, you can find a link to Dr. Soberg's excellent book, Winter Swimming. If you're learning from and or enjoying this podcast, please subscribe to our YouTube channel. That's a terrific zerocost way to support us. In addition, please subscribe to the podcast on Spotify and Apple. And in addition, on both Spotify and Apple, you can leave us up to a five-star review. If you have questions for me or topics you'd like me to cover on the Hubman Lab podcast or guests that you'd like me to consider inviting on the Hubman Lab podcast, please put that in the comments on YouTube. I do read all the comments. In addition, please check out the sponsors mentioned at the beginning and throughout today's episode. That's the best way to support this podcast. Not so much on today's episode, but on various previous episodes of the Hubberman Lab podcast, we discuss supplements. While supplements aren't necessary for everybody, many people derive tremendous benefit from them for things like enhancing sleep, focus, and hormone support. The Hubberman Lab podcast is proud to have partnered with Momentous Supplements. If you'd like to hear more about the supplements discussed on the

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about the supplements discussed on the Hubberman Lab podcast, please go to livemus, spelled o us. com / huberman. Again, that's livemus. com / huberman. If you're not already following the Hubman Lab podcast on social media, we are Huberman Lab on Instagram, Twitter, Facebook, and LinkedIn. And on all those places, I focus on material that somewhat overlaps with content from the Hubberman Lab podcast, but often is distinct from the content covered on the Huberman Lab podcast. So again, it's Huberman Lab on all social media channels. For those of you that haven't already subscribed to our so-called neural network newsletter, this is a completely zerocost monthly newsletter that has summaries of podcast episodes and so-called toolkits. Toolkits are lists of about a page to two pages long that give the critical tools for instance for optimizing sleep or for neuroplasticity or deliberate cold exposure or deliberate heat exposure, optimizing dopamine. Again, all available to you at zero cost. You simply go to hubbermanlab. com, go to the menu tab in the corner, scroll down to newsletter, you provide us your email. We do not share your email with anybody. And in addition to that, there are samples of toolkits on the hubinlab. com website again under newsletter. And you don't even have to sign up to access those, but I think most people do end up signing up for the newsletter because it's rich with useful information and again completely zero cost. Thank you once again for joining me for today's discussion with Dr. Susanna Soberg. And last, but certainly not least, thank you for your interest in science.