Full Transcript: The Science & Health Benefits of Deliberate Heat Exposure
This is a full transcript of Andrew Huberman's deep dive into the science of heat exposure, sauna protocols, and thermal regulation. Click any timestamp to jump to that moment in the YouTube video.
0: 00 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 ophthalmology at Stanford School of Medicine. Today we are talking about the science of heat, and more specifically the science of heating, the verb, meaning how our body heats up from both the outside and the inside. Heat is a remarkable stimulus, meaning when we are in a hot environment, it has a profound effect on our biology. And heating up from the outside, or as you'll soon learn, from the inside, has a profound effect on many different aspects of our health, including our metabolism, both in the immediate and long term; our cognition, meaning our ability to think more or less clearly.
1: 00 And if you're immediately thinking that heating up makes you less capable of thinking, you're wrong. Heat applied properly as a stimulus can engage certain neurochemical systems in your brain and body that can allow your brain to function far better. We will talk about those data today. So we're going to talk about the science of heat and heating, both in terms of their mechanisms. And, as I know many of you are interested in, the tools related to the use of heat, things like sauna, how often to do sauna, how long to be in the sauna, how hot to be in the sauna for particular goals and outcomes.
2: 00 We're also going to talk about the very exciting new science around local heating. That is the use of heat applied to specific areas of the body in order to heal or improve tissues at that location that you are heating as well as your biology and health overall. In fact, we are going to talk about one very recently published paper that came out in the journal Cell. Cell is one of the three apex journals, meaning three of the most competitive, most rigorous scientific journals. Those are Nature, Science, and Cell. This particular paper was published in Cell, and I will go into it in more detail later, but basically what this paper shows is that by locally heating up skin and fat, you can change the identity of certain fat cells at that location and elsewhere.
3: 00 We have three kinds of fat, white fat, beige fat, and brown fat. And as you will learn more about soon, white fat is not very metabolically active. It's more of a fuel reserve. It's what we typically think of as blubbery fat. Beige fat and brown fat are rich in mitochondria, and those mitochondria provide a sort of furnace or heating mechanism for your entire body and increase your metabolism and the burning of white fat. So in other words, having more beige fat and brown fat is a good thing. And it turns out that the proper application of heat to specific areas of your body can increase the conversion of white fat to beige fat. In other words, turn an innocuous fuel source into a metabolically active tissue that can help you burn off more white fat.
10: 00 So here's how this circuit is structured. You have this shell, which is basically skin, and within the skin you have neurons, nerve cells. Those nerve cells have channels or receptors on them. They're called trip channels. There's some other ones as well, which basically sense changes in heat. So if I were to put a hot object on your hand or your arm, or for instance, if I were to put a hot object on your hand or arm, and then remove that hot object, those neurons would respond to that. They would send electrical signals into your spinal cord. And that's where the next station of the circuit resides.
11: 00 In your spinal cord you got a little cluster of neurons that exists at the top part of your spinal cord called the dorsal horn. The name, again, doesn't matter. And those neurons specifically relay heat information up to another area of your brain. Now here's where we get into some fancy names. It's the lateral parabrachial area. You don't need to know lateral parabrachial area, but it's a relay station. The lateral parabrachial area sends electrical signals to the POA. And I would like you to know POA. The POA stands for preoptic area. Neurons in the preoptic area basically reside over the roof of your mouth. These are neurons within the hypothalamus, and neurons in the preoptic area have the ability to send signals out to the rest of your brain and body to get you to heat up and actually to change your behavior so that you heat up.
27: 00 This is one of several papers that clearly demonstrate that regular use of sauna or other forms of deliberate heat exposure can reduce mortality to cardiovascular events, but also to other events, things like stroke and other things that basically can kill us. What I like so much about this and the related studies, and yes, I will provide a link to these in the show notes, is that they involve a lot of participants. So for instance, in this particular paper, which was published in BMC Medicine, they looked at a sample of 1, 688 participants who had a mean age of 63, but there was a range of ages around 63, and of whom 51. 4% were women. The rest were men. So it's a pretty nicely varied study in terms of the populations that they looked at.
28: 00 And basically what they found was the more often that people do sauna, the better their health is and the lower the likelihood they will die from some sort of cardiovascular event. What do we mean by sauna? We need to define some of the parameters around sauna, and I promise to provide you some alternative ways to access some of the health benefits that were observed in this and related studies without the need to have a sauna,' cause I do realize that a lot of people don't have access to sauna. First off, the temperature ranges that were used in this and pretty much all the studies that I'm going to talk about, unless I say otherwise, are between 80 degrees Celsius, meaning 176 degrees Fahrenheit, and 100 degrees Celsius, meaning 212 degrees Fahrenheit. So somewhere in that range.
30: 00 In subjects that did this two hour a day, 80 degree Celsius protocol experienced 16-fold increases in growth hormone. So they measured growth hormone before the sauna and after the sauna and growth hormone levels went up 16 fold, which is obviously an enormous, and it turns out statistically significant, effect. What they observed was on day one, there was a 16-fold increase in growth hormone. On day three, however, there was still a significant effect on growth hormone as compared to before sauna, but that effect was basically cut by two thirds. So now instead of getting a 16-fold increase, it was more like a three or four-fold increase, which is still a huge increase, but not as great as the increase observed on day one.
31: 00 In this case, the effects of sauna exposure on growth hormone actually went down the more often that people did this deliberate heat exposure. So as I mentioned, they did this two hour a day divided into 30 minute sessions protocol on day one, day three, and day seven of a week. And what they found was on day one, there was a 16-fold increase in growth hormone. On day three, however, there was still a significant effect on growth hormone as compared to before sauna, but that effect was basically cut by two thirds. And then on day seven, there tended to be a two, maybe a threefold increase, but not as great as the one observed on day one. What does this mean? And why does this happen? Well, the reason this happens is because heat, just like cold, is a shock or a stressor to the system.
40: 00 So what kind of mechanisms are activated in your brain and body that allow for the various health benefits of sauna or other forms of deliberate heating? Well, we talked about reduced risk of cardiovascular event related mortality and all-cause mortality. As you'll soon learn, there are also tremendous benefits in terms of increases in growth hormone, reductions in cortisol, et cetera. I will detail those. So what happens when you do get into a hot environment? What are the mechanisms that allow for the various health effects of that? Well, your shell, your skin, senses that. And through the circuit that I described earlier, activates neurons in the POA, the preoptic area, which in turn activates mechanisms in your autonomic nervous system, like vasodilation. So blood flow increases.
42: 00 One of the more striking examples of that comes from a study that was published in 2021. The title of the study is" Endocrine Effects of Repeated Hot Thermal Stress and Cold Water Immersion in Young Adult Men." And indeed, this study was, in this case, just done on men. I'll just briefly describe the protocol they used. They had these men attend four sauna sessions of 12 minutes each. So again, well within that range of five to 20 minutes, 12 minutes. The temperature of those saunas was 90 to 91 degrees Celsius. So I'll just quickly do the calculation, admittedly, not in my head. That's 194 degrees Fahrenheit, and they did that four times. Afterwards they had a six-minute cool down break during which they did get into some cool water or cold water of about 10 degrees, which is, 10 degrees Celsius is 50 degrees Fahrenheit. And then they measured hormones at various times throughout this study, before, during, and after.
45: 00 One of the more dramatic and important effects of going into a hot environment for some period of time is the activation of so-called heat shock proteins, or HSPs. Heat shock proteins are a protective mechanism in your brain and body to rescue proteins that would otherwise misfold. What do I mean by this? Well, most of you're familiar with the fact that if you have protein in the kitchen, like a steak or a piece of chicken or a piece of fish, and you heat it up, it changes it's texture. Raw meat is different than cooked meat, to be quite blunt about it. Heat changes the quality of proteins, not just in terms of how they taste, but the way in which they are configured. It changes it right down at the molecular level.
58: 00 They used an 80 degree Celsius environment, so that's 176 degrees Fahrenheit, and they had subjects do the sauna for 30 minutes, four times per day. So that's two hours total in one day, 30 minutes in the sauna, a period of cool down rest, 30 minutes in the sauna again, cool down rest, a third, and a fourth time. So, two hours total in this 80 degree Celsius environment. So that's a lot, but what they observed was really quite significant. So they had subjects do this protocol, and I should mention they had both male and female subjects in this study, and the entire study lasted a week. They did this two hours of sauna exposure on day one, day three, and day seven of that week.
60: 00 In subjects that did this two hour a day, 80 degree Celsius protocol experienced 16-fold increases in growth hormone. So they measured growth hormone before the sauna and after the sauna and growth hormone levels went up 16 fold, which is obviously an enormous, and it turns out statistically significant, effect. Now, one important caveat here. Remember earlier when I talked about people who did sauna once a week versus two to three times a week versus four to seven times a week, and the more often people did sauna, the less likely they were to die of cardiovascular events or other things of that sort? Well, in this case, the effects of sauna exposure on growth hormone actually went down the more often that people did this deliberate heat exposure.
70: 00 Your body temperature follows a 24-hour rhythm. It's lowest two hours before you wake, peaks in the late afternoon, and drops again as you prepare for sleep. That drop in core temperature is what allows you to fall asleep and stay asleep. Enter the sauna in the evening, and your body heats up. But when you step out, your body continues to cool down — amplifying that natural temperature drop. For many people, an evening sauna session becomes a sleep aid. The heat primes your body for rest. Cold exposure, by contrast, increases your core temperature in the hours after exposure. If you plunge into an ice bath late at night, you may find yourself wired when you should be winding down. Save the cold for the morning. Save the heat for the evening.
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