A four-week heat acclimation protocol helped trained runners stay cooler, shift fuel use, and improve key aerobic markers during exercise in the heat.
The Heat Protocol
Heat changes the cost of running. It asks the body to manage pace, temperature, oxygen demand, and fuel at the same time. In this study, 18 trained male middle- and long-distance runners entered that demand with a precise question: could deliberate heat acclimation improve aerobic capacity and metabolic adaptation during exercise in the heat.
The runners were randomized into two groups. One group continued regular exercise training in a thermoneutral environment, with wet bulb globe temperature above 20 degrees Celsius and up to 25 degrees Celsius. The heat acclimation group trained five days a week, once per day, for four weeks, completing 20 sessions in total.
The heat protocol was intentionally demanding. The target core temperature sat between 39 and 40 degrees Celsius, creating a sustained internal load rather than a casual exposure to warmth. That distinction matters. Acclimation is not simply about enduring discomfort; it is about teaching the system to organize itself under pressure.
We read this kind of protocol as a study in adaptation. Heat becomes a controlled stressor, and the body learns to protect equilibrium while continuing to work. For an endurance athlete, that balance is central. Performance in the heat depends not only on effort, but on how efficiently the body carries the cost of that effort.
A Cooler System Under Load
After four weeks, the heat-acclimated runners finished the incremental treadmill test with a lower core temperature than the control group. The difference was modest and meaningful: 38.2 degrees Celsius compared with 38.6 degrees Celsius, a reduction of about 0.4 degrees Celsius after the intervention.
That cooler response reflects improved thermoregulatory adaptation. The runners still worked in the heat, but the internal temperature cost was lower. Under demanding treadmill conditions, less heat strain gives the body more room to sustain quality movement. You are not escaping the environment. You are meeting it with a better-organized system.
This is where heat training becomes more than tolerance. Tolerance can mean staying present through discomfort. Adaptation means the same work produces a different internal response. The body carries the load with more composure, and that composure becomes a foundation for output.
For runners, this matters because heat rarely announces itself as one single limit. It accumulates. Temperature rises, breathing changes, fuel demand shifts, and pace begins to feel more expensive. A cooler system protects balance before performance begins to unravel. It gives resilience a practical shape.
Thresholds Moved in the Right Direction
The heat-acclimated runners also improved key aerobic markers. At the first ventilatory threshold, oxygen uptake increased to 44.7 mL/min/kg compared with 43 mL/min/kg in the control group. Running velocity at that same threshold rose to 12.9 km/h compared with 12.4 km/h. The signal is clear: they could do more work before reaching the same ventilatory marker.
The second ventilatory threshold moved as well. Oxygen uptake increased to 55.9 mL/min/kg in the heat acclimation group compared with 53.9 mL/min/kg in the control group. These numbers point toward improved aerobic capacity in hot conditions, not through maximal effort language, but through better sustainable intensity.
The 4-week HA decreased the 0.4°C core temperature.
Thresholds matter because they describe where the body begins to change how it supports work. When those markers improve, the runner gains more usable range. Pace can stay steadier. Breathing can remain more controlled. The protocol did not ask the athletes to perform louder; it helped them perform with greater precision.
That is the quiet value of acclimation. You build capacity before the decisive moment arrives. In heat, mastery often looks like restraint: a system that wastes less, manages more, and keeps its shape when conditions become less forgiving.
Fuel Use Became More Efficient
The metabolic finding gives the study its sharper edge. During submaximal exercise in the heat, carbohydrate oxidation decreased in the heat acclimation group. At 75 percent V̇O2 max, carbohydrate oxidation was 2.5 g/min compared with 3.1 g/min in the control group. At 85 percent V̇O2 max, it was 3.4 g/min compared with 4 g/min.
Lower carbohydrate oxidation at those intensities points toward improved muscle glycogen utilization efficiency. In plain terms, the runners appeared to rely less heavily on carbohydrate at demanding but submaximal workloads. That can protect reserves during prolonged exposure to heat, where pacing discipline and fuel management become part of the same protocol.
This does not make heat easy. It makes the response more refined. The body still faces thermal strain, but it spends fuel with greater care. For endurance work, that efficiency supports resilience. You preserve capacity for later moments, when fatigue and heat begin to press into decision-making.
The practical takeaway is simple. Deliberate heat acclimation can help trained runners stay cooler, sustain higher aerobic intensity, and shift fuel use during exercise in the heat. Recovery and performance share the same foundation here: repeated, precise exposure; enough stress to create adaptation; enough structure to keep the work intentional.
Four-week heat acclimation reduced carbohydrate oxidation during submaximal exercise in the heat