Thermoregulation
Evidence: strong
Running generates far more heat than it converts to movement, and most of it is shed by evaporating sweat. In the heat, the body sends blood to the skin to lose that heat, which competes with the muscles and drives heart rate up. Rising core temperature and dehydration are what limit running in the heat.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
Thermoregulation is how the body holds core temperature near 37 °C while running floods it with heat. What follows is the physiology of staying cool; heat illness is what happens when the system is overwhelmed, and heat acclimation is how the body adapts to cope better.
Heat balance: production versus loss
Core temperature is set by the balance between heat made and heat lost. Running makes a great deal of heat because muscle is an inefficient engine. Only around 20 to 25% of the energy released by the working muscles becomes mechanical movement; the rest appears as heat (Nybo et al. 2014). The faster the running, the more metabolic heat is produced, so the cooling system has to work harder precisely when the runner is asking most of their muscles. If production outpaces loss, core temperature climbs.
The four routes of heat loss
Heat leaves the body by four routes. Radiation and convection move heat from the warm skin to cooler surroundings and passing air, and conduction passes it to anything cooler in direct contact. These dry routes work well at rest in cool air, but they fade as the air warms toward skin temperature, and they reverse in genuine heat, when the environment adds heat to the body instead of taking it away (González-Alonso et al. 2008).
That leaves evaporation as the dominant route during exercise. Turning sweat from liquid to vapour on the skin draws a large amount of heat from the body, and in warm conditions evaporative cooling does the overwhelming majority of the work of keeping a runner from overheating (Nybo et al. 2014).
Why humidity matters most
Evaporation depends on the air’s capacity to take up water vapour. In dry air sweat evaporates freely and cools efficiently; in humid air the surrounding air is already near saturation, so sweat drips off without evaporating and carries away little heat (Racinais et al. 2015). This is why a humid 28 °C is more dangerous than a dry 35 °C, and why wet bulb globe temperature, which weights humidity heavily, is the standard measure of heat stress for racing (Casa et al. 2015).
Cardiovascular strain and the rising heart rate
Shedding heat by sweating has a cardiovascular cost. To carry heat from the core to the skin, the body diverts blood to the skin surface, and this demand competes with the muscles’ need for blood (González-Alonso et al. 2008). During prolonged exercise, especially in the heat, the result is cardiovascular drift: stroke volume falls and heart rate rises to defend cardiac output, so the same running pace costs more beats per minute as the effort wears on (González-Alonso et al. 2008). This is a large part of why heart rate climbs through a steady long run even when pace holds, and it links directly to durability and the field measure of decoupling. It is also one of the cardiovascular adaptations that training and heat acclimation blunt, by expanding plasma volume and lowering heart rate at a given effort (Tyler et al. 2016).
Core temperature and the limit to performance
Performance in the heat is limited by rising core temperature itself, not only by the muscles running out of oxygen. Fatigue during prolonged exercise in the heat tends to arrive at a core temperature slightly above 40 °C, largely regardless of how warm the runner started (González-Alonso et al. 2008). A rising core temperature also impairs the brain’s drive to the muscles, a central component of hyperthermic fatigue, and the body pre-empts the danger by slowing the pace it selects before any hard ceiling is reached (Nybo et al. 2014). The practical consequence is that hot races call for more conservative early pacing than cool ones.
Individual and sex differences in sweating
Sweating capacity varies widely between people, which is one reason heat tolerance does. Sweat rate, the sodium concentration of sweat, and the temperature at which sweating begins all differ substantially across individuals (Racinais et al. 2015). On average women sweat somewhat less for a given heat load and rely a little more on skin blood flow for cooling, though body size, fitness and acclimation status explain much of the apparent sex difference (Besson et al. 2022). Acclimation shifts every runner in the same direction: earlier, heavier and more dilute sweating (Tyler et al. 2016).
Dehydration degrades cooling
Sweating to cool down depletes the very blood volume the cooling system relies on. As dehydration progresses toward about 4% of body mass, systemic, muscle and skin blood flow all fall, core temperature rises faster and cardiovascular strain worsens (González-Alonso et al. 2008). Dehydration and hyperthermia together account for most of the strain of exercising in the heat. This is the physiological case for sensible hydration, though the size of the performance penalty from modest, self-paced fluid loss is contested (Goulet 2011), and overdrinking carries its own serious risk.