Heat acclimatization

Why does an athlete who has trained through two weeks of heat finish a hot race with a lower core temperature while sweating far more?

Sweating looks like the symptom. The athlete who is struggling in the heat is the one dripping; the one who looks comfortable seems to be sweating less. Yet the athlete who has spent a fortnight training in heat finishes the same hot race wetter than before and with a lower core temperature. Both of those cannot be side effects. One of them is the mechanism — so which way round does the causation run?

Start with the heat balance, because it settles the direction immediately. A runner working hard is a heat engine of poor efficiency: perhaps a fifth to a quarter of the metabolic energy becomes mechanical work and the rest becomes heat. So an athlete metabolising at roughly a kilowatt is producing something like 750 to 800 watts of heat that has to leave the body or core temperature climbs.

Where can it go? Radiation and convection only work while the skin is cooler than the air, which on a hot day it is not. That leaves evaporation. Vaporising water takes about 2.4 megajoules per kilogram at skin temperature, so dumping 800 watts by evaporation alone needs 800 divided by 2.4 million kilograms per second — about a third of a gram every second, which is roughly 1.2 litres an hour. That number falls out of physics with no physiology in it at all, and it tells you what the sweat gland is for.

Now notice the crucial qualifier: evaporated sweat. Sweat that rolls off the chin has cost the body water and salt and removed almost no heat, because the latent heat is only paid at the moment of the phase change. So the sweating athlete is not necessarily the cooling athlete.

That reframes the puzzle. If the acclimatized athlete is cooler while wetter, the plausible reading is that they have made evaporation happen earlier and faster, and that the extra fluid is buying extra cooling rather than merely being lost. What would that require?

Three things, and they are what a fortnight of heat exposure actually produces. First, the sweating response starts at a lower core temperature — the threshold shifts down, so cooling begins before the heat has accumulated rather than after. Second, sweat rate rises and the sweat spreads over more of the skin, which matters because evaporation is limited by wetted area as much as by supply. Third, and least intuitive, the sweat gets more dilute: the duct reabsorbs more sodium on the way out, so the athlete loses less salt per litre. That one is not about cooling at all. It is about being able to afford the water.

And it is affordable only because of a fourth change that happens fastest of all: plasma volume expands within the first days of exposure. More circulating fluid means the body can send blood to the skin for heat transfer and to the working muscle without the two competing so sharply — which is why the acclimatized athlete's heart rate at a given pace drops as well.

So the causal chain runs: expanded plasma and earlier, larger, more dilute sweating produce more evaporation per minute, which removes more heat per minute, which holds core temperature lower. The wetness is the cause. The coolness is the effect.

Is any of this a special heat adaptation, or is it just fitness? Partly the latter, honestly — trained athletes are already somewhat heat-adapted, and the boundary is blurry. But the sodium-sparing and the threshold shift are specific to repeated heat exposure, and they decay within weeks of stopping.

Two clarifications, and one comparison worth making explicitly.

The first is that this adaptation is conditional on the air. Evaporation is driven by the water-vapour pressure gradient between wet skin and the surrounding air. In near-saturated air that gradient collapses, and sweat that cannot evaporate is pure cost. So heat acclimatization is not a general upgrade; it is a large advantage in hot dry conditions and a much smaller one in hot humid conditions, which is the single most useful thing the mechanism predicts.

The second is honesty about the evidence. The direction of these adaptations is well established, but the numbers are not portable: the size of the core-temperature difference, the time course, and how much of it survives into cool-weather racing all vary by protocol, by how hard the sessions were, and by the individual. Reported group means say little about any particular athlete, and the claim that heat training improves performance in cool conditions via plasma volume is actively contested rather than settled.

Now the comparison. Altitude acclimatization has the same shape — stress, then adaptation — but a structural difference worth naming. At altitude the stimulus is simply being there, so it can be separated from training: live high, train low. Heat cannot be split that way, because the stimulus is the raised core temperature the work itself produces. The cost and the stimulus are the same event.

• Why hot humid conditions defeat the adaptation, and what cooling strategies remain when evaporation is capped.
• How the sodium-sparing change interacts with drinking strategy, and where over-drinking becomes its own hazard.
• Whether the plasma-volume component genuinely transfers to cool-weather performance, which is the contested part.

Every term the collection defines is gathered in the glossary.