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The fitter you get, the more you sweat during exercise in order to dissipate heat more quickly. That’s the conventional wisdom among scientists, and I’ve certainly repeated it many times here and elsewhere. So I was surprised to see a new study posted online in the American Journal of Physiology, from Ollie Jay and his colleagues at the University of Ottawa’s Thermal Ergogenics Laboratory, that contradicts this conventional wisdom. His results suggest that your sweat rate simply depends on how much physical work you’re doing, and how much skin surface area you have. Previous studies have been confused because fitter people are able to do more physical work (thus generating more heat and responding with more sweat) at the same effort level.
Let’s say I’m running at a given intensity (say 60% of VO2max) that corresponds to 6:00/km. In order to move my legs, my body is burning a combination of carbs and fat, producing heat as a metabolic byproduct. In order to dissipate that metabolic heat, I’ll sweat a certain amount.
Now let’s say I accelerate to 5:00/km (so I’m at 70% of VO2max). I’m moving my legs faster, so I generate more metabolic heat, and in response, I sweat more than at the slower pace.
The question is: what happens if I go away and train for a year, and improve my fitness so that I can run at 5:00/km (the faster speed) and have it correspond to 60% of VO2max (the lower intensity). How much will I sweat compared to my untrained state? Will it depend on my intensity, or my speed? The current conventional wisdom says it’ll depend on intensity: so running at 60% VO2max will produce the same amount of sweat whether I’m running at 6:00/km (unfit) or 5:00/km (fit). But Jay’s new study found the opposite: I’d sweat the same at 5:00/km regardless of whether my intensity is at 70% VO2max (unfit) or 60% VO2max (fit).
Confused yet? In actual fact, the study took a slightly different approach, comparing two groups matched for body mass and surface area but with dramatically different aerobic fitness (VO2max): one group averaged 40.3 mL/kg/min, the other 60.1 mL/kg/min. He had them perform cycling tests, fixing either the relative intensity (i.e. 60% of VO2max) or the metabolic heat production, and found that sweat rates depended on heat production, not aerobic fitness.
There is one important caveat, though: the study was conducted in relatively comfortable temperatures of 26 C (79 F) and 26% relative humidity:
Maximal sweating capacity and subjective tolerance to the heat are no doubt improved by aerobic fitness, and therefore individuals with a high VO2peak would certainly have a distinct advantage during exercise at a fixed heat production in a physiologically uncompensable (i.e. hot and humid) environment.
So under “normal” conditions, the amount you sweat depends only on how much physical work you’re doing (and how big you are). But if the conditions are so hot that it’s impossible for you to dissipate all your metabolic heat through sweating, less fit people will hit their maximum sweat rate earlier than fit people.