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Getting fitter doesn’t make you sweat more after all

August 1st, 2011

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.

  1. Richard Ayotte
    August 1st, 2011 at 13:12 | #1

    Can a fitter person store more glycogen though? If they can, it would explain how they seem to sweat more because they have the energy to do more work.

  2. mark
    August 1st, 2011 at 14:05 | #2

    Too many undefined terms for me to follow this one. Maybe someone can translate all of this to me in terms of efficiency: if I am running at the same speed as my less fit friend (i.e., my muscles are more “efficient” as is my stride technique) then I expect I would be burning less energy. Thus less heat production. Thus less sweat? Seems to me the issue doesn’t exactly relate to VO2max since one could have a middling VO2max but be uber efficient, no?

  3. August 1st, 2011 at 15:17 | #3

    Richard: Our fitter people could likely store more glycogen and, along with other factors associated with high fitness; this would mean they would be able to work at a greater work rate (and therefore metabolic heat production) during maximal effort. But when heat production was clamped between the fit and unfit groups, there was no difference in sweating (or how much core temperature changed).

    Another factor to consider is the difference between how much someone seems to sweat and how much someone is actually sweating. One tends to equate the amount of sweat someone is producing with how many drops you can see forming (e.g. on the forehead) or how much is soaked through their shirt. However when running outside we generate our own convective current (air movement) which is dependent upon how fast we are running. A less fit individual who runs slower (and likely has a much lower running economy than a fitter runner – which means a greater heat production for a given running speed and vice-versa) will therefore have a lower convective current across the body. Since this convection aids the evaporation of sweat (what actually cools you) more sweat will not evaporate and end up dripping off the body in the unfit person.

    But again this is not because of a low aerobic fitness per se; rather the conditions are different.

    We wanted to dissociate VO2max (aerobic fitness) from everything else and see if there was an independent effect. So we had our participants cycle on a stationary bike with a fixed airflow and environmental conditions (this nullified any of the aforementioned problems with differences in air flow etc) and we matched the fit and unfit people for body mass and surface area – this showed absolutely no differences in sweating or changes in core temperature between groups.

    You can also find out more about our lab on http://www.facebook.com/thermalphysiology

  4. Jp
    August 1st, 2011 at 21:47 | #4

    Hi Alex, I’ve always wondered about this cause I’m a sweater! I have gone on bushwalks with my dad and a friend who are at similar fitness levels (in the middle of a humid Australian summer!) and my dad and I are dripping with sweat while the friend is practically dry. Based on this (and multiple other similar experiences) I assumed there was a significant genetic component determining the individual nature of thermal regulation and level at which sweating kicks ib as a compensatory mechanism.

  5. Richard Ayotte
    August 2nd, 2011 at 21:42 | #5

    @Ollie Thank you for your response. I find this very interesting because I’m a heavy perspirer. I can lose 8-9 lbs in a 80min run when I’m fit, and 3-4 lbs when not so fit.

    If I understand this correctly, VO2max has no affect on sweat rate but core body temperature does. We are also talking sweat rate which should not be confused with total sweat and I’m stating this because the “sweat more” statement in the title is ambiguous.

    Can we say then that a more fit person can produce more total sweat but not at a faster rate than a less fit person?

    Can a more fit person bring their core temp higher and increase their sweat rate accordingly?

  6. alex
    August 3rd, 2011 at 00:45 | #6

    @Rich: Thanks for the questions (and @Ollie, thanks for the great answers!). In response to Rich’s last question (“Can a more fit person bring their core temp higher and increase their sweat rate accordingly?”), here’s a excerpt from an earlier e-mail that Ollie sent me about this research:

    “The only other thing I would potentially add is perhaps mentioning that there were no changes whatsoever in how much core temperature in the fit and unfit people went up during exercise after we matched them for body mass and made sure they produced the same amount of heat – this was kind of cool too.”

    So I think the answer — holding heat production equal — is no. But in practical terms, of course, a fit person can go faster and/or longer and thus produce more sweat overall.

    Going back to your earlier statement (“I can lose 8-9 lbs in a 80min run when I’m fit, and 3-4 lbs when not so fit.”): my understanding of Ollie’s results is that if you were running the same time (80min) AT THE SAME PACE, you’d expect sweat output to be roughly the same (notwithstanding changes in efficiency, which I’d expect to be small). Are you running more slowly when you’re less fit?

  7. alex
    August 3rd, 2011 at 00:55 | #7

    @mark
    Thanks for raising this point, Mark. In my attempt to explain the study, I ignored the role of efficiency (how much energy it takes to run at a given speed). Ollie’s study considered “metabolic heat production” by measuring the gases inhaled and exhaled by the subjects, allowing him to calculate exactly how much carbohydrate and fat they were burning (and thus how much heat was generated by metabolic reactions). He found that sweat rate (for two people who are the same size) depended only on metabolic heat production, rather than on relative intensity, fitness, or other factors.

    You wrote: “if I am running at the same speed as my less fit friend (i.e., my muscles are more “efficient” as is my stride technique) then I expect I would be burning less energy.”

    What’s incorrect in your statement is the assumption that you’re burning less energy that your less fit friend. He may be trying much harder than you, but that doesn’t mean he’s burning more energy. Your superior fitness allows you to burn larger amounts of energy with less effort.

    Now, there ARE differences in efficiency. At any given pace, some people burn more energy than others. But these differences are only loosely correlated with fitness. (Trying to figure out how to increase efficiency is one of the great mysteries that exercise scientists are trying to unravel, and not everyone believes it’s possible.) It likely has more to do with whether you have a bouncy stride, or wasted side-to-side motion, not how fit you are.

    Bottom line: the biggest factor determining your energy consumption (and thus your heat production and sweat rate) is the basic physics of moving a body of mass M along a distance D at a velocity V. That doesn’t change based on how fit you are or how hard it feels.

  8. alex
    August 3rd, 2011 at 01:02 | #8

    @Jp
    This is an interesting point, JP. What you say about genetic differences in sweating is what I’ve always assumed to be true. But could it be that there are genetic differences in heat production, and THAT’s what drives differences in sweating? i.e. your engine runs warmer that your friend’s does, and so your body responds by sweating sooner and more copiously?

    Even if that’s true, though, I’d still assume there must also be genetic differences in the threshold at which sweating kicks in. After all, there’s human variation in all traits — we weren’t manufactured in a factory. Perhaps this study just tells us that, on average, getting fitter doesn’t change sweat rates: wherever you sit on that bell curve of human variation, that’s where you’ll be no matter how fit you are.

  9. August 4th, 2011 at 06:13 | #9

    Can anyone explain to me how the relation is with having lactate in the body and sweating during the night?
    My teammates and I concluded all the same: with stiffness in the body, we all sweat more during the night. Are we actually moving more during the night or is there a ‘system’ behind this?

    Once again a good topic for discussions, thanks!

  1. August 1st, 2011 at 17:26 | #1