At the American College of Sports Medicine meeting this spring, Matthew Cramer of the University of Ottawa and Ollie Jay of the University of Sydney presented some results that challenged those ideas. That data has now been published in the Journal of Applied Physiology, and it has some surprising twists.
Answers have traditionally focused on factors like body fat percentage (more fat insulates you and makes you overheat sooner) and aerobic fitness (the fitter you are the less you sweat). Some people sweat more than others. Exercise with a group, and the differences become obvious.
But what determines these variations?
The problem with previous studies is that body fat and aerobic fitness (VO2 max) tend to correlate with other factors. People with lots of body fat tend to weigh more—so is it the insulative properties of fat that matter, or is it simply being bigger and having to haul around more weight?
Similarly, people with high relative VO2 max (expressed as the maximum amount of oxygen their muscles can use per kilogram of body mass) tend be smaller overall—so maybe it’s the body size, not the fitness, that makes the difference.
To tease out the key factors, Cramer and Jay assembled 28 volunteers with widely varying fitness and body sizes and put them through a series of 60-minute cycling tests at different intensities while measuring sweat rates and changes in body temperature.
Sure enough, the change in core temperature was mostly explained by how much heat they generated in pedaling the bike per unit of body mass, with no “insulation effect.”
Heat production accounted for 50 percent of the variability in core temperature, and adding body fat percentage (which varied from 6.8 to 32.5 percent in the subjects) only explained another 2.3 percent of the variability.
This suggests that two people who weigh the same and pedal at the same pace should heat up at the same rate, even if one of them is short and fat and the other is tall and lean.
The same was true for overall sweat rate: body fat percentage explained only 1.3 percent of the variation.
It’s important to note that this doesn’t conflict with the general observation that people with lots of body fat tend to sweat more. But it’s not because of the thermal properties of fat itself; it just takes more work to haul more weight around.
Similarly, VO2 max turns out not to make a big difference on its own, accounting for only 4 percent of variation in sweat rate. That seems counterintuitive—but again, it comes down to how much heat you’re generating.
If you ask two people with different VO2 max to exercise at, say, 70 percent of their max, the fitter person will be pedaling or running much faster, and thus generating more heat. They’re sweating more, but that’s because they’re doing more work, not because of some magical property of VO2 max itself.
The bottom line? If you’re trying to figure out whether you’re likely to overheat on a hot day, or how much you’re likely to sweat, simple rules of thumb about fatness and fitness aren’t that useful.
In the end, there’s so much variability in thermoregulatory responses that you have to rely on your own experiences and on simple tests like weighing yourself before and after a run to get a sense of how much fluid you’re losing.