Ever wondered why the guy next to you in your spin class barely has a bead of sweat on his forehead when you resemble a particularly soggy version of Aquaman?
Well, when all other factors are equal (i.e. environmental conditions, relative exercise intensity, clothing, mode of exercise etc) then a person will sweat more when they’re fit than when they’re out of shape.
This might seem counter-intuitive at first, but there’s a good reason for it and it’s to do with keeping core body temperature under control when we’re exercising.
Heat production and dissipation during exercise
Exercise is not particularly efficient as a large proportion of the energy burned during the process goes into producing heat and not into motion. So, even on a cold day, when you need a thick coat to stay warm during periods of inactivity, you can get seriously hot when running or cycling in just light clothing.
This poses a problem because your core body temperature has very little wiggle room before overheating becomes a life-threatening issue – a rise from just 37 C (98.6 F) to 40 C (104 F) is enough to put you in real trouble. You have to be able to offload heat to the external environment effectively if you want to work hard for extended periods of time.
Sweating is the body’s most effective method of dissipating heat as the evaporation of water away from the skin takes a lot of heat energy away with it. This is why you feel very cold stepping out of the pool or sea, even on a warm day, as the initial covering of water evaporates off rapidly into the atmosphere, taking heat with it.
As you get fitter, a major, desired side-effect is that you can burn more energy, more quickly, and therefore hold a higher intensity of exercise for a longer period of time. Because heat production is linked to exercise intensity, your body heats up more quickly when you are fitter and needs to become better at cooling itself to cope with this demand.
One major way it does this is to decrease the core temperature at which you start to sweat (i.e. you almost begin to sweat in anticipation of the fact that your body temperature is going to rise when you begin working out). Your body also increases the overall rate at which sweat can be produced.
Put simply, as you get fitter, you can work harder and produce more heat, so the body responds to this by ‘ramping up’ its sweating response and by starting to sweat earlier in order to reduce the risk of overheating.
A major reason that some people find this concept counter-intuitive is that if you imagine two people training together - one fit, one unfit – it’s easy to visualize the unfit guy sweating profusely whilst the pro athlete looks as fresh as a daisy. Whilst this image might not be far from the truth, it obscures the fact that in relative terms the two athletes are working at very different intensities to one another.
For example, if we took an elite marathon runner and someone who had just started training and got them to run together at 9 minutes per mile, this would be a very low relative intensity of exercise for the elite runner in comparison to their maximum speed. But it might be literally as hard as the novice can go.
In this scenario, the novice will certainly sweat more because he is working relatively harder.
On the other hand, if we also got the pro athlete to run at a similar relative intensity (e.g. 5 minutes per mile) they would sweat more than the novice due to having a better-developed sweating response overall.
Other factors that can influence sweat rate
Genetics also clearly plays a big role in determining an individual’s sweat rate (check out How to measure your sweat rate for more information) and fitness level; some people just sweat more than others, full stop.
Body size tends to have an effect too. Bigger people generally have the capacity to produce more sweat than smaller folk simply because they have a larger body surface area and more muscle mass to generate heat.
Men, on average, sweat more than women. Whilst the exact reasons for this aren’t fully understood they have been studied, including in an interesting paper published in 2010.
Additionally, acclimation to the heat promotes an increase in sweat rate for the same basic reason that increased fitness levels do (i.e. it's a coping mechanism to prevent too much of a rise in core body temperature).
And finally, some drugs, medical conditions, foods and other factors (e.g. psychological stress) can exert influence on sweat rate. As anyone taking cocaine, suffering hyperthyroidism or eating a hot curry whilst sitting an exam will be able to tell you!