The surprising science behind who sweats more

Share

A brainteaser to mark the merciful arrival of warm weather: A couch potato and a gym rat line up for a race. Who will sweat more?

The answer matters, because sweat – inconvenient and occasionally unpleasant as it might seem – is a crucial tool to avoid overheating, and those who adjust to hot conditions by ramping up their sweat rate gain an advantage. But figuring out who will sweat the most is surprisingly complex, as a pair of apparently conflicting studies makes clear, depending on factors like how big you are, how hard you push and how fast you go.

The first study, published by researchers from Soonchunhyang University in South Korea in the journal Plos One earlier this month, used a neurotransmitter called acetylcholine to chemically stimulate maximal sweating in two groups of volunteers: 16 trained long-distance runners and 20 sedentary controls with similar age, height and weight.

The results showed a clear effect of fitness: The runners had more sweat glands than the non-runners; their sweat glands kicked into action more quickly after the stimulus was applied; and each sweat gland produced more sweat. Even within the two groups, fitter subjects (as measured in a treadmill test of maximal oxygen consumption) showed enhanced sweating ability compared with less-fit subjects.

This paints a simple picture – the fitter you are, the more you sweat – that agrees with conventional wisdom. And studies dating back to the 1970s have indeed found that trained athletes produce more sweat at any given effort level compared with their untrained peers, suggested that you can "train" your sweat glands to pump out more coolant. But this picture, as recent research at the University of Ottawa's Thermal Ergonomics Lab has shown, is a little too simple.

The problem is that sweat comparisons between fit and unfit people are distorted by differences in other factors like body size, skin surface area and, most importantly, the ability to generate internal heat, according to Dr. Ollie Jay, the former head of the Ottawa lab (now at the University of Sydney in Australia). If the couch potato and the gym rat are both exercising at the same effort level, say 60 per cent of maximum, the gym rat might easily be going twice as fast and thus generating twice as much heat.

To get around this problem, Jay and his colleagues assembled a unique group of 14 volunteers, half with very high aerobic fitness and the other half with very low aerobic fitness. Crucially, the two groups were carefully matched to have almost identical body weight and skin surface area despite their fitness differences (a daunting logistical challenge).

The results, first published in 2011, showed that when the two groups exercised at the same relative intensity (60 per cent of max), the fitter group indeed produced much more sweat, which is not surprising since the fitter group was pedalling much faster. But when they exercised at the same absolute intensity, generating the same amount of metabolic heat, their sweat rates were identical.

So training doesn't make you a better sweater after all – at least, not directly. Instead, training allows you to work harder for longer, which in turn heats you up and causes you to sweat more.

How can the seemingly conflicting South Korean and Ottawa results be reconciled? The key, Jay explains, is to distinguish between maximal sweat rates, as measured in the chemically stimulated South Korean experiment, and sub-maximal sweat rates as observed during actual exercise. Only in extremely hot and humid conditions do we approach truly maximal sweat rates, and in that case, even if trained runners really can sweat more, it may not be an advantage for cooling.

"The greater volume of sweat produced [in extreme conditions] could simply lead to more sweat dripping off the body, and not contribute to evaporation," Jay says.

There is, however, one effective way to ramp up your sweat rate: After as few as six days of moderate hot-weather exercise, your body makes a series of adjustments to handle the heat, including increasing your blood volume and boosting your sweat rate. This heat acclimation process enables you to stay cooler and exercise longer in the summer heat. It's also an important reason to give yourself time to adjust before leaping into heavy exercise during the first hot days of summer.

As for the sweating contest between the couch potato and the gym rat, the most accurate answer is "it depends," but the smart money is on the gym rat. Because whatever the underlying physiology, you can be pretty sure that if you're dripping sweat, you're having a good workout.

Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com.