It seems like common sense: if you leave a fitness class looking as though you’ve just ridden a log flume, you’ve probably worked harder than if you’re barely glistening. But that’s not always the case, says Adam Collins, a researcher from the Centre for Nutrition, Exercise and Metabolism at the University of Bath.
Sweating, he says, is part of the thermoregulation process. When your body temperature rises, it signals to your brain to sweat in order to cool you down. As the sweat evaporates, it helps regulate your core temperature.
One reason temperature rises is exercise. “When we exercise, the energy we produce to fuel that activity creates heat as a byproduct,” says Collins. The more intense the workout, the more heat is generated – and usually, the more you sweat. But sweat levels can be misleading.
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If you went on an easy run on a hot, humid day, you’d probably sweat a lot. But on a cold day, you might barely sweat at all
“There are many other factors that influence how much you sweat,” he adds. These include the temperature of the environment, your clothing and humidity. In humid conditions, sweat evaporates less efficiently, making you feel sweatier than usual, whether or not you’re working hard.
“If you went on an easy run on a hot, humid day, you’d probably sweat a lot,” he says. “But on a cold day, you might run at a much higher intensity and barely sweat at all.”
Sweat also varies from person to person. Your fitness level, age, sex, body size and even whether you’re acclimatised to the climate you’re in all play a role.
So, no, sweat isn’t a reliable measure of how hard you’ve worked out. Better indicators of exertion include how hard you’re breathing, your perceived effort, and your heart rate, which you can track with a chest strap monitor or smart watch. That said, “sweat is a good marker of dehydration”, says Collins. So if you come out of a workout dripping, you’ll need to grab a drink.
