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This is What Happens When You Work Out in the Heat

2 MIN READ • 2nd October 2022

Hotter temperatures can throw your workout routine off kilter – find out how to navigate them with our expert-backed guide

Fluids in flux

As the weather warms up, you’ll probably want to max the time you spend outside. However, if you’re considering taking your workout outdoors, bear in mind that hotter days mean your body will have to work overtime. “When working out outside in the heat, your core body temperature rises more quickly than it would in milder climates and therefore it produces more sweat in order to keep cool,” says PT Tom Opper (top-fitness.uk). “If left unchecked, the fluids lost through sweating can lead to you becoming dehydrated, which is why it’s important to drink regularly when exercising in warm weather.” Dehydration can lead to feeling weak, faint, susceptible to headaches, dizzy or confused, so make sure you grab a bottle if you plan to exercise in the heat.

Topping yourself up

“When working out in the heat, staying on top of your hydration and drinking water regularly is key to replace the fluids lost through sweating,” says Tom. “Staying hydrated is also critical for performing at your best during your session. In fact, research suggests that fluid losses of five percent and above can decrease your work capacity by as much as 30 percent.” It’s also important to consider when you work out during the day. Swap your midday run for a morning one, and you could avoid the scorching heat altogether. “Plan your session ahead so you’re able to avoid exercising in the hours around midday (around 10am-3pm), which is when the sun will be at its hottest and exercise is therefore likely to be at its most strenuous,” advises Tom. “When it comes to what you wear, light, loose-fitting clothes are advised to let your skin breathe and lightercoloured clothes will help reflect the sun’s rays.”

Calorie burn

With all that extra sweat, you’re probably wondering if exercising in the heat increases your calorie output, but the answer isn’t as simple as you might think as Tom explains. “While you do burn more calories in the heat, the increase to your calorie expenditure is relatively minimal, so this shouldn’t be a factor behind whether you choose to work out in warmer weather,” he says. “The increased caloric expenditure is a side effect of your body working harder to stay cool through mechanisms such as sweating. This increased effort from your body requires energy, which is why you’ll burn slightly more calories.” If your personal bests take a nosedive when the weather warms up, then make sure you’re fueling properly before your exercise and make hydration your number one priority – this will help you account for the extra effort your body uses to keep you cool.

Check in

Our bodies can find the heat taxing, especially if we’re used to living in cooler temperatures for the majority of the year. “Working out in hot weather will initially feel more strenuous than the conditions you’re likely used to, and can leave you more susceptible to heat-related illnesses such as sunstroke,” says Tom. “If you feel any related symptoms, such as headaches, nausea or dizziness, stop immediately, find some shade and drink some water.” Having a snack to hand, such as a piece of fruit or a handful of nuts, can give you a real energy boost. “In terms of how hard you should work, you may need to dial back your session intensity for a week or two as summer hits so your body can acclimatise to the warmer weather, and then gradually build intensity back up over time as your body adjusts.”

Meet the writer
Stacey Carter
Senior content writer

When I’m not at my desk writing a feature or researching the latest trends in women’s health, you’ll probably find me deep in a book or doing a LISS-style session in the gym in the evening. I have... Discover more

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