Bad Workout? Blame the Heat

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As athletes start gearing up for fall sports, summer heat may mean more than dips in the lake and lemonade stands: Exercising in the heat has its own unique set of challenges.

The benefits of exercising outdoors are well-documented and numerous, so the forecast shouldn’t be an excuse to skip workouts. Whether you’re trying to make the varsity football team or training for your first 5K, though, monitoring your body as it adapts to the heat is critical. Adjusting to the heat is highly individual, so there’s no one-size-fits-all solution.

Michael Bergeron, a professor of pediatrics at Sanford School of Medicine of the University of South Dakota, once went for a run with a woman who told him she rarely sweats. Before the run, they both ingested tiny body sensors to record their body temperatures.

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“After 20 minutes, her temperature went up and we had to stop,” said Bergeron, whose own temperature didn’t waver. “It really varies a lot, and if you don't tolerate the heat, it’s very dangerous” -- no matter how much you hydrate or wear Dri-FIT.

He’s also seen cases at the opposite end of the spectrum: He tested one man on a treadmill who lost 4.1 liters of sweat in an hour, in 72 degree heat. Bergeron advised him to steer clear of any races longer than 10K.

One thing humans share when it comes to temperature regulation: We're horribly inefficient at it. When we exercise, 20 percent of our energy goes into moving our muscles. The remaining 80 percent gets released as heat. Since that percentage remains the same regardless of the temperature, the environment helps determine how our bodies get rid of the heat. If it’s cool and breezy, for example, sweat will evaporate quickly and keep body temperature in control. But if you’re in a hot, humid environment with no breeze, a vapor barrier can prevent sweat from evaporating.

And body temperature isn’t the only physiological effect of exercising in the heat: Your body undergoes cardiovascular, metabolic, central nervous system and behavioral changes as well. In fact, the cardiovascular changes may be the most significant, said Mike Sawka, who researched heat stress physiology at the U.S. Army Research Institute of Environmental Medicine and now teaches at Georgia Tech. When the skin dilates in the heat, he says, you’ve got the same volume of blood, but it needs to move to a bigger space -- so the heart works harder.

In the worst cases, exertional heat stroke can end in death.

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As dehydration kicks in, even the temperature in your brain rises, Bergeron said. That makes it harder to decipher what your body is dealing with and knowing when to stop.

“The worst person to make that decision (to stop) is the person that’s overheating,” Bergeron said. “People finish marathons who you would have thought would stop, but it’s related to those changes in the brain.”

People exercise in the heat for a variety of reasons, Sawka said, including cultural, occupational and athletic.

But while performance takes a negative hit as the mercury mounts, there may be some benefits of training in the heat, similar to training at altitude: “There’s emerging data that people who exercise in the heat may augment their training,” Sawka said, performing better when they return to cooler temps.