It sounds like some new pharma drug, but it's really something you add to your drinks that helps your kidneys retain water, which in turn helps your body stay cool, says Karp; the idea is similar to the way that sodium functions in sports drinks like Gatorade. He recommends drinking a 20 percent glycerol solution (you can find them where vitamins and supplements are sold) no sooner than about 30 minutes before you exercise, since it doesn't always sit well on people's stomachs.
Get used to the heat first
Karp says it's important to acclimate yourself to the heat with less-intense exercise before heading out for your usual workout. Giving your body time to adjust lowers the stress on your heart and can reduce the amount you sweat. Two weeks is usually enough for most people get acclimated, Karp says, and you should do it in increments. "If you're used to exercising 30 minutes at a time, cut that by a third and then slowly increase," he says. Exercise for 10 minutes outside for the first few workouts, then add another 10 after a few outdoor sessions. If you're just starting an exercise program, on the other hand, stay in the gym to start with and begin exercising outdoors only after you've reached a decent fitness level, says Karp.
Stick to morning workouts
"Exercise performance actually peaks in the late afternoon," Karp says, "but when you add the stress of heat, that changes things." Research has shown that cool morning air boosts your endurance more so than trying to work out after the sun goes down at night, he adds.
Invest in synthetics
You'll do yourself a favor by investing in sweat-wicking synthetic shirts rather than reaching for that old cotton T-shirt, which traps sweat next to your skin and doesn't allow it to evaporate, he says. Just be sure to avoid synthetic clothes treated with antibacterial chemicals or silver nanoparticles, both of which have iffy health effects and damage the environment when you toss them in the washing machine.
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