Heat has to factor into your summer exercise plans if you expect to do any shaping up outdoors. Heat has a big impact on you while you exercise, and it’s important to listen and respond to your body’s cues before you wind up with a case of heat stroke.

The details: Because your body is mostly water, dehydration impacts every aspect of your physiology, writes Jason Karp, PhD, exercise physiologist and running coach, in the most recent issue of the IDEA Fitness Journal. And when you’re working out on a really hot day, the amount of water you lose can be double the amount you’d lose on a normal day. Not only does that water loss increase your body’s internal temperature, which puts you at risk for getting overheated, it also reduces the amount of energy that’s fed to your cells, which means your muscles aren’t getting the energy they need. Adding to the stress on your body, humid air doesn’t allow for sweat to evaporate from your skin and cool you off, so your internal core temperature rises even higher, putting you at risk for heat stroke. Finally, all that sweating speeds up your heart rate, causing it to rise three to five beats per minute for every 1 percent of water loss you experience. Essentially, you feel like you’re working out harder than you really are.

What it means: Don’t stop exercising outside this summer just because it’s hotter than normal. An outdoor run, bike ride, or romp in a pool or lake can be more convenient and less expensive than hitting the gym. And being in nature is a natural mood elevator, even if it is 100 degrees and getting hotter by the minute. What’s key, says Karp, is staying hydrated and being smart about your workouts.

Here are his tips for staying healthy and still enjoying the great, if steamy, outdoors:

Weigh yourself before and after your workouts. The key to staying hydrated is to match your fluid intake with your sweat loss, he says. “The best way to do that is to weigh yourself before and after your workout and see how much water is lost,” he says. One pound of weight difference means you lost about 16 ounces of water.

Hydrate before your workout, and avoid the sprinklers. Running through a sprinkler or biking past a gushing hydrant may feel good, but it doesn’t really cool you off. “Getting fluid inside you is what helps with thermal regulation,” Karp says. Drinking water while you exercise is essential, but it’s even better to hydrate before you start, he says, because it increases your body’s ability to maintain a proper temperature and can help your heart maintain a steady pace so you won’t feel like you’re exerting more effort than you really are.

Try glycerol. 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.