Follow a safe summer exercise plan. Hydrate before, during, and after your workout, and exercise at the right time of day.

Don't wait until you're already sweating; drink water before you work out.

RODALE NEWS, EMMAUS, PA—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.