If you're working out at a low level for 60 to 75 minutes or less, drinking water is just fine, says sports nutritionist Monique Ryan, author of Sports Nutrition for Endurance Athletes (Velo Press, 2002). But if you're exercising for a longer period of time, or if the intensity is high—for example, running a half-marathon or competing in a triathlon—then you probably need a sports drink to rehydrate. "The sodium and the carbs in sports drinks help you absorb the fluid through the small intestines better than plain water, and these drinks empty from your stomach just as quickly as water," says Ryan.

For low to moderate exercise> Choose a sports drink with 6 percent to 8 percent carbohydrate concentration, or 50 to 70 calories, and a modest amount of sodium (about 90 mg) per 8-ounce serving.

For hard-core exercise > If you work out for three to four hours or more, Ryan suggests using a sports drink with a higher sodium concentration (usually about 200 mg of sodium per 8-ounce serving).