Sweating it out in sweltering heat

As you strive to ensure that you get enough exercise, don't forget to add water. A study presented to the American College of Sports Medicine shows that many children in summer sports camps are dehydrated, despite the availability of water and sports drinks and encouragement to drink them.

The study did not examine adult hydration habits, but experts' advice on drinking enough fluids before and during exercise applies equally to children and adults.

The study of 34 boys and 24 girls aged 10 to 14 enrolled in four-day soccer camps found that most of the children were dehydrated by the second day of camp and that 59 percent of boys and 70 percent of girls were significantly dehydrated by the last day of camp.

Study author Douglas Casa, director of athletic training education at the University of Connecticut and a member of Gatorade's board of advisers, said there is no easy way to say exactly how much each child should drink to be adequately hydrated.

"The key message is that you don't want to lose weight [from fluid loss] during exercise, so whatever you sweat out must be replaced," Casa said. The best strategy, for both children and adults: Drink 20 to 30 extra ounces of water or a sports drink at least 30 minutes before you start exercising and, if you will be exercising for more than an hour, drink more fluids during your workout.

Finding ways to keep cool outdoors can be tough, but "short dips in cool water and running through water sprays briefly cools the skin and feels good," said Larry Kenney, professor of physiology and kinesiology at Pennsylvania State University. But those splashes "provide minimal whole-body cooling effects."

High air temperatures make your heart work harder than it otherwise would to pump blood to your skin to keep you cool. "When you hear about old people dying during heat waves, it is almost always from a heart attack," Kenney said. "Their hearts are overworked."

High humidity impedes the evaporation of sweat from the body, which also strains your heart.

"More sweat drips from the body than evaporates, and that makes the heart pump harder to transfer blood to the skin," Kenney said.

When most of your sweat evaporates, as it does at lower humidity levels, heat is transferred away from the skin. "Without evaporation, you would only be able to exercise for 10 or 15 minutes without overheating," Kenney said.

Dehydration is a danger for exercisers at any temperature.

, whether on land or in water.

"Swimmers sweat even though they do not perceive it," Kenney notes. "Fluid lost in sweating must be replaced."

Heat exhaustion, characterized by nausea, dizziness, fatigue and headaches, is often caused by dehydration. Heat stroke results from such a rapid rise in body temperature that the body stops sweating -- it loses the ability to regulate body temperature -- and that affects the nerves of the brain. "That's why the effects of heat stroke are mostly neurological -- confusion, not knowing where you are," Kenney said.

Does that mean you can't exercise outdoors in high heat and humidity? Not at all.

Almost any exercise is fine in moderation, even on the hottest of days, Kenney said. Allow your body to gradually adjust to a heat wave. On the first day, exercise for 15 minutes, then 20 minutes on the second day, then a half-hour on the third day.

"When football players collapse during summer training camp, almost 100 percent of those cases occur on the first or second day of camp," Kenney notes.

Drink more fluids -- before, during and after exercise -- than you would in milder weather. That way, the 103 heat index is less likely to get you. And wear light-fitting, light-colored clothing designed to wick sweat away from the body. Cotton, though comfortable, isn't advised because it holds sweat.