While some athletes advocate drinking "electrolyte replacement" beverages during hot weather, experts say that -- for the majority of exercisers -- plain water is sufficient.

"Sweat turns out to be more dilute than your own bodily fluids," says Ed Coyle, exercise physiologist at the University of Texas at Austin. "As you're sweating, you're really drawing off proportionately more water from your body than anything else. What's left behind is actually more concentrated electrolytes. What you need to do is not replace the electrolytes, but replace fluids, to restore your body's own electrolyte concentration back to a normal level."

The electrolyte beverages versus water question was tested by Carl Gisolfi, exercise physiologist at the University of Iowa. He tested a variety of carbohydrate beverages on distance runners in 95 degree heat with 25 percent humidity last summer, running two hours on a treadmill at 70 percent maximum oxygen uptake. Every 20 minutes they drank 200 milliliters of fluids.

He found no significant differences in plasma volume, heart rate or core body temperature between the beverage drinkers and the water drinkers.

But Coyle adds that, although "you have enough of your own body glycogen stores to last for at least two or 2 1/2 hours," people exercising longer than that may benefit from the carbohydrate solutions.

Working with competitive cyclists, Coyle found that a 50 percent carbohydrate solution after 20 minutes of exercise and then a more diluted solution every 20 minutes apparently slowed "the rate at which these endurance athletes used up their own body's muscle glycogen stores."

Says Gisolfi: "The most critical thing is that you drink something."