Newswise — We've all been told it's important to drink plenty of fluids during exercise. But now it seems too much water can be very dangerous. So which is right? Both. Good hydration is important, but overhydration can be hazardous, even lethal. Common sense and moderation can help protect you from both extremes, reports the July 2008 issue of Harvard Men's Health Watch.

Dehydration increases the risk of muscle cramps, heat exhaustion, and heat stroke during exercise in warm weather. And even in mild weather, dehydration can leave exercisers groggy for hours afterward. When the hazards of dehydration became apparent, experts began to encourage drinking fluids during exercise. Guidelines were formulated to meet the needs of elite male athletes whose high-intensity exercise produced lots of fluid loss in sweat. As a result, athletes began to increase fluid intake, and some drank too much, leading to water intoxication and hyponatremia (low blood sodium levels). Hundreds of cases and a number of deaths have been recorded in medical journals.

Caution is justified, but it's not easy to drink enough to get into trouble. The typical victim of water intoxication is a runner who is out on a marathon course for over four hours and who consumes enough fluids to gain weight during the race.

So, how much should you drink? Harvard Men's Health Watch suggests that you plan to drink two to three cups of water an hour, but boost the amount if you are sweating heavily. Unless you get way behind in your fluid replacement, sports drinks won't be any better than water. Drink when you feel thirsty, but don't force down huge amounts. Remember, if you gain weight, feel bloated, or experience nausea and vomiting, you're on your way to trouble.

Also in this issue:

"¢ Active surveillance of prostate cancer"¢ Adult asthma"¢ Grape juice and your health

Harvard Men's Health Watch is available from Harvard Health Publications, the publishing division of Harvard Medical School, for $24 per year. Subscribe at or by calling 877-649-9457 (toll-free).