Whether you’re a weekend warrior looking to lose a few extra pounds or a perennial marathoner running toward a personal record, one key to surviving daily workouts is proper hydration.

Regular water intake and the use of sports drinks help the body regulate its core temperature during exercise, as well as replace essential salts lost through perspiration.

But sports medicine experts are warning exercise enthusiasts, from marathon runners to health club hounds, against overhydrating. Too much drink during long-term exercise— such as a triathlon, or even a long afternoon gardening— can actually dilute the blood, resulting in what doctors call hyponatremia (HIGH-po-nuh-TREE-mee-uh). Diluted blood cells can fail to function altogether, depriving the heart of needed nutrients. This condition can be fatal.

Findings from a recently released study of runners who participated in the 2002 Boston Marathon concluded that thirteen percent of the athletes who finished the race had hyponatremia, and these were runners who thought they had not overhydrated. Ironically, experts say overhydration has been linked to more problems, including death, than dehydration during running events. They stress, however, that short of an obsession with hydration, most athletes are not in danger of experiencing hyponatremia.

To make sure you don’t overdo it on your water breaks, doctors recommend sports drinks like Gatorade during prolonged exercise. These beverages hydrate while restoring salts lost during a workout. As for water intake, exercisers should drink a maximum of eight ounces of water for every twenty minutes of activity.