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Thirst Doesn't Always Guide Need for Water: Sports Experts

Reuters 26 February 2004

Merritt McKinney

NEW YORK (Reuters Health) - Are you getting enough water? Relying on thirst to tell you may not always be the best guide, sports medicine experts say.

Earlier this month, the Institute of Medicine said people can generally depend on thirst to let them know how much fluid to drink each day, but the American College of Sports Medicine (ACSM) is emphasizing that this may not be optimal during prolonged exercise or hot weather.

"Our fluid intake is mostly behavioral," said Dr. W. Larry Kenney, the president of the ACSM. "On a daily basis, people drink when they eat meals, when they get a little thirsty, when they walk past a water fountain."

Most of the time, people can safely rely on thirst to judge how much fluid they need, but "during heavy or prolonged exercise or long summer periods of hot weather, thirst doesn't accurately tell the body how much fluid has been lost via sweating," Kenney told Reuters Health. Under those circumstances, drinking should be encouraged, said Kenney, who is a professor of physiology and kinesiology at Pennsylvania State University.

To prevent drinking too little or too much water, Kenney advised exercisers to weigh themselves before and after exercising.

"Strive to drink enough to get back to beginning body weight," he said.

The Institute of Medicine report declared that, despite concerns about being properly "hydrated," most Americans consume plenty of fluids each day. The Institute, an independent group of experts that advises the federal government on health issues, recommends that women consume about 2.7 liters of water a day and that men drink about 3.7 liters per day.

According to the Institute, water contained in food, coffee, beer and other drinks all count toward the recommended daily amount.

In line with the ACSM recommendation, the Institute of Medicine noted that people who are very physically active or who live in hot climates may need to drink more water.

In the mid-1990s, the sports medicine group issued recommendations on exercise and fluid replacement. The group advises exercisers to eat a nutritionally balanced diet and drink adequate fluids during the day before exercise or an athletic event. About 2 hours before exercising, people should drink about 500 milliliters of fluid, according to the guidelines.

During exercise, people should drink early and at regular intervals to make sure that they replace all water lost through sweating, according to the ACSM. To make drinking more appealing, the group recommends consuming cold water. Flavored drinks may also encourage athletes to drink enough fluid, the guidelines note.

For people who exercise for less than an hour, there is little evidence that sports drinks with electrolytes and carbohydrates are necessary, according to the ACSM. Sports drinks may be appropriate for people exercising for longer than an hour, however, the group notes in the recommendations.