Fitness Nutrition and Hydration

Water Works: How Much is Enough?

By Christine Haran

In an age when many people are armed with a bottle of spring water at all times, it's generally thought that the more water you drink, the healthier you are. But new research suggests that most people, including the elderly, meet their hydration needs simply by drinking when they are thirsty.

Although conventional wisdom has advised that people drink eight 8-ounce glasses of water daily, little research about water turnover in the body has been done. In an effort to address that question, the authors of a recent study, published in the American Journal of Physiology-Renal Physiology, examined total water intake and excretion patterns in 458 healthy people between the ages of 40 and 79. The researchers found that water intake and urinary output varied greatly from person to person, and was unrelated to height or weight.

"We speculate that water intake is more of a behavior than something that's caused by metabolism," said coauthor Dale Schoeller, PhD, a professor of nutritional science at the University of Wisconsin-Madison.

Dr. Schoeller noted that most people observed in the study were not drinking 64 ounces of water a day. "Since most people are not doing that and are not dehydrated, it's probably not needed. The advice would be to obey your thirst and to be careful in situations where you're exposed to high temperature or low humidity or when you're physically active. Those are things that cause water loss and affect physical performance and health."

This is particular important advice for older people, even though Dr. Schoeller and his colleagues did not find that the older people in the study were dehydrated. In fact, the researchers were surprised to find that there was little difference in water intake or water output between the ages of 40 and 70, though water intake decreased after age 70.

"Dehydration in the elderly is still a concern," Dr. Schoeller cautions. "But it's not because they're bumping along with barely enough water intake. It's that when they're in a situation where the water need increases dramatically—like out in the sun at a picnic—they don't get as thirsty as young people."

Other people who may be at risk for dehydration include children, people with diabetes and people taking certain medications such as diuretics, which lower blood pressure, laxatives and some anti-psychotic and anti-epileptic drugs.

Still, a recent Institute of Medicine report also concluded that that most people "adequately meet their daily hydration needs by letting thirst be their guide." The report did offer general guidelines, recommending that women drink approximately 2.7 liters (91 oz) of total water a day and that men consume about 3.7 liters (125 oz). This water can be obtained from food or from fluids, including caffeinated and alcoholic beverages, according to the report's authors.

So while it's important to stay hydrated, unless you're exercising or out in hot or arid weather, the idea that healthy people have to drink water all day can probably be hung out to dry.