How much water do we need, anyway?

Do we really need to drink eight eight-ounce glasses of water a day? The bottled water industry heavily promotes this prescription, but a growing number of experts are beginning to question the standard's scientific validity and even safety, making it hard to know how much water we actually need.

If you haven't thought about how much water you're drinking lately, you're probably drinking enough, says Dr. Heinz Valtin, a retired professor and kidney specialist at the Dartmouth Medical School, who recently spent 10 months studying the question. Valtin cautions that drinking too much water may increase exposure to water-borne and may increase the risk of water intoxification, also called hyponatremia, a sometimes fatal condition that is the result of replacing lost fluids much faster than sodium. CNN Medical Correspondent Elizabeth Cohen reported, "After talking to the U.S. Department of Agriculture, the National Academy of Sciences, the National Institutes of Health (NIH) and people at various universities, [the eight-glass standard] appears to be kind of a myth. We can't find a single study saying that's what people ought to do."

Other experts insist that the majority of Americans are chronically under hydrated, and should drink more water. "While medical and nutrition assertions require scientific scrutiny, it is reckless to confuse consumers by leading them to believe that they could be endangered by a suggested guideline of drinking eight eight-ounce servings of water each day," says Dr. Barbara Levine, director of the Nutrition Information Center at Cornell University's medical college.

A healthy adult in temperate climates typically loses between a half gallon and three quarters of a gallon of water a day through body functions. Under strenuous conditions in tropical climates one could lose up to six-and-a half gallons. High fluid intake has been shown to reduce incidences of urinary bladder cancer, colorectal cancer and coronary heart disease, with results even at five or six glasses of water a day. Some people claim that it also aids in weight loss, and in relieving constipation, headaches, fatigue, arthritis and depression.

Symptoms of dehydration may include nausea, fatigue, headaches, dry mouth and reduced mental acuity. Dr. Charles Peterson of the NIH says dehydration is mainly a problem among Americans suffering from other illnesses, such as diabetes, or those who undergo extreme exertion. Valtin says people can obtain water from many beverages and foods, from sodas to fruits. A 2000 study suggests that, contrary to popular belief, caffeinated drinks might not actually have diuretic effects that lead to dehydration, unless a person hasn't had caffeine in a week or more. Peterson and Valtin say thirst may be the best indicator of how much water a person should drink.

Fitness organizations have toned down their hydration messages. In April of this year, the U.S.A. Track and Field association changed its guidelines from the recommendation that long-distance athletes "stay ahead of thirst" by drinking as much water as possible, to the caution that athletes should not consume more than 100 percent of fluid lost through sweat.

While all of these different messages may be confusing, what now seems to be clear is that each person should probably work out his or her own daily water requirements, preferably with the help of a medical professional. Factors involved include weight, climate and level of physical activity. The National Academies are in the process of evaluating water needs, and are expected to make a recommendation soon. We can only hope that this will shed more light on the subject and not serve to confuse us even further. CONTACT: The National Academies, Institute of Medicine, [phone], www.iom.edu.