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Everyone from womens magazines to Colon Health Network and the Georgia Rural Water Journal extols the virtues of drinking at least eight, eight-ounce glasses of water a day. Bottled-water makers heavily market their products based on this theory so commonly held that nutritionists refer to it simply as "eight-by-eight."
The International Bottled Water Association (IBWA) even has a "hydration calculator" on its Web site for people to determine whether they need even more than that.
But a growing number of health experts say the advice may not hold water, reports Betsy McKay in The Wall Street Journal
Researchers who have tried to pinpoint the origin of the eight-by-eight rule have come up dry. "This idea is being promoted terrifically by the bottled-water industry, but I have found no scientific evidence that supports the claim," says Heinz Valtin, a kidney specialist at Dartmouth Medical School. Later this year, the American Journal of Physiology is expected to publish the results of nine months of research by Dr. Valtin that led to that conclusion.
The National Academy of Sciences, one of the perpetuators of the eight-by-eight wisdom, recently formed a panel to develop new guidelines. Theyre due out in March. "We need to look at it again," concedes Allison Yates, director of the academys Food and Nutrition Board.
The only studies researchers have found that bolster the eight-by-eight rule were performed on soldiers at high altitude, hospitalized patients, and others in "nontypical environments," says Ann Grandjean, a nutrition specialist at the University of Nebraska and a consultant to the U.S. Olympic Committee. For the average American adult who rarely breaks a sweat and works in a climate-controlled office, eight-by-eight may be more than enough.
For most people, the consequences of drinking too much water are simply extra trips to the bathroom. But for a few, excessive amounts can be harmful, even fatal. Some diabetics and others take a high dose of an antidiuretic hormone to prevent the body from losing water. If such individuals consume too much liquid, they can develop "water intoxication" the excess water can dilute sodium in the blood, causing a swelling of cells that in extreme cases can lead to seizures and death.
No one disputes that drinking plenty of water is a good idea. A study published this month in the American Journal of Epidemiology asserts that drinking water can help reduce the risk of fatal heart attacks.
But for bottled-water makers, hydration translates into sales. On a Web site for its Dasani brand (www.dasani.com), Coca-Cola Co. urges people who exercise to "consume at least 16 ounces of fluid [two glasses] before you start your activity and then five to eight ounces every 15 minutes while youre working out." The label of Zephyrhills, a Nestle SA water brand, urges consumers to "drink a 24-oz. Zephyrhills Brand Sport-Pack [three glasses] two hours before you exercise." (The company says hydration tips are being phased out on packaging, in favor of graphics and wording that promotes its brand.)
The IBWA, which represents the $6.4 billion bottled-water industry in the U.S., has a "hydration calculator" on its Web site that recommends a 140-pound person drink 70 ounces, about 8 1/2 glasses, of water on a day in which he or she doesnt exercise. An hourlong workout raises the recommended daily intake to 110 ounces nearly 14 glasses. The association cites the National Academy of Sciences as well as hydration experts and various studies as the sources of its hydration tips.
"Americans still arent drinking enough water," says Stephen Kay, a spokesman for the bottled water association. While water isnt the only source of hydration, its the healthiest, he argues. "Its caffeine-and calorie-free."
How much is enough actually depends on the individual, nutritionists say. People who exercise need more liquids than couch potatoes to help replenish the fluids theyve lost. The average adult loses about two liters of fluid a day but about half of that amount can by replaced through a healthy diet and the bodys own metabolism, nutritionists say.