Distilling the water myth: experts now say the eight-glasses-a-day rule is wrong. Here, new guidelines to determine how much you really should be drinking

Shape, May, 2004 by Richard Laliberte

Dehydration was K.C. Guevara's main concern when she started the 2003 Boston Marathon. The weather was in the low 70s and sunny--easily 20 degrees hotter than usual for April. To keep hydrated, the then 26-year-old drank about 3 liters of water before the race and, from miles 5-22, started tossing back 3-ounce cups of water at hydration stations set up along the course. But Guevara was worrying about the wrong thing.

Through most of the race, she felt unusually tired. By the time she crossed the finish line, "I knew something was wrong," she says. "I felt dizzy and light-headed, and it was difficult to think straight." Guevara made a beeline for the medical tent, where she found her mouth wouldn't cooperate with her brain. "I babbled like a 5-year-old," she says. Though she was still on her feet, a blood test revealed she was entering a state known as hyponatremia, the result of way too much water and way too little sodium.

Unchecked, hyponatremia can cause brain cells to become swollen with water, potentially leading to confusion, seizures, coma--and even death. (In fact, a participant in the previous year's marathon did die of the condition.) Fortunately for Guevara, doctors were ready with doses of intensely salty bouillon, and after two hours she was fine. "It changed my mind about my fluid needs," Guevara says. "During the summer I used to drink 2 gallons a day. I just thought that's what my body needed."

The new water recommendations

Obviously, Guevara over did it. She drank the equivalent of about 19 cups of water over a six-hour period!

Drinking yourself to death with water used to be considered virtually impossible; hyponatremia is certainly still rare in healthy people, and far less common than dehydration. But stories like Guevara's have caused race directors, researchers, coaches, everyday exercisers and the Institute of Medicine (IOM) of the National Academies of Sciences to look more seriously at how much fluid the body really needs.

In February the IOM published the first-ever Dietary Reference Intakes for water and for electrolytes such as sodium and potassium chloride (which control the movement of water in and out of the cells of your muscles and organs). "[The guidelines are] part of an ongoing effort to emphasize nutrition, not just for preventing disease, but for making people optimally healthy," says Stella L. Volpe, Ph.D., R.D., a member of the IOM panel and the Miriam Stirl Term Endowed Chair in Nutrition at the University of Pennsylvania School of Nursing in Philadelphia.

To establish the new water-intake recommendations, the IOM panel spent two years examining hundreds of studies from peer-reviewed scientific journals on everything from normal fluid balance and kidney function to fluid needs for those who are ill.

The panel concluded that healthy, sedentary women ages 19-50 who live in a temperate climate are adequately hydrated when they get:

* 2.7 liters, or 91 ounces, of water a day (for men, it's 3.7 liters). Divided into 8-ounce portions, that's the equivalent of 11.4 glasses a day.

* 81 percent of hydration from drinking water and other beverages, including coffee and tea (that's about nine 8-ounce glasses a day) and the remaining 19 percent from foods, especially fruits and vegetables. A diet that includes plenty of produce also helps maintain a healthy electrolyte balance by adding potassium (turn to "New Rules: Less Salt, More Potassium" on page 116 to learn about the new IOM intake recommendations for these minerals).

You may be thinking, "So what else is new?" After all, these guidelines come pretty close to matching the eight 8-ounce glasses of water a day "rule" that we've accepted--and tried to comply with--for the past 30 years. The so-called "8X8" guideline directed us to drink eight glasses of water on top of everything else we drank. So it was actually more difficult to follow than these new recommendations. For example: Let's say you drink two cups of coffee and one cup of juice every morning. That's already 24 ounces--one-third of your 74-ounce liquid needs under the new guidelines--before you've even left the house! The best thing about these new Dietary Reference Intakes is that you can meet your hydration needs through a variety of sources.

And, for the first time, there is a scientific basis for the recommendation that we require about nine glasses of liquid a day.

Perhaps just as important is the IOM's finding that all beverages count toward the nine-glass goal (coffee and tea included) and that we can get part of our hydration needs met from food. "Every food you eat contains water--including dry foods like bread," explains Leslie Bonci, R.D., director of the sports nutrition program at the University of Pittsburgh Medical Center. In fact, most fruits and vegetables are 80-99 percent water, but even cake, bread and cheese are more than 20 percent water each.

Follow your thirst

Under normal circumstances, however, hydration isn't something we need to think about. "The minute you drink too much fluid, the kidneys want to pee it out," says kidney expert Heinz Valtin, M.D., a retired professor of physiology at Dartmouth Medical School in Hanover, N.H. (Valtin tried to find scientific studies to support the widely cited 8X8 water guideline, but his 2002 published report came up empty.) "When you don't have enough fluid, the kidneys retain it. The system that regulates the body's fluid balance is accurate, sensitive and fast. On its own, it's remarkably efficient," he says.