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

The new water-intake report backs up Valtin's observation that our bodies know what they're doing: Most people meet their daily hydration needs simply by drinking when they're thirsty--and that includes beverages other than water, since they're almost entirely [H.sub.2]O. That means your day's tally should factor in soft drinks, coffee and tea, which will come as a great relief to anyone who remembers the old hydration adage to drink an extra cup of water for every cup of caffeinated beverage consumed. That too is passe, it turns out: A study at the University of Nebraska published in 2000 found no significant differences in hydration when subjects drank caffeinated beverages and when they drank the same beverages without caffeine.

One size doesn't fit all

Helpful though they are, however, the new guidelines are just averages and wouldn't have helped marathoner K.C. Guevara, since they don't account for activity or weather. In its report, the IOM panel grants that "higher intakes of total water will be required for those who are physically active or are exposed to [a] hot environment." Yet it doesn't say what those amounts are.

So if you're running in the heat, for example, it's tricky to figure your needs down to the ounce because each person's body handles fluid and electrolytes differently. "You can have two identical runners next to each other--same height, weight and conditioning--and one will lose more fluid than the other," says Douglas Casa, Ph.D., A.T.C., director of athletic training education at the University of Connecticut in Storrs. "You can't apply one-size-fits-all standards to [hydration and] exercise."

The conventional wisdom that you're already dehydrated by the time you're thirsty is an exaggeration that borders on being a myth in itself. When a lack of fluids makes minerals and other components in your blood more concentrated by about 2 percent, you naturally become thirsty. But you're not actually considered dehydrated until your blood concentrates by 5 percent. These numbers sound small, but "that's a large amount of leeway," says Valtin. "If you're healthy, you could do nothing but follow your thirst and probably be OK."

Still, Casa says that when doing an aerobic activity for extended periods of time (as with long bike rides, hikes and runs), be careful not to consume too much fluid. Extended exercise, he says, makes the kidneys less efficient at eliminating excess fluid because blood is shunted away from organs to working muscles--a problem that may have affected Guevara during her marathon.

On top of that, some people are more prone to sodium loss when they exercise heavily. "I'm a significant sodium loser," says University of Pittsburgh's Bonci. "I get a gritty feeling on my skin, and I notice white caking around my waistband, sports bra--anyplace I sweat more." If you notice such signs after an extended workout, it's an indication your body is excreting a lot of sodium, so go ahead and down a sports drink to be safe.

Such drinks can help replace electrolytes and sodium, but Casa recommends that vigorous athletes ensure proper fluid balance by taking another step: Weigh yourself before and after your workouts, and drink what you think is best while you exercise. "If you're lighter when you're done, you've lost fluid and you need to drink more. If you're heavier, you need to drink less," he explains. "With practice and training, you'll see how your fluid needs change with intensity and heat." (See the recommendations in "What to Drink for a Hot-Weather Workout" on page 138.)