Drink More Water, Says FLOTUS—But How Much?

"I've come to realize that if we were going to take just one step to make ourselves and our families healthier, probably the single best thing we could do is to simply drink more water," First Lady Michelle Obama said in a statement issued this week. She went on to recommend downing an additional glass a day.

Which raises the question: How many glasses of water a day should you drink?

It's commonly believed that eight 8-ounce glasses of water should be guzzled each day. You sure won't get any arguments about that from the bottled water industry. But hydration experts aren't sure where the "8 x 8" rule came from—or whether it holds water.

Mike Sawka, a U.S. Army research scientist, thinks the origins lie in a 1933 study on rodent hydration. The research led to a recommendation of 2.5 liters a day, or 84.5 ounces of liquid, for a moderately active human to make up for water lost to sweat and excretions. He says that 20 percent of those ounces come from foods that contain a lot of water: soup, ice cream, celery. That leaves 67.6 ounces of water, or roughly "8 x 8."

Only you don't really need eight daily glasses. Other beverages count, even if they're caffeinated. "The body's need to keep fluid trumps the small influence caffeine might have on losing fluid," says University of Connecticut exercise physiologist Douglas Casa.

Besides, it's not like you need to line up eight glasses and down 'em or risk dehydration. The basic rule: Drink if you feel thirsty. If not, don't. The exception would be folks about to embark on an intense workout. Drinking beforehand is helpful.

And if you're worried that you're not drinking enough, check your urine. Dark yellow, says University of Pennsylvania nutritionist Stella Volpe, is the color of dehydration.