How much do you need?

For most people, not eight glasses. In 2002 Heinz Valtin, M.D., a kidney specialist and a professor emeritus at Dartmouth Medical School, spent nearly a year searching for evidence to back up the “eight glasses a day” dictum — and came up dry. “We didn’t find a single scientific report supporting the recommendation,” Valtin says.

So how much should you drink? “It depends on your size and activity level,” says Kristin Reimers, a registered dietitian and the associate director of the Center for Human Nutrition, in Omaha. You can tell if you’re getting enough by “looking at the color of your urine. A light lemon color means you’re well hydrated.” Even easier, just follow your thirst. “Our bodies do an excellent job of telling us when we need water,” Valtin says. “No need to force it.”

Do other beverages count?

Yes. Juice, milk, soda, and other liquids also help keep you hydrated. Even caffeinated beverages — long blamed for siphoning fluid from our bodies — seem to count. In a study conducted by the Center for Human Nutrition in 2000, subjects were given plain water or a combination of water and noncaffeinated soda, caffeinated soda, or coffee. No matter what they drank, they all stayed equally hydrated. Food is another ample source of liquid. Fruits and vegetables can be up to 95 percent water. All told, nutritionists estimate that we get two to four cups of water a day from food.