This is from the September 2011 issue of Shape magazine.

According to Cynthia Sass, R.D. Thirst is your body’s way of letting you know that your optimal water level is down 2 percent. The Institute of Medicine recommends taking in 11 cups of fluid a day and even more if you’ve been sweating. (I always thought it was 8.) All liquids count. You can even drink caffeinated beverages as long as you don’t drink more of them than you usually do. Apparently your body gets accustomed to processing caffeinated beverages which means you won’t experience a diuretic effect. Fruits and veggies count as liquids since they’re 80 percent water (many of them, not all). For ex: A cup of watermelon has about half a cup of water. So DRINK UP!