How it builds muscle: Whether it's in your shins or your shoulders, muscle is approximately 80 percent water. "Even a change of as little as 1 percent in body water can impair exercise performance and adversely affect recovery. For example, a 1997 German study found that protein synthesis occurs at a higher rate in muscle cells that are well hydrated, compared with dehydrated cells. English translation: The more parched you are, the slower your body uses protein to build muscle.
Not sure how dry you are? "Weigh yourself before and after each exercise session. Then drink 24 ounces of water for every pound lost.
How it keeps you healthy: Men who drank five or more 8-ounce glasses of water a day were 54 percent less likely to suffer a fatal heart attack than those who drank two or fewer.