Sedentary folks might do fine using this mantra, but anyone who occasionally feels the urge to be active need not subscribe. "Exerciseblunts your thirst mechanism," says Leslie Bonci, R.D., director of sports nutrition at the University of Pittsburgh Medical Center. "You lose fluid so rapidly that the brain can't respond in time." In fact, a recent study from Maastricht University in the Netherlands found that women lose more water during exercise than men. An hour before you hit the gym, grab an extra 20 ounces to hydrate before you dehydrate. "It takes 60 minutes for the liquid to travel from your gut to your muscles," Bonci says.