Dr. Schwartz: A recent study found that dehydrated people work out for significantly less time than those who drink water before and during exercise. Although most people know that it's important to stay hydrated during exercise, what is less well known is that the amount of water you drink has a huge effect on both the quality and quantity of your workouts. Even a slight case of exercise-induced dehydration can cause your aerobic capacity to drop.
Water will also optimize your weight loss.