Your body is 60% water -- and not drinking enough can wreak havoc from head to toe. Up to half of all Americans suffer chronic mild dehydration that can cause fatigue, headache, loss of appetite, dry eyes and mouth. As we age, thirst becomes a less sensitive indicator of dehydration, so don't wait until you're thirsty to grab a glass. If you work out, drink more during intense physical activity, when dehydration is a real threat -- your body rapidly loses water through sweat and vapor in exhaled air. A rapid water loss of 5% of body weight can reduce muscular work by 20 - 30%.