The body is dehydrated when it has lost more fluid than it’s taken in, resulting in not having enough water to allow it to perform at optimal level. People won’t feel it right away as most of them drink water only when they feel thirsty. However, you’re already dehydrated at that point. Moderately active people won’t see a difference immediately, but studies have shown that athletes’ performance worsens even at 2 percent loss in water volume, according to Dr. William Roberts, founder of the International Institute of Race Medicine. Your skin, muscles and mood are not the only victims. The kidneys, heart and cholesterol levels also suffer.