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 suffers 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.
Overdrinking water is possible and bad for your health, Dr. Roberts says. "Symptoms are dizziness, vomiting, progressive headaches and confusion." But while it happens more often than you think, he adds, not drinking enough H2O is by far the bigger problem.
Adequate hydration is dependent on many different factors such as your size, activity levels and sweat rate. Severe dehydration – losing 25-30 percent of total water volume – can even lead to death. “It’s very rare for someone to dehydrate to that level in normal conditions,” Dr. Roberts adds. But the lack of fluids in your system is still hurting you in many ways.