Hydrate for good health: drinking when you're thirsty is not enough. find out why your body needs fluid and learn how to make sure you're getting the right amount.(FOOD: EAT SMART)

It's a hot summer day and you're thirsty--very thirsty. Just drink a glass of water and the problem is solved, right? Not so fast. According to W. Larry Kenney, PhD, president of the American College of Sports Medicine, "Thirst alone is not the best indicator of dehydration or the body's fluid needs."

When you drink to quench your thirst, you are not satisfying your total fluid needs. "Unfortunately, our brain signals us after it is too late," says Susan Ryan, MD, an emergency room physician in Denver, CO. According to Ryan, "Most people don't drink enough water and therefore are not operating at peak efficiency."

Your Body's Cries for Water

The human body is made up of approximately two-thirds water. About 75 percent of the brain is water, and water makes up 75 percent of muscle and more than 90 percent of blood. As a result, nearly every bodily function depends on water, including the transportation of oxygen and nutrients to cells, regulation of body temperature, and lubrication and cushioning of joints and key organs.

Brain function, immunity and elimination all rely on water. Water even helps us breathe. Having youthful skin also requires proper hydration.

"Dehydrated skin is a common problem," explains Myra Eby, president and founder of MyChelle Dermaceuticals, a Frisco, CO-based manufacturer of nontoxic skin care products. "A key cause of aging skin is lack of moisture. In order to have supple, soft skin, you need to drink plenty of fresh water frequently throughout the day," she says.

Water is especially beneficial to weekend warriors, exercisers and athletes. According to Ryan, studies demonstrate that as little as 2 percent dehydration significantly decreases athletic performance. "I have found that even sedentary people have improved vitality after they increase their daily water intake," she says.