Water -- a basic thirst quencher -- is a beverage people often take for granted. Whether it is a sweltering summer day or the dead of winter, the body slows down without adequate water intake.

For an average adult weighing 160 pounds, the body contains 40 to 50 quarts of water. If the body loses 5 percent of body water -- about 2.5 quarts -- muscles weaken and the person feels fatigued, dizzy, irritable and drained of energy. It is estimated that an adult cannot live for more than 72 hours without water.

Unfortunately, thirst -- according to nutrition experts -- is not much help. When the thirst signal kicks in, the body is already partially dehydrated. "Once you have the desire to drink, you have already lost 3 percent of body weight in water," said Evelyn Tribole, a registered dietitian and spokeswoman for the American Dietetic Association.

But people don't automatically want a cool glass of water to quench thirst. Their inclination is to reach for drinks that cause the body to lose more water -- diuretics such as caffeinated tea, coffee, soda or beer.

On a hot day, cold water is the best choice to rehydrate the body, said Tribole. The body absorbs cold water faster than hot.

Children and older people are at greater risk of dehydration because they are less sensitive to feelings of thirst, said Nancy Clark, a sports nutritionist with SportsMedicine, an athletic-injury clinic in Brookline, Mass. A 1984 study of active men ages 67 to 75 showed that after being deprived of water for 24 hours, they were less thirsty and voluntarily drank less water than men ages 20 to 31.

Although people consume a variety of foods that contain water, nutrition experts still recommend that people drink at least six to eight ounces of water or some other hydrating beverage -- like milk or fruit juice -- daily. Most fruits and vegetables -- oranges, lettuce, cucumbers, tomatoes -- are 80 to 90 percent water. Starchy vegetables like potatoes are about 75 to 80 percent water. Eggs have 75 percent, while cooked meat and fish are about 50 to 60 percent water. While cakes are 25 to 30 percent water, a cookie has only 5 percent.

Vigorous exercise causes the body to lose more water. To determine how much water is lost, sports medicine experts recommend that people weigh themselves before and after they exercise. If a person loses 3 pounds after exercise, he or she has usually lost water weight, not fat, according to Clark. A good rule of thumb is to drink one pint of water -- 16 ounces -- for every pound lost during a workout.