Almost everyone has heard the rule that we need eight 8-ounce glasses (about two liters) of water daily, along with the other part of the rule that beverages containing caffeine or alcohol do not count. It turns out this rule has never been scientifically tested, and there is little evidence to support it.
Diet surveys of thousands of adults suggest that such a large amount of liquid is not needed because these people were healthy and did not consume this much fluid. Likewise, there are dozens of studies that show the precision of the system that regulates water balance. The evidence was summarized in a review published in the American Journal of Physiology: Endocrinology and Metabolism in August 2002.
HERE'S WHAT YOU NEED TO KNOW: Remember that most foods you eat are loaded with fluid. Uncooked meat is two-thirds water; most fruits are many vegetables are 90 percent water. Some people may need large amounts of fluids, especially those physically active in hot climates. Caffeine and alcohol are diuretics, that is they increase urine production, but much of the water in beverages that contain them does get used by the body.