Valtin, a kidney specialist and author of two widely used textbooks on the kidney and water balance, sought to find the origin of this dictum and to examine the scientific evidence, if any, that might support it. He observes that we see the exhortation everywhere: from health writers, nutritionists, even physicians. Valtin doubts its validity. Indeed, he finds it, "difficult to believe that evolution left us with a chronic water deficit that needs to be compensated by forcing a high fluid intake."

The 8 x 8 rule is slavishly followed. Everywhere, people carry bottles of water, constantly sipping from them; it is acceptable to drink water anywhere, anytime. A pamphlet distributed at one southern California university even counsels its students to "carry a water bottle with you. Drink often while sitting in class..."

How did the obsession start? Is there any scientific evidence that supports the recommendation? Does the habit promote good health? Might it be harmful?

Valtin thinks the notion may have started when the Food and Nutrition Board of the National Research Council recommended approximately "1 milliliter of water for each calorie of food," which would amount to roughly two to two-and-a-half quarts per day (64 to 80 ounces). Although in its next sentence, the Board stated "most of this quantity is contained in prepared foods," that last sentence may have been missed, so that the recommendation was erroneously interpreted as how much water one should drink each day.

He found no scientific studies in support of 8 x 8. Rather, surveys of fluid intake on healthy adults of both genders, published as peer-reviewed documents, strongly suggest that such large amounts are not needed. His conclusion is supported by published studies showing that caffeinated drinks, such as most coffee, tea and soft drinks, may indeed be counted toward the daily total. He also points to the quantity of published experiments that attest to the capability of the human body for maintaining proper water balance.

Valtin emphasizes that his conclusion is limited to healthy adults in a temperate climate leading a largely sedentary existence -- precisely, he points out, the population and conditions that the "at least" in 8 x 8 refers to. At the same time, he stresses that large intakes of fluid, equal to and greater than 8 x 8, are advisable for the treatment or prevention of some diseases, such as kidney stones, as well as under special circumstances, such as strenuous physical activity, long airplane flights or hot weather. But barring those exceptions, he concludes that we are currently drinking enough and possibly even more than enough.

Despite the dearth of compelling evidence, then, What's the harm? "The fact is that, potentially, there is harm even in water," explains Valtin. Even modest increases in fluid intake can result in "water intoxication" if one's kidneys are unable to excrete enough water (urine). Such instances are not unheard of, and they have led to mental confusion and even death in athletes, in teenagers after ingesting the recreational drug Ecstasy, and in ordinary patients.