12 August 2002
Recommended Water Intake A Myth
by Kate Melville
It has become accepted wisdom: "Drink at least eight glasses of water a day!" Not necessarily, says a DMS physician Heinz Valtin, MD. The universal advice that has made guzzling water a national pastime is more urban myth than medical dogma and appears to lack scientific proof, he found. In an invited review published online by the American Journal of Physiology August 8, Valtin, professor emeritus of physiology at Dartmouth Medical School, reports no supporting evidence to back this popular counsel, commonly known as "8 x 8" (for eight, eight-ounce glasses). The review will also appear in a later issue of the journal.
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.
And he lists other disadvantages of a high water intake: (a) possible exposure to pollutants, especially if sustained over many years; (b) frequent urination, which can be both inconvenient and embarrassing; (c) expense, for those who satisfy the 8 x 8 requirements with bottled water; and (d) feelings of guilt for not achieving 8 x 8.
Other claims discredited by scientific evidence that Valtin discusses include:
- Thirst Is Too Late. It is often stated that by the time people are thirsty, they are already dehydrated. On the contrary, thirst begins when the concentration of blood (an accurate indicator of our state of hydration) has risen by less than two percent, whereas most experts would define dehydration as beginning when that concentration has risen by at least five percent.
- Dark Urine Means Dehydration. At normal urinary volume and color, the concentration of the blood is within the normal range and nowhere near the values that are seen in meaningful dehydration. Therefore, the warning that dark urine reflects dehydration is alarmist and false in most instances.
Is there scientific documentation that we do not need to drink "8 x 8"? There is highly suggestive evidence, says Valtin. First is the voluminous scientific literature on the efficacy of the osmoregulatory system that maintains water balance through the antidiuretic hormone and thirst. Second, published surveys document that the mean daily fluid intake of thousands of presumably healthy humans is less than the roughly two quarts prescribed by 8 x 8. Valtin argues that, in view of this evidence, the burden of proof that everyone needs 8 x 8 should fall on those who persist in advocating the high fluid intake without, apparently, citing any scientific support.
Finally, strong evidence now indicates that not all of the prescribed fluid need be in the form of water. Careful peer-reviewed experiments have shown that caffeinated drinks should indeed count toward the daily fluid intake in the vast majority of persons. To a lesser extent, the same probably can be said for dilute alcoholic beverages, such as beer, if taken in moderation.
"Thus, I have found no scientific proof that absolutely every person must 'drink at least eight glasses of water a day'," says Valtin. While there is some evidence that the risk of certain diseases can be lowered by high water intake, the quantities needed for this beneficial effect may be less than 8 x 8, and the recommendation can be limited to those particularly susceptible to the diseases in question.