The Christmas edition of the British Medical Journal usually has a series of light-hearted articles in it. Last month's copy was no exception. In one of the articles, entitled "Medical myths" (Vreeman & Carroll 2007), the authors wrote "We generated a list of common medical or medicine related beliefs espoused by physicians and the general public, based on statements we had heard endorsed on multiple occasions and thought were true or might be true. We selected seven for critical review". The first myth critiqued was "People should drink at least eight glasses of water a day". Searches on Medline and Google apparently yielded no hard evidence supporting this advice. There are many sources stating that it's good health practice, but they could find no worthwhile evidence to back it up.

This state of affairs is explored very carefully by Valtin in a review published in the American Journal of Physiology (Valtin 2002). He writes "No scientific studies were found in support of 8 x 8 (drink at least 8 x 8 ounce glasses of water daily). Rather, surveys of food and fluid intake on thousands of adults of both genders, analyses of which have been published in peer-reviewed journals, strongly suggest that such large amounts are not needed because the surveyed persons were presumably healthy and certainly not overtly ill. This conclusion is supported by published studies showing that caffeinated drinks (and, to a lesser extent, mild alcoholic beverages like beer in moderation) may indeed be counted toward the daily total, as well as by the large body of published experiments that attest to the precision and effectiveness of the osmoregulatory system for maintaining water balance. It is to be emphasized that the conclusion is limited to healthy adults in a temperate climate leading a largely sedentary existence, precisely the population and conditions that the "at least" in 8 x 8 refers to. Equally to be emphasized, lest the message of this review be misconstrued, is the fact (based on published evidence) that large intakes of fluid, equal to and greater than 8 x 8, are advisable for the treatment or prevention of some diseases and certainly are called for under special circumstances, such as vigorous work and exercise, especially in hot climates. Since it is difficult or impossible to prove a negative - in this instance, the absence of scientific literature supporting the 8 x 8 recommendation - the author invites communications from readers who are aware of pertinent publications."

Valtin, H. (2002). ""Drink at least eight glasses of water a day. "Really? Is there scientific evidence for "8 x 8"?" Am J Physiol Regul Integr Comp Physiol 283(5): R993-1004. [Free Full Text]
Vreeman, R. C. and A. E. Carroll (2007). "Medical myths." BMJ 335(7633): 1288-1289. [Free Full Text]