Some central tenets of good health: more vegetables, less soda, lots of exercise. And let’s not forget water: at least eight glasses a day. Much ink is spilled over the first three of these recommendations, but the last sometimes seems to be taken for granted by all the people lugging around Nalgene bottles. Is drinking so much water necessary? Is reaching eight glasses per day crucial to good health?
The short answer — at least to the specific question of eight glasses versus, say, seven or nine — is no, there is nothing special about eight. This threshold appears to be a long-standing medical myth. It’s not even clear where it started. The best answer I can find (based on this review) is that the source was a 1945 publication by the National Food and Nutrition Board, a government advisory agency, that stated this: “A suitable allowance of water for adults is 2.5 liters daily in most instances. … Most of this quantity is contained in prepared foods.” The theory is that people read this, ignored the last sentence, and the eight glasses a day (about 2.5 liters) recommendation was born.
So let’s dispense immediately with glass-counting attempts to reach this magical threshold. If we take a more charitable view of the goals of the water lobby, however, its goal is not to get us to some specific cutoff but to increase water consumption in general. So really the question shouldn’t be so much about eight glasses versus seven, but whether there is evidence that drinking more water makes you healthier.