Nobody needs eight glasses of water a day

In 1945, the US Food and Nutrition Board —the body that sets recommended allowances— published a sentence that has survived eighty years: an adult needs about 2.5 litres of water a day. The next sentence, the one nobody repeated, said that most of that amount already comes inside prepared food.

In 2002, Heinz Valtin, professor of physiology at Dartmouth Medical School, set out to find the study behind the eight glasses. He reviewed the scientific literature and published the result in the American Journal of Physiology: it does not exist. Not a trial, not a cohort, not a measurement. The 8×8 rule was born from a misquoted recommendation and spread because it sounds like hygiene.

What has been measured: between 20 and 30 % of the water entering the body arrives as soup, fruit, vegetables and bread. Coffee and tea count as fluid, because at normal doses their diuretic effect does not cancel out what they provide. And thirst, in a healthy person, kicks in long before there is any hydration problem.

Drinking too much is not free either. At the 2002 Boston Marathon, 488 runners were tested as they crossed the finish line: 13 % came in with hyponatraemia, blood sodium below normal from too much water.