Eight Glasses a Day

In 2002 Heinz Valtin, a kidney physiologist at Dartmouth, went looking for the science behind drinking eight glasses of water a day. He searched the published literature and reported that he found no scientific studies in support of it. The rule everyone knew had no study sitting underneath it.

One origin he reported, credited to P. Thomas, was a 1945 circular from the Food and Nutrition Board of the National Research Council. It set an allowance for adults of 2.5 liters daily in most instances, and a standard of 1 milliliter for each calorie of food. The sentence after that read: "Most of this quantity is contained in prepared foods."

The suggestion was that the last sentence went unheeded and the allowance was misread as eight glasses of water to drink each day. Valtin named a second candidate. Fredrick Stare's 1974 nutrition book put the average adult at around 6 to 8 glasses, counted coffee and beer among them, and called fruits and vegetables good sources of water. Both ancestors came with food attached.

The rule kept the glasses and dropped the food. Valtin also found published evidence that caffeinated drinks and moderate beer may count toward daily fluid. He limited his conclusion to healthy adults in a temperate climate living largely sedentary lives, granted that disease, hard work and heat call for more, and said he could not prove a negative.

In February 2004 the Institute of Medicine tried to settle the matter. It set Adequate Intakes of 3.7 liters of total water for men and 2.7 for women, counting every beverage and all food, and said most healthy people meet their needs by letting thirst guide them. Those totals now circulate as drinking targets.