Trying to do the "right" thing by drinking eight full glasses of water a day may do little more than make a person run to the bathroom, a researcher said on Friday.
Newspapers and health and beauty magazines all advise drinking at least 8 full glasses of water a day totaling 64 ounces for optimal health — an approach called "8x8" by proponents.
But Dr. Heinz Valtin of Dartmouth Medical School in New Hampshire said there is no scientific evidence to back up this advice, which has helped create a huge market for bottled water.
"After 10 months of careful searching I have found no scientific evidence that supports '8x8'," Valtin, who has written textbooks on the subject of human water balance, said in a telephone interview.
Writing in the American Journal of Physiology, Valtin, a kidney specialist, said people forget that the food they eat also contains some water.
The Food and Nutrition Board of the National Research Council has recommended that people take in about one milliliter of water for each calorie of food eaten.
This adds up to two liters, or 74 fluid ounces on an average 2,000-calorie diet. But the National Research Council also noted that much of this is already contained in food.
"I did 43 years of research on that system — the osmoregulatory system. That system is so precise and so fast that I find it impossible to beleve that evolution left us with a chronic water deficit," Valtin said.
If a person gets low on fluid, the body compensates by bringing fluid back out of the kidneys and by slowing the loss of water through the skin, Valtin said. Thirst kicks in long before dehydration starts, he added.
"It does it very quickly and very accurately and it does so in minutes," Valtin said.
He said he and colleagues became concerned after seeing dozens of newspaper and magazine articles urging people to sip water all day. "I started talking to my colleagues and asking them 'Do you know of any evidence for this?'. Invariably, they said, 'No I think it's a myth'," Valtin said.
The journal asked him to review all the scientific studies he could find and he concluded that someone misinformed has been telling people to drink large amounts of water when most do not need to.
"I am referring to healthy adults in a temperate climate leading a largely sedentary existence," Valtin said. "Persons with certain diseases must have large volumes of water — kidney stones are probably the most common example."
The rest can just drink enough to slake thirst — and this includes coffee, tea, and even beer — despite their diuretic effects, Valtin said.
He hopes people will be relieved of the guilt of not getting enough water, and of the expense of buying bottled water to drink throughout the day.
"There is also the possibility that if you drink a lot of water that happens to be polluted then of course you get more pollutants," Valtin said.
"Then there is the inconvenience of constant urination, the embarrassment of having to go to the bathroom all the time," he added.
And overdoses of water can cause water intoxication that can lead to confusion and even death. Water intoxication is one deadly effect of taking the drug Ecstasy, for instance, because it makes people thirsty beyond their physical needs.