Not a drop of evidence eight glasses of water is good for you, say experts
By DAVID DERBYSHIRE
The belief that we need eight glasses of water a day to stay healthy is a myth, researchers say.
There is no solid evidence that drinking plenty of pure water is good for the skin, wards off weight gain or helps rid the body of toxins.
Instead, most of us get all the fluids we need to avoid dehydration from food and other drinks, including tea and coffee.
The idea that those who fail to drink enough water suffer health problems became widespread in the 1990s.
Some health gurus and self-proclaimed nutritionists have insisted that the typical person needs to drink eight glasses, or two litres, of water every day.
Rather than just drink when they are thirsty, many now feel the need to keep a water bottle with them all day long.
Yet, according to a report published today, there is no evidence of any benefits from drinking so much.
Dr Dan Negoianu and Dr Stanley Goldfarb from the University of Pennsylvania in Philadelphia reviewed every published clinical study into the subject.
They found evidence that those in hot, dry climates have an increased need for water, as do athletes.
Patients with some diseases also benefit from additional fluid.
"But no such data exist for average, healthy individuals," said a spokesman for the scientists, who are based at the university's renal, electrolyte and hypertension division.
No study indicated any real need to drink the recommended eight glasses of water each day.
"Indeed, it is unclear where this recommendation came from," the spokesman added.
Some studies have shown that water intake affects the rate that kidneys clear salt and urea from the body.
But they failed to show any health benefits, the researchers said.
Other studies showed that water retention in the body varied hugely - and depended on the speed with which water is swallowed.
If gulped quickly, water leaves the body more quickly.
If it is sipped, it is more likely to be retained. But no studies showed any benefit to organs from increased water intake.
The scientists also looked at the theory that drinking more water makes people feel full and helps them lose weight.
Again, the studies were inconclusive.
And while water has been touted as an elixir for improved skin tone, no studies showed any clinical benefit as a result of drinking more.
The scientists concluded there was no clear evidence that drinking more water was healthy.
"There is simply a lack of evidence in general," they reported in the Journal of the American Society of Nephrology.
One source for the eight glasses claim is a 1945 study which concluded that most people need two litres of fluids each day.
However, this came from other drinks and food.
Ursula Arens, a spokesman for the British Dietetic Association, said the typical man needed around 2.5 litres of fluid a day while a woman needed around two litres.
"Most people can rely on their thirst to tell them when they need to drink, although as we get older perception of thirst is not so good," she said.
"Tea and the sort of coffee you get in Britain are fine for rehydrating - the diuretic effects of caffeine are massively outweighed by the fluids in those drinks. "There are millions of people who get most of their fluids from tea without any ill effects."