WASHINGTON (AFP) - - Drinking enough to quench your thirst is sufficient for the body's needs, and there is no evidence to support the common advice to drink eight glasses of water a day, according to a new study.

Doctors from the University of Pennsylvania said "there is simply a lack of evidence in general" that everyone should drink the recommended 1.5 litres of water a day.

Researchers Dan Negoianu and Stanley Goldfarb had examined the theory that drinking more water helped the kidneys flush more toxins from the body.

"So the question is does drinking more water increase these normal important functions of the kidneys. And the answer is no," Goldfarb told NPR radio.

"In fact, drinking large amounts of water, actually and surprisingly, tends to reduce the ability of the kidney to function as a filter."

Water has also been touted as an ideal aid to those who want to lose weight. But while drinking more helps dieters feel full, no clinical study has proved that it will help keep the weight off.

"There were some studies that suggested that in fact, calorie intake was reduced when individuals were given water prior to eating. Other studies suggested that it wasn't," Goldfarb added.

Increased water intake was only really justified in extreme cases, such as for athletes, people living in hot, dry climates and those suffering from particular illnesses.