It has become a deeply entrenched belief among the public - drink at least eight glasses of water a day to maintain health and wellbeing.
Bottled water companies often repeat the claim to boost their sales but it is bunkum. There is no evidence that drinking eight glasses of water a day improves skin tone, aids dieting or prevents headaches (except those induced by hangovers), scientists say.
US researchers who reviewed the evidence concluded most people do not need to worry about their water consumption, as they will be getting plenty of fluid in other ways – from tea, coffee and other drinks and from the food they eat.
The misunderstanding is believed to have arisen from a 1945 recommendation that adults should consume 2.5 litres of water daily, one millilitre for every calorie consumed, which was highlighted by the British Medical Journal in December. The crucial part of the recommendation, however – "most of this quantity is contained in prepared foods" – is usually ignored.
That error is compounded by the belief that caffeinated drinks such as tea, coffee and cola, and alcohol do not count towards the total. But that, too, is baseless. Tea and coffee and weak alcoholic drinks such as beer can contribute to the daily total, in moderation, despite being mildly diuretic (stimulating the production of urine).
Less well known are the dangers of drinking too much water, which can be dangerous, resulting in water intoxication, hyponatraemia (low salt levels) and even death. The obsession with maintaining hydration has led to an increasing rate of collapse among distance runners in the last decade who drink so much they suffer water intoxication.
The review of research by Dan Negoianu and Stanley Goldfarb, from the University of Pennsylvania, in Philadelphia, found not a single study included the recommendation to drink eight 8oz glasses of water a day.
Although one small study suggested that drinking water could result in fewer headaches, the results were not statistically significant.
The benefits of drinking a pint of water before bed after a night's drinking to ward off a hangover are, separately, well established.
No studies showedany benefit to skin tone as a result of increased water intake. Dehydration can make skin less plump, but there was no solid evidence to back up the claim water helps people maintain a youthful appearance.
The authors also found no evidence that drinking lots of water benefits the body's organs.