Two litres of water a day tip a myth: scientists

Drinking two litres of water a day to maintain one’s health is a myth, and people may need up to six litres daily overall, depending on job, gender and environment, scientists say.

In recent decades, the need to drink eight glasses of water a day has become standard advice, yet there is little evidence to back it up.

U.S. researchers studied thousands of people in 26 countries to find out how much water they needed and discovered it varied greatly. They found that daily averages ranged from one litre per day to six litres, including water derived from other drinks and water in food.

“The science has never supported eight glasses as an appropriate guideline, if only because it confused total water turnover with water from beverages, and a lot of your water comes from the food you eat,” said Dale Schoeller, a professor emeritus of nutritional sciences at the University of Wisconsin-madison. “This work is the best we’ve done so far to measure how much water people actually consume on a daily basis — the turnover of water into and out of the body — and the major factors that drive water turnover.”

THE SCIENCE HAS NEVER SUPPORTED 8 GLASSES AS ... GUIDELINE.

Previous studies had asked people to self-report water intake, but this study measured water as it moved through the body. Participants drank water containing hydrogen and oxygen isotopes that could be tracked through their bodies. The results, published in the journal Science, found the amount they needed depended on temperature, gender and levels of activity.

An 154-pound man aged 20, who lived at sea level in a developed country where the mean air temperature was 50 F, and whose physical activity was average, needed around 3.2 litres per day. A 126-pound woman the same age, in the same area, whose activity level was average, needed 2.7 litres. An 112-pound person needed 2.5 litres a day, while someone weighing 210 pounds needed five litres.

When people doubled their daily energy expenditure, they required an extra litre, the researchers found, while a 50 per cent increase in humidity meant they needed to drink a further 0.3 litres a day to stay hydrated.

The advice to drink eight glasses of water a day appears to originate from Dr. Fredrick J Stare, an American nutritionist, who in 1974 suggested six to eight glasses, although he said that could include other drinks and food.

Drinking too much water can dilute the sodium content of blood and trigger hyponatremia, a potentially life-threatening condition.