Do we really need to drink 8 glasses of water a day?

By Mark Waghorn via SWNS

The idea we need eight glasses of water a day is nonsense, according to new research.

Humans can be perfectly healthy on less than half as much. Requirements vary on an individual and national scale, nutritionists say.

The study is based on more than 5,600 people from 26 countries - ranging between eight days to 96 years old.

It found water needs peak for men in their 20s - while they remain the same for women from the age of 20 to 55.

But newborns turn over the largest proportion - replacing about 28 percent of water in their bodies every day.

"The science has never supported the old eight glasses thing 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 lead author Professor Dale Schoeller, of the University of Wisconsin-Madison.

"But 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."

Men's and women's requirement differ by about two glasses - or half a liter, say the international team.

The average 20 year old man of normal weight living in a temperate climate like the UK
would take in and lose about 3.2 liters every day - and a female peer 2.7.

Doubling the energy a person uses will push their expected turnover up by about a liter - or four glasses.

Fifty kilograms (110 pounds) more body weight adds 0.7 liters a day. A 50 percent increase in humidity pushes use up by 0.3 liters. And athletes use about a liter more.

Schoeller, who has been studying water and metabolism for decades, said: "There are outliers, too, that are turning over as much as 10 liters a day.

"The variation means pointing to one average doesn't’'t tell you much. The database we’ve put together shows us the big things that correlate with differences in water turnover."

Commuters are encouraged to take bottles onto the London Underground and schoolchildren advised to bring water into their lessons. Few office meetings commence without a giant jug sitting in the middle of the desk.

Fuelling this appetite is the '8x8 rule' - the unofficial recommendation we drink eight 240ml glasses a day, totalling just under two litres - on top of any other drinks.

Now the most comprehensive study of its kind has revealed consumption varies wildly around the world - from daily averages of one to six liters.

Previous studies relied largely on volunteers to recall and self report water and food consumption or involved small focus groups - such as soldiers working in the desert.

Schoeller and colleagues analyzed the time it took to move through the bodies of participants by following turnover of labelled water.

Subjects drank a measured amount containing trackable hydrogen and oxygen isotopes - distinguishable chemical atoms.

Schoeller, whose lab invented the method in the 1980s, said: "If you measure the rate a person is eliminating those stable isotopes through their urine over the course of a week, the hydrogen isotope can tell you how much water they're replacing and the elimination of the oxygen isotope can tell us how many calories they are burning."

His team, including colleagues at the University of Aberdeen, collected and analysed the data.

They compared environmental factors such as temperature, humidity and altitude of volunteers' hometowns to measured water turnover, energy expenditure, body mass, sex, age and fitness.

They also incorporated the United Nations' Human Development Index, a composite measure of a country that combines life expectancy, schooling and affluence levels.

Water volume peaked for men during their 20s, while women held a plateau from 20 through 55 years of age. Newborns, however, turned over the largest proportion daily, replacing about 28 percent of the water in their bodies every day.

Physical activity level and athletic status explained the largest proportion of the differences in water turnover, followed by sex, the Human Development Index, and age.

The researchers found hunter-gatherers and farmers in developing nations had higher water turnover than those in industrialzsed economies.

In all, the lower your home country’s Human Development Index, the more water you go through in a day.

"That's representing the combination of several factors. Those people in low HDI countries are more likely to live in areas with higher average temperatures, more likely to be performing physical labour, and less likely to be inside in a climate controlled building during the day," Schoeller said.

"That, plus being less likely to have access to a sip of clean water whenever they need it, makes their water turnover higher."

The results in the journal Science have implications for global warming. It is hoped they will improve our ability to predict more specific and accurate future water needs - especially in dire circumstances.

"Look at what is going on in Florida right now, or in Mississippi - where entire regions have been exposed by a calamity to water shortages," Schoeller said.

"The better we understand how much they need, the better prepared we are to respond in an emergency."

It also opens the door to being better prepared for long-term needs - and wary of short term health concerns.

"Determining how much water humans consume is of increasing importance because of population growth and growing climate change," said co-author Dr. Yosuke Yamada, section head of the National Institute of Biomedical Innovation, Health and Nutrition in Japan.

"Because water turnover is related to other important indicators of health, like physical activity and body fat percent, it has potential as a biomarker for metabolic health."