The idea we need eight glasses of water a day is nonsense, according to new research.
It turns out humans can be perfectly healthy on less than half as much, and requirements vary on an individual and national scale, say nutritionists.
The study is based on more than 5,600 people from 26 countries - ranging between eight days old 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.
Newborns turn over the largest proportion – replacing about 28 per cent of water in their bodies daily.
Lead author Professor Dale Schoeller of the University of Wisconsin-Madison explained: "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.
"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 requirements differ by about two glasses - or half a litre, 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 litres every day - and a female peer 2.7.
Doubling the energy a person uses will increase their expected turnover by about a litre - or four glasses.
Fifty kilograms (7.9 stone) more body weight adds 0.7 litres daily. A 50 per cent increase in humidity pushes uses up by 0.3 litres. And athletes use about a litre more.
Prof Schoeller, who has been studying water and metabolism for decades, said: "There are outliers, too, that are turning over as much as 10 litres a day. The variation means pointing to one average doesn’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 school children are advised to bring water into their lessons. Few office meetings commence without a giant jug 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 litres.
Previous studies relied mainly on volunteers to recall and self-report water and food consumption or involved small focus groups - such as soldiers working in the desert.
Prof Schoeller and colleagues analysed the time it took to move through participants' bodies by following the turnover of 'labelled water.' Subjects drank a measured amount containing trackable hydrogen and oxygen isotopes - distinguishable chemical atoms.
Prof 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 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 measure 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.
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 industrialised economies. Overall, the lower your home country’s Human Development Index, the more water you go through daily.
Prof Schoeller said: "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.
"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.
Prof Schoeller said: "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. 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.
Co-author Dr Yosuke Yamada, section head of the National Institute of Biomedical Innovation, Health and Nutrition in Japan, said: "Determining how much water humans consume is of increasing importance because of population growth and growing climate change.
"Because water turnover is related to other important indicators of health, like physical activity and body fat per cent, it has potential as a biomarker for metabolic health."
Credit: Mark Waghorn, SWNS.
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