Lucy Devine
Scientists have revealed why drinking eight glasses of water each day might not be the correct advice after all.
Researchers at the University of Aberdeen collaborated with other scientists to survey 5,604 people from across 23 different countries, aged between eight days old and 96, to understand how much water people really need to be drinking.
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The research saw participants drink a glass of water in which some hydrogen molecules were replaced by a stable isotope of the element, deuterium, allowing scientists to understand how quickly water in the body turns over.
Those with a high turnover often drink - and require - more water, for example those living in hotter countries and pregnant and breastfeeding women.
Men aged between 20-35 turned over an average of 4.2 litres per day, which steadily decreases with age.
Women meanwhile, aged 20-40, turned over 3.3 litres, which also declines as they get older.
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The findings - published in Science this week - revealed that eight glasses of water might be more than what people actually need, especially as around half of our water intake comes from food.
"The original estimate of two litres a day comes from a slight miscalculation," Professor John Speakman from the University of Aberdeen told BBC Radio's Good Morning Scotland.
"The water that we'd need to drink is the difference between the total water that we need to ingest and the amount that we get from our food.
"The way they estimated the amount from food was by asking people how much they eat.
"Because people under-report how much they eat, there's a misestimate and so you overestimate the amount of water that's needed."
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Prof Speakman added that high turnover rate does not mean people must drink the same amount of water.
"Even if a male in his 20s has a water turnover of 4.2 litres per day, he does not need to drink 4.2 litres of water each day," he added.
"About 15 per cent of this value reflects surface water exchange and water produced from metabolism.
"The actual required water intake is about 3.6 litres per day. Since most foods also contain water, a substantial amount of water is provided just by eating.
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"This study shows that the common suggestion that we should all be drinking eight glasses of water is probably too high for most people in most situations and a 'one-size-fits-all policy' for water intake is not supported by this data."
It's estimated that people only need around 1.5 to 1.8 litres per day.