One minute story, a glass of water a day (Walla system)
The recommendation to drink eight glasses of water a day is one of the most well-known nutritional recommendations and it even appears in the recommendations of government websites.
But a new study, the most stringent yet on water, reveals that people consume a wide range of amounts of water.
Some of them need only 1.5 to 1.8 liters a day, less than the two liters usually recommended, the study shows.
"The current recommendation is not scientifically supported at all," said Yusuke Yamada of the National Institute of Biomedical Innovation, Health and Nutrition in Japan, and one of the paper's first authors.
"In fact, most scientists are not sure where this recommendation came from."
One problem is that previous estimates of water requirements have tended to ignore the water content of food, which can contribute a significant portion of our total intake.
"If you only eat bread, sausage and eggs you won't get much water from food, but if you eat meat, vegetables, fish, pasta and rice you can get about 50 percent of your water needs from food," said Yamada.
High water content.
Salad (Photo: ShutterStock)
The study, published in the journal Science, assessed the water intake of 5,604 people aged eight days to 96 from 23 countries.
The study involved people drinking a glass of water in which some of the hydrogen atoms were replaced with deuterium, or heavy hydrogen, which is a stable (non-radioactive) isotope of hydrogen found naturally in the human body and is harmless.
The additional deuterium removal rate reveals how quickly water in the body evaporates, and the study found that the measure varied greatly depending on a person's age, sex, activity levels and environment.
Doubling the energy a person uses pushes their expected cycle up by about a liter, or four glasses.
Fifty kilograms more body weight adds 0.7 liters a day.
A 50 percent increase in humidity increases water consumption by 0.3 liters.
When it comes to sports, athletes use about a liter more.
An average 20-year-old man of normal weight living in a temperate climate would pump and lose about 3.2 liters daily.
For women, that number is closer to 2.7.
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Energy expenditure is the largest factor in water turnover, with the highest values observed in men aged 20 to 35, with an average of 4.2 liters per day.
This decreased with age, averaging 2.5 liters per day for men in their 90s.
Women aged 20 to 40 had an average cycle of 3.3 liters, which dropped to 2.5 liters by age 90. Newborn babies cycled the largest proportion, replacing about 28 percent of their body water each day.
"This research shows that the common suggestion that we should all drink eight glasses of water - or around two liters a day - is probably too high for most people in most situations, and a 'one-size-fits-all' policy for water consumption is not supported by these data," said Prof. Van Speakman of the University of Aberdeen, co-author of the study, "I think this is a recommendation that many people just ignore and follow what their body is telling them," he said.
Although drinking more water than the body requires is not expected to harm health, clean drinking water is not free to produce, the authors note.
"There is a price for drinking more than we need, even if it's not a health cost," said Speakman, "If 40 million adults in the UK followed the guidelines and they drank half a liter of clean water more than they should every day, that's 20 million liters of water wasted every a day."
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