Drinking two liters of water a day to stay healthy and hydrated is a myth, with people needing up to six liters depending on their job, climate and gender, researchers have found.
In recent decades, the need to drink eight glasses of water a day has become standard advice, but there is little evidence to support it.
Researchers at the University of Wisconsin-Madison in the US studied thousands of people from 26 countries to find out how much water they needed, and discovered that it varied widely.
They found that daily averages ranged from as little as one liter per day to six liters, which included water from other drinks such as tea and coffee, and also water in food.
“Science has never supported the old 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 Professor Dale Schoeller, Emeritus Professor of Nutritional Sciences.
“But this work is the best we’ve done so far to measure how much water people actually use on a daily basis – the turnover of water in and out of the body – and the key factors that drive water turnover.”
Unlike previous studies that had asked people to self-report water intake, researchers measured water as it moved through the body.
The participants drank special “traceable” water containing hydrogen and oxygen isotopes, so the researchers could tell when it had passed through.
They found large differences depending on temperature, gender and levels of physical activity.
Weight gain and exercise are an important factor
For example, a 20-year-old man weighing 11 stone living at sea level in a developed country such as the UK where the average air temperature was 10C (50F), and doing average physical activity, would need around 3.2 liters per day.
A nine stone woman of the same age and activity level, living in the same area, only needs 2.7 litres.
When people doubled their energy consumption in a day, they needed an extra liter, the researchers found, while a 50 percent increase in humidity required an extra 0.3 liters a day.
Weight gain was also a major factor, with the average eight stone person using around 2.5 liters a day while the average 15 stone person used five litres.
It was found that athletes used about a liter more than non-athletes. One male athlete in the study was found to use 10 liters of water a day, even though experts admitted he was an outlier.
The researchers found that hunter-gatherers, mixed farmers and agriculturalists had higher water needs than people living in industrialized economies.
“Those people in countries with a low human development index are more likely to live in areas with higher average temperatures, more likely to do physical work and less likely to be inside a climate-controlled building during the day,” Professor Schoeller added.
“That, plus being less likely to have access to a sip of clean water when they need it, makes water turnover higher.”
The origin of eight glasses a day
The rule of eight glasses a day seems to originate from Dr Fredrick J Stare, the American nutritionist who in 1974 proposed a figure of six to eight glasses. He said this could include coffee, tea, milk, soft drinks and beer along with the water content of fruit and vegetables.
Most nutritionists now accept that the body controls water levels well, with the body urinating out what it does not need and triggering thirst when it requires replenishment.
Experts argue that encouraging people to drink more water than their bodies require is the same as deliberately breathing more, simply because oxygen sustains life.
Drinking too much water can be dangerous. If the kidneys can’t get rid of the excess, it dilutes the sodium content of the blood, triggering a condition called hyponatremia, which can be life-threatening.
The University of Wisconsin-Madison’s research was published in the journal Science.