It’s unclear exactly where the myth that humans need to drink eight glasses of water a day comes from – but we’ve probably all heard it at some point in our lives.
The evidence for this claim has been largely debunked. Previous studies relied on people remembering how much water they drank, which has low accuracy.
To provide a more accurate estimate of how much water we really need, a new study recruited more than 5,600 people of all ages from 26 countries around the world.
Researchers gave participants 100 milliliters of water fortified with 5% “double-labeled water.”
Double-labeled water is often used for metabolism experiments because it tracks the rate at which chemicals move through the body.
This type of water contains unusual isotopes of hydrogen called deuterium. They have an extra neutron in their nucleus, which makes the individual atoms twice as heavy as a normal hydrogen atom which has only one proton and no neutrons.
The resulting heavy water, which is 10% heavier than normal water, is drinkable in small quantities.
To make it doubly labeled, this heavy water is also mixed with water containing an oxygen isotope, Oxygen-18, which has 8 protons and 10 neutrons inside each atom (instead of the normal 8 of each). It is a stable, natural type of oxygen that makes up 0.2% of the air we breathe.
“If you measure the rate at which a person eliminates these stable isotopes through their urine over the course of a week, the hydrogen isotope can tell you how much water it replaces, and the elimination of the isotope of oxygen can tell us how many calories they’re burning,” says Dale Schoeller, a nutrition scientist who co-authored the study.
The University of Wisconsin-Madison lab, where Schoeller works, first pioneered the doubly labeled water experiment in humans in the 1980s.
In their recent study, published in Sciencethe team shows that daily water intake varies widely by age, gender, activity levels and climate.
“The current study clearly indicates that one size does not fit all for drinking water guidelines, and the common suggestion that we should drink eight 8-ounce glasses of water per day (~2 liters) is not supported by objective evidence,” the researchers write.
Water turnover was highest in men aged 20-30 and women aged 20-55 and decreased after age 40 in men and after age 65 in women.
Newborns had the highest water turnover as a percentage of all their body water – replacing about 28% each day.
Under similar conditions, men consume about half a liter more water each day than women.
For example, a 20-year-old man who is not athletic, weighs 70 kg, and lives in a developed country at sea level with 50% humidity and an average air temperature of 10°C will have a renewal water about 3.2 liters. per day.
A non-athletic woman of the same age living in the same place will have a water turnover of approximately 2.7 liters per day.
Using twice as much energy per day increases daily water turnover by approximately one litre.
For every additional 50 kilograms of body weight, water turnover increases by 0.7 liters per day.
A 50% jump in humidity increases water consumption by 0.3 litres.
Some people in the study had extremely high water turnover: 13 women consuming more than 7 liters of water per day, they were either athletes, pregnant women or experiencing a hot climate, and nine men who consumed more than 10 liters per day.
Again, these were very active people, athletes or foragers from the Ecuadorian Amazon.
“Variation means pointing to an average doesn’t tell you much,” says Schoeller.
Water turnover increased in pregnant women in the third trimester of pregnancy and during lactation.
People living a sedentary lifestyle in temperature-controlled indoor environments in developed countries had lower water turnover than people working as laborers or hunter-gatherers in developing countries.
“Improved guidelines are of increasing importance due to the explosive population growth and climate change the world is currently facing, which will affect the availability of water for human consumption,” the researchers write.
This article was published in Science.