It’s unclear exactly where the myth that humans need to drink eight glasses of water a day came from – but we’ve probably all heard it at some point in our lives.
The evidence for this claim has largely been debunked. Past studies relied on people recalling how much water they drank, which has low precision.
To provide a more accurate estimate of how much water we actually need, a new study recruited over 5,600 people of all ages from 26 countries around the world.
Researchers gave participants 100 milliliters of water enriched with 5 percent ‘doubly labeled water’.
Doubly labeled water is often used for metabolism experiments as it provides a way to track how rapidly chemicals are moving through the body.
This type of water contains unusual isotopes of hydrogen called deuterium. They have an extra neutron within their nucleus, making individual atoms twice as heavy as a normal hydrogen atom which has just one proton and no neutrons.
The resulting heavy water, which is 10 percent heavier than normal water, is safe to drink in small amounts.
To make it doubly labeled, this heavy water is also mixed with water containing an isotope of oxygen, Oxygen-18, which has 8 protons and 10 neutrons inside each atom (instead of the normal 8 of each). This is a stable, naturally occurring type of oxygen that makes up 0.2 percent of the air we breathe.
“If you measure the rate a person is eliminating those stable isotopes through their urine over the course of 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,” says Dale Schoeller, a nutritional 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 Science, the team shows that daily water intake varies greatly with 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 backed up by objective evidence,” the researchers write.