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 been debunked in large part. Past studies relied on people recalling how much water they drank, which had 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 in 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 in 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