Do You Need to Drink Eight Glasses of Water a Day? It's Complicated

It's not clear 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.

There is little evidence for this claim. People recall how much water they drink, which has low accuracy.

A new study recruited over 5,600 people from 26 countries around the world to provide a more accurate estimate of how much water we actually need.

100 million liters of water was given to participants by researchers.

Accurately labeled water is often used for metabolism experiments as it gives a way to track how quickly chemicals move through the body.

Unusual amounts of hydrogen are found in this type of water. Individual atoms are twice as heavy as a normal hydrogen atom because they have an extra neutron in their nucleus.

It is safe to drink small amounts of the heavy water.

To make it doubly labeled, this heavy water is mixed with Oxygen-18, which has 8 protons and 10 neutrons inside each atom, which is different from the normal 8 of each atom. This type of oxygen makes up 0.2 percent of the air.

"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 oxygen isotope can tell us how many calories they are burning."

The University of Wisconsin–Madison lab pioneered the doubly labeled water experiment in humans in the 1980's.

The team found that the daily water intake varies greatly with age, gender, activity levels, and climate.

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 is not backed up by objective evidence.

After the age of 40 in men and after the age of 65 in women, water turnover decreased in men and increased in women.

Newborns have the highest turnover of water as a percentage of their body's water.

Men consume half a liter more water than women.

A 20-year-old man who is not athletic, weighs 70 kilograms and lives in a developed country with a mean air temperature of 10C will have a water turnover of around 3.2 liters per day.

A non athletic woman of the same age living in the same location will have a water turnover of over 2 liters per day.

The daily water turnover is increased by about a liter if you use twice as much energy as usual.

Water turnover goes up by 0.7 liters a day for every additional 50 kilograms of body weight.

The jump in humidity causes water use to go up.

13 women who got through over 7 liters per day were either athletes, pregnant women, or experiencing warm weather, and 9 men who consumed over 10 liters a day were also in the study.

Highly active people, athletes, and foragers were once again present.

Pointing to one average doesn't tell you much.

Water turnover went up for pregnant women in the third quarter.

People living a sedentary lifestyle in developed countries have lower water turnover than people working in developing countries.

Population growth and climate change will affect the availability of water for human consumption, which is why improved guidelines are important.

The paper was published in a scientific journal.