Do you need to drink eight glasses of water a day? It’s complicated: ScienceAlert

This is not clear 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 proof of this assertion has largely debunked. Previous studies have relied on people reminder 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 5% fortified water’double labeled water‘.

Double-labeled water is often used for metabolism experiments because it provides a way to track 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 safe to drink 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 normal 8 of each). It’s a stable, of natural origin 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 burn” said Dale Schoeller, a nutrition scientist who co-authored the study.

The University of Wisconsin-Madison lab, where Schoeller works, first pioneer doubly marked water experience in humans in the 1980s.

In their recent studyPosted 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 said. 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 water in their bodies – replacing approximately 28% every 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 of about 3.2 liters per day.

A non-athletic woman of the same age living in the same location 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 rotation: 13 women consuming more than 7 liters per day, whether sporty, pregnant or experiencing hot temperatures, and 9 men consuming 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,” said 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.

“Improving guidelines is of increasing importance due to the explosive growth of the population 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.