It’s not exactly clear where the myth that humans should drink eight glasses of water a day came from, but we’ve probably all heard it.
The evidence of this claim has been found to be largely incorrect. Past studies have relied on people remembering how much water they drank during the day. These types of studies have little accuracy.
To get a more accurate estimate of how much water we need, a new study enrolled 5,600 people of all ages from 26 countries. The researchers gave the participants 100 ml of water enriched with 5% doubly labeled water.
Doubly labeled water is commonly used for metabolism experiments because it provides a way to follow the rate at which chemicals move through the body. This type of water contains an unusual isotope of hydrogen called deuterium (heavy hydrogen). This isotope has an extra neutron in its nucleus, which makes a heavy hydrogen atom twice as heavy as a normal hydrogen atom, which has only one proton and no neutrons. Drinking small amounts of the resulting water, which is 10% heavier than normal water, does not pose a health risk.
In order to produce doubly labeled water, this heavy water is also mixed with water that contains an isotope of oxygen, called oxygen 18, which has 8 protons and 10 neutrons in its nucleus (instead of the normal 8 of each). This isotope is one of the stable and natural isotopes of oxygen that makes up 0.2% of the air we breathe. “If you measure the amount of stable isotopes that a person excretes through urine over the course of a week, the hydrogen isotope can tell you how much water is being replaced, and how much oxygen isotope is being excreted,” says Dale Schuller, a nutritionist and one of the study’s authors. “It tells us how many calories they burn.”
The University of Wisconsin lab where Schuler works began testing doubly labeled water in humans in the 1980s.
In a recent study published in the journal Science, Tim Schuler’s team shows that daily water consumption varies greatly depending on age, gender, activity level, and weather. The researchers write in their study: “The current study clearly shows that the guidelines for drinking water are not the same for everyone, and the common suggestion that we should drink eight glasses of water (about two liters) per day is not supported by objective evidence.”
Water circulation was higher in men aged 20 to 30 and women aged 20 to 55, and it decreased after 40 years in men and after 65 years in women. Infants had the highest water turnover as a percentage of the total water in their body and about 28% of their body water was replaced daily.
In the same situation, men consume half a liter of water more than women. For example, a 20-year-old man who is not an athlete, weighs 70 kg and lives in a developed country at sea level with 50% humidity and an average temperature of 10 degrees Celsius, the water circulation is about 2.3 liters per day. A non-athletic woman of the same age who lives in the same area will have a water circulation of about 2.7 liters per day.
If the amount of energy that a person consumes in a day is doubled, the daily water circulation will increase by about one liter. For every 50 kg of extra weight, water circulation increases by 0.7 liters per day. A 50% increase in humidity increases water consumption by 0.3 liters.
Some people in the study had very high water circulation: 13 women who received more than 7 liters of water per day. These female athletes were either pregnant or in hot weather. 9 men consumed more than 10 liters of water per day, and they were very active people, athletes or indigenous people who had high activity. “This variation means it’s meaningless to point to an average,” Schuler says.
Water circulation increased for pregnant women in the third trimester of pregnancy and during breastfeeding. People living in temperature-controlled indoor environments in developed countries had lower water turnover than people living as workers or hunter-gatherers in developing countries.
The researchers write that due to the explosive growth of the population and the climate changes that the world is currently facing and that affect human access to drinking water, it is very important to prepare an accurate guideline of the amount of water that humans need in different situations.
This article in the magazine Science Released.
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