How much water should you drink in a day? It’s complicated.

Are you drinking enough water?

The question seems like it should have a straightforward answer — a specific amount of water you need to drink daily to combat dehydration.     

But the rate and way in which the human body takes in and excretes water is not as universal as you might expect. By studying more than 5,000 people living in 23 countries and ranging in age from 8 days to 96 years, researchers have found that the turnover of water in a person’s body varies widely depending on the individual’s physical and environmental factors.

The results, published in the Nov. 24 Science, suggest that the idea that a person should ideally consume eight 8-ounce glasses of water a day is not a one-size-fits-all solution to peak hydration.    

Even within the calculations, “individual variabilities could be huge,” says biomedical engineer Kong Chen, director of the metabolic research program at the National Institutes of Health’s Clinical Center.

Yosuke Yamada, a physiologist at the National Institute of Biomedical Innovation, Health and Nutrition in Japan, and colleagues used a stable isotope of hydrogen known as deuterium to track the movement of water through people’s bodies. Drinking water accounts for only half of the total water intake by humans, with the rest coming from food. Simply measuring the amount of water that a person drinks in a day is not enough to accurately gauge water turnover or the amount of water used by the body daily.

The researchers found that men ages 20 to 30 and women ages 20 to 55 had the highest water turnover. These numbers varied significantly depending on humidity, altitude, latitude and physiological factors, such as whether a person was athletic. For men and women, the low end of water turnover averaged around 1 to 1.5 liters a day and the high end averaged around 6 liters a day. 

But the findings laid out in the study are not a road map for how much water individuals in certain populations should drink daily, Chen says. “There’s still a lot of complex relationships that need to be teased out,” he says. Instead, the data raise more questions about the effects of particular environments on an individual’s water turnover.      

“The most unexpected finding is that people who are living in poor countries … , or lower human development index countries, have a higher water turnover,” Yamada says. Even when the researchers adjusted for climate, body size, sex and other factors, people living in low-HDI countries — which, for this study, included Ghana, Kenya, Nigeria and Tanzania — still had higher water turnover rates than those in high-HDI countries, including Belgium, Japan and the United States. The disparity may be due to the frequent use of indoor climate control in the wealthier countries, the researchers suggest.

The water turnover rate could also represent a significant marker of an individual’s metabolic health. Ten percent of a person’s total water body water is lost daily to the metabolic processes occurring in our cells. For people with less access to safe drinking water, this loss can also be a “huge issue,” Yamada says.

More than 2 billion people in the world don’t have access to safe drinking water and that number is projected to grow, according to a 2018 United Nations report (SN: 8/16/18). Hopefully the research will help the people of the world fight against dehydration in the face of water shortages, Yamada says.