The Once Clear Water-Intake Theory Becomes Murky With New Research Finding

By: Yosef Scher  |  February 15, 2023

By Yosef Scher, Science and Technology Editor

It’s common knowledge that people must drink water to survive. Drinking water has numerous benefits, including stabilizing a person’s normal body temperature, lubricating joints, protecting tissues and vital organs, and expelling waste through sweating and urinating. For many people, one common concern is whether they are drinking enough water. For years, scientists have argued that a person should drink eight 8-ounce glasses of water over a day. However, a recent study published in Science, has proven that theory is not ideal for everyone.

By performing a large-scale study that accounted for various distinguishable variables such as age, location, climate, and body size, Yosuke Yamada and his team discovered that the amount of water an individual needs depends on many factors. Yamada, a physiologist at the National Institute of Biomedical Innovation, Health, and Nutrition in Japan, used deuterium, a stable isotope of water, to track where water flowed throughout a person’s body. While they would have ideally liked to see where all of a person’s water intake was traveling to, that was not a feasible task, as more than half of a person’s water intake comes from the food they consume. Yamada and his team of researchers analyzed the results of the water turnover, the replacement of body water lost in a given period of time, from over five thousand people living in twenty-three countries, ranging from eight days to ninety-six years old. They discovered that the “turnover of water in a person’s body varies widely depending on the individual’s physical and environmental factors.” While the obvious factors, such as the outside temperature and amount of shelter a person could obtain, did not surprise the researchers, the team was intrigued to find that “men ages 20 to 30 and women ages 20 to 55 had the highest water turnover.” Even within this range, the data revealed that water turnover varied significantly depending on “humidity, altitude, latitude, and physiological factors, such as whether a person was athletic.” Yamada and his team disproved the recommended eight 8-ounce glasses of water over a day theory by showing that some people may need more or less water, depending on the factors affecting their current situation. In their study, the “low end of water turnover averaged 1 to 1.5 liters a day and the high end averaged around 6 liters a day.” While water turnover does not determine the amount of water a person needs to drink, obtaining the proper body water turnover leads to improved body water homeostasis––a necessary network of physiological controls that maintain stable water levels in the body.

Interestingly, the researchers found that people who live in lower human development (LDI) countries, i.e., countries that do not have adequate access to resources, had a much higher water turnover. While that, unfortunately, may not seem unexpected, the researchers were surprised that this was still the case even after they “adjusted for climate, body size, sex and other factors.” Although there has not been a definitive reason posed to answer this anomaly, Yamada believes that “frequent use of indoor climate control in … wealthier countries” may help explain it.

With more than two billion people lacking access to safe drinking water and the number projected to grow in the future, Yamada is optimistic that his research “will help the people of the world fight against dehydration in the face of water shortages.”