The commonly held belief that eight, 8-ounce glasses of water per day are enough to meet a person’s daily needs has been disproved by a new study involving thousands of participants. The results show that people’s water consumption varies widely around the world and across their lifetimes.

According to co-author Dale Schoeller, a longtime researcher of water and metabolism at the University of Wisconsin-Madison, “the science has never supported the old eight glasses thing as an appropriate guideline if only because it confused total water turnover with water from beverages and a lot of your water comes from the food you eat.” 

“But this work is the best we’ve done so far to measure how much water people actually consume on a daily basis — the turnover of water into and out of the body — and the major factors that drive water turnover,” the author adds.

That doesn’t mean that the new results set a new rule. More than 5,600 individuals from 26 nations, ranging in age from 8 days to 96 years old, had their water turnover assessed for the research, which was published today in the journal Science. The results showed that the average water turnover ranged from 1 to 6 liters on a daily basis.

“There are outliers, too, that are turning over as much as 10 liters a day,” adds the author. “The variation means pointing to one average doesn’t tell you much. The database we’ve put together shows us the big things that correlate with differences in water turnover.”

Previous studies of water turnover relied heavily on volunteers recalling and self-reporting their water and food consumption, or they were focused observations, such as a small group of young, male soldiers working outside in desert conditions, of questionable use as representative of the majority of people.

By monitoring the turnover of “labeled water,” the new study was able to quantify how long it took water to pass through the bodies of study participants. The participants drank standardized amounts of water containing radioactive isotopes of hydrogen and oxygen.

Isotopes are atoms of the same element with slightly varying atomic weights, allowing them to be distinguished from other atoms of the same element in a sample.

“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 elimination of the oxygen isotope can tell us how many calories they are burning,” explains Schoeller.

The study, which included more than 90 researchers, was led by a team that included John Speakman, a professor of zoology at the University of Aberdeen in Scotland, and Yosuke Yamada, a former postdoctoral researcher in Schoeller’s lab at the University of Wisconsin-Madison and current section head of the National Institute of Biomedical Innovation, Health and Nutrition in Japan. They collected and reviewed participant data, comparing measured water turnover, energy expenditure, body mass, sex, age, and athletic status to environmental characteristics including temperature, humidity, and altitude of the participants’ hometowns.

The team also took into account the Human Development Index (HDI), a composite indicator of a nation that incorporates economic, educational, and life expectancy data.

In the study, men’s water turnover volume peaked in their 20s, but women’s water turnover volume remained steady from 20 to 55 years of age. However, newborns had the highest rate of daily water replacement, restoring nearly 28% of their body weight each day.

Differences in water turnover were first explained by gender, then by the Human Development Index and age, and last by one’s degree of physical activity and athleticism.

If everything else is the same, men and women have a difference of about 0.5 liters of water turnover. As a baseline, the study’s findings suggest that a non-athlete male of average physical activity who is 20 years old, weighs 70 kilograms (154 pounds), lives at sea level in a developed country with an average air temperature of 10 degrees Celsius (50 degrees Fahrenheit), and a relative humidity of 50% would consume and lose approximately 3.2 liters of water per day. In the same environment, a 60 kg (132 lb) lady of the same age and activity level would drink 2.7 liters (91 ounces) of water each day.

If a person’s energy consumption doubles, their water turnover is projected to increase by around a liter per day. An extra 50 kg adds 0.7 l/d. Water usage increases by 0.3 liters for every 50% rise in humidity. Athletes drink about a liter more than people who don’t do sports.

The researchers found that “hunter-gatherers, mixed farmers, and subsistence agriculturalists” all used more water than people who live in industrialized economies. Overall, daily water use rises in direct proportion to a country’s HDI ranking.

“That’s representing the combination of several factors,” Schoeller adds. “Those people in low HDI countries are more likely to live in areas with higher average temperatures, more likely to be performing physical labor, and less likely to be inside in a climate-controlled building during the day. That, plus being less likely to have access to a sip of clean water whenever they need it, makes their water turnover higher.”

According to Schoeller, the data will allow for more precise forecasting of water requirements in the future, which will be particularly helpful in emergency situations.

“Look at what’s going on in Florida right now, or in Mississippi — where entire regions have been exposed by a calamity to water shortages,” he adds. “The better we understand how much they need, the better prepared we are to respond in an emergency.”

The experts think this will help us better plan for the future and pick up on any immediate health issues.

“Determining how much water humans consume is of increasing importance because of population growth and growing climate change,” adds Yamada. “Because water turnover is related to other important indicators of health, like physical activity and body fat percent, it has potential as a biomarker for metabolic health.”

Source: 10.1126/science.abm8668

Image Credit: Joshua Roberts/Getty Images