Water consumption is closely related to human health. You may have heard the recommendation that people should drink eight glasses of water a day. But is that really true for everyone? A new study has suggested that individual water needs vary tremendously. So much, in fact, that there is no universal recommendation for how much water should be consumed per day. The report has been published in Science.
In this study, the researchers aimed to find out how much water people were turning over every day - how much water they took in and how much they put out, and what was influencing that turnover rate, explained study co-author and emeritus professor Dale Schoeller of the University of Wisconsin–Madison who has studied water metabolism for decades.
“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,” said Schoeller. Previous studies have relied on self-reports or unusual groups, such as soldiers toiling in the desert, which would hardly be an accurate reflection of how much water most people might need on an average day.
This study involved over 90 researchers who measured water turnover in over 5,600 people between the ages of 8 days and 96 years in 26 countries. Data about age, body mass, fitness, and other characteristics were considered. Environmental conditions were also recorded, such as temperature and humidity, as well as sociological factors like the economy or schooling rate of nations - measure by the UN Human Development Index. The daily averages ranged from one to six liters per day. Some people were outliers too, added Schoeller, so "one average doesn’t tell you much."
The researchers did reveal the factors that correlate with water turnover, however. In this work, participants drank water containing labeled hydrogen and oxygen isotopes, so the researchers could track the fluid as it moved through bodies.
This effort revealed that water turnover peaks in men in their early 20s, and in women, turnover plateaus from about the age of 20 to 55. Newborns have the highest rate of turnover, replacing about 28 percent of the water in their bodies daily. The biggest differences from one individual to another were caused, unsurprisingly, by how athletic the people were. Sex was the next biggest factor, then the development rating of the country, followed by age.
Water turnover in men and women differs by about half a liter. An average, 20-year-old male non-athlete weighing 70 kilograms (154 pounds), and living in a well-developed country with a mean temperature of 10 degrees C (50 Fahrenheit), humidity of 50 percent, at sea level, takes in and loses roughly 3.2 liters (108 ounces) of water per day. A woman in the same place who weighs 60 kg (132 pounds) goes through about 2.7 liters (91 ounces).
Athletes, or anyone who doubles their daily energy expenditure needs about one liter more per day. If they weigh an additional 50 kilograms, it also adds 0.7 liters to the daily turnover. Weather has an influence too; if humidity increases by 50 percent, an additional 0.3 liters is replaced.
The study also suggested that people living in less developed conditions, like hunter-gatherers or subsistence agriculturalists had higher water turnover than people in industrialized economies. As a country’s Human Development Index drops, the water needs of their citizens increases. That may be because these people are more likely to work as laborers, and live in hot climates without climate controlled buildings, noted Schoeller.
This research may help society prevent problems in the future by accurately predicting water needs now. “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. The better we understand how much they need, the better prepared we are to respond in an emergency,” Schoeller added.
Sources: University of Wisconsin-Madison, Science