Do you really need 8 cups of water a day? UW study weighs in – Madison.com

It depends on what food and other beverages you consume, along with factors such as your age, sex, size, physical activity and climate, a study involving UW-Madison researchers says.

The study analyzed 5,600 people in 26 counties, looking at their water turnover, or the amount they took in and lost each day. Subjects drank water labeled with hydrogen and oxygen isotopes, a method first used in people at UW-Madison in the 1980s, allowing scientists to track water replacement and calories burned.

People generally turned over one to six liters of water per day, the researchers reported recently in the journal Science. That’s about four to 25 cups.

The often-cited advice to drink eight cups of water a day stems from a 1945 recommendation from the Food and Nutrition Board of the National Research Council. It said adults should consume 64 ounces, or eight cups, of water a day. But that includes water in food and other beverages, not just from drinking plain water.

Many fruits and vegetables and other foods contain water, and most people drink other beverages. “The science has never supported the old eight glasses thing as an appropriate guideline, if only because … a lot of your water comes from the food you eat,” Dale Schoeller, a retired UW-Madison professor of nutritional sciences involved in the new study, said in a statement.

In the study, several factors affected people’s daily water turnover. Men typically turned over half a liter more than women, and doubling the energy used through physical activity increased water turnover by a liter, researchers found. Water needs peaked for men during their 20s and women from age 20 to 55.

People in hotter climates required more water, as did those in countries with lower scores on the United Nations’ Human Development index, which combines life expectancy, schooling and economic factors.

People in places with low HDI scores are more likely to engage in physical labor and less likely to be in climate-controlled buildings, Schoeller said. “That, plus being less likely to have access to a sip of clean water whenever they need it, makes their water turnover higher,” he said.

The findings could help officials better prepare for and respond to emergencies involving water shortages, said the researchers, who include Yosuke Yamada, a former UW-Madison postdoctoral researcher in Schoeller’s lab now at the National Institute of Biomedical Innovation, Health and Nutrition in Japan.

“Determining how much water humans consume is of increasing importance because of population growth and growing climate change,” Yamada said.