8 Glasses of Water a Day? Nonsense, Says a New Study

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A woman drinking water
Kateryna Onyshchuk / Shutterstock.com

It’s been something like an article of faith for many years: Drinking 8 ounces of water eight times a day is ideal for staying hydrated.

But a new study is pouring that theory down the drain.

Researchers say individual water needs vary from person to person, and blanket recommendations for water intake should be forgotten. Their findings were published in Science, a journal of the American Association for the Advancement of Science.

For decades, study co-author Dale Schoeller, a University of Wisconsin-Madison emeritus professor of nutritional sciences, has studied how water intake impacts the body.

In a summary of the study findings, he says:

“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.”

He says the new research is the best to date for measuring daily water turnover and the factors that influence it. Water turnover is the amount of water going into and out of the body, so it serves as a measurement of how much water the body actually uses each day.

In the study, the researchers measured water turnover in more than 5,600 people of all ages and from 26 countries. They found that daily averages of water consumption range from 1 liter per day to 6 liters per day. (There are about 33.8 ounces in 1 liter.)

The study measured the time it took water to move through a person’s body by tracking the turnover of what researchers call “labeled water.” In short, the study participants drank water containing trackable isotopes, which enabled researchers to know how much water went into and out of their bodies.

The biggest factors that impacted water turnover in the participants included:

  • Activity level
  • Sex
  • Human Development Index of the county a participant lives in (developed by the United Nations, this index is a measure of a country that combines life expectancy, schooling and economic factors)
  • Age
  • Pregnancy
  • Socioeconomic status
  • Environmental characteristics (latitude, altitude, air temperature and humidity)

In the end, the study findings suggest that given how much variation there is in people’s water turnover patterns, “pointing to one average doesn’t tell you much,” Schoeller says.