“We have guidelines that tell people how much water to drink,” says Pontzer, who wrote a book on metabolism called Weight Loss Burn. “But the reality is people kind of made it up.”
To see how much water people really need, Pontzer and his co-authors analyzed data from 5,600 people in 26 countries who ranged in age from eight days old to 96 years old. Participants included people from all walks of life, such as agricultural workers, athletes and non-athletes, sedentary office workers in Europe and the United States, and people from farming and hunter-gatherer communities in South America and Africa.
The participants were monitored with a gold standard technique called “double-labeled water,” which uses water laced with tracers that can be used to track the body’s production of carbon dioxide, allowing the researchers to get accurate measurements of daily energy expenditure. of the participants. . It also allowed them to estimate the amount of water the participants generated from their metabolism and the water they consumed.
“It measures very accurately how many calories you burn each day, as well as how much water you take in and how much goes out,” Pontzer said.
Using this method, the researchers determined how much water the participants lost and replaced each day, a measurement known as water turnover. They found that a person’s daily water turnover was largely determined by their size and their level of body fat, which contains less water than muscle and other organs.
The more “fat-free” mass a person has, the more water they need. Because men generally have larger bodies and less body fat than women, they generally consume more water. “Men use more water every day because we have a bigger system for staying hydrated,” Pontzer said.
The study showed that the amount of water you need changes throughout your life. In general, our water requirement peaks between the ages of 20 and 50 and then decreases in parallel with the slowing down of our metabolism. The amount of water you need partly depends on your metabolism and how many calories you burn.
“All the work your cells do every day is water-based,” Pontzer said. “The ratio of the amount of water you use to the number of calories you burn remains fairly constant throughout your life.”
Two other important factors that determine your water needs: the climate and whether you lead a sedentary lifestyle. People who live in warmer climates and exercise more have higher water turnover.
The study found that people from less developed regions of the world have higher water turnover than people from developed countries. “We think that’s because if you’re in a poorer country, you’re probably working outside every day and you have less access to climate control,” Pontzer said. “People in poorer countries also tend to do jobs that require more physical activity.”
He noted that as the world warms, our water needs will increase, exacerbating problems for the 2 billion people around the world who currently have limited or inadequate access to clean water.
Asher Y. Rosinger, an expert not involved in the study, said the study was “incredibly impressive” and the findings “made a lot of sense.” He said that, for the most part, people tend to consume the amount of water their bodies need.
But he cautioned that water turnover isn’t necessarily a precise measure of how hydrated a person is.
“In those hot environments, people use more water, but we don’t necessarily know if it’s enough to meet their water needs,” said Rosinger, the director of the Water, Health and Nutrition Lab at Penn State.
So how much water should you drink? The answer is simple: drink when you are thirsty. Prioritize water and try to avoid sugary drinks, which can cause metabolic problems. Coffee and tea are also fine.
While the caffeine they contain may increase urination, they’ll still hydrate as long as you consume less than 400 milligrams of caffeine, Rosinger said. Keep in mind that you also remove water from your food. Some water-rich foods include fruits, vegetables, beans, yogurt, brown rice, and soups.
“If you pay attention to your body and drink when you feel you need to, you should be fine,” Pontzer said.
The Washington Post