Your body actually needs that much water

Water makes up more than half of your body weight, and insufficient water intake is a risk factor for heat stroke, kidney disease, and heart failure. And yet, for something so important, it’s hard to quantify exactly how much of it we need every day.

Now researchers from an international research team have developed an equation to predict how much water our body actually uses and how much we really need to use.

You may have heard of the 8×8 guideline: we should drink eight, 8-ounce glasses of water every day. But how accurate is that?

“The dogma of eight glasses a day overestimates most people’s water needs,” said Herman Pontzer, a Duke University professor who worked on the study news week. “It’s not based on any real evidence as far as we can tell. It’s more of a marketing slogan that seems to have caught on. Also, it doesn’t have to be pure water. Other drinks also count towards our water intake.”

The US National Academies of Sciences, Engineering and Medicine recommends that men consume 3.7 liters (125 ounces) of water daily, while women are recommended 2.7 liters (91 ounces). However, these numbers include water ingested through food, which accounts for at least 20 percent of your daily water intake.

Our bodies lose water not only through urine, but also through sweat, feces, evaporation from the surface of the skin, and water vapor on our breath. However, Kyoto University’s Yosuke Yamada, who led the study, said the amount of water your body uses, known as water turnover, isn’t the same as the amount you need to consume.

“Drinking water accounts for about half to 40 percent of water turnover,” he said news week. “We use up the water content of food, and our body itself also produces some water during the energy-metabolizing process. If you multiply water turnover by about 0.4, you might get an answer as to how much water you need to drink in a day, although it depends on what you eat.”

Water requirements also vary from person to person. Pontzer said that on average, men’s bodies use about 4.3 liters of water per day, while women use about 3.4 liters. “But it’s not uncommon for male or female water needs to vary by plus or minus 1 liter per day,” he said.

Yamada said that this variation exists between and within individuals. “The variability in water turnover is incredibly large. The low end for adults is about 1.5 liters per day and the high end is about 6 liters per day… Even within an individual, when the mean air temperature is 30 degrees Celsius, The water turnover is 1.0 liters per day more than at 10 degrees Celsius.

“A unified approach is a big issue both between and within individuals,” he said.

Yamada and his team measured the water turnover and body water content of 5,604 people aged between 8 days and 96 years from 26 different countries. To make these measurements, participants were asked to drink a small amount of “heavy water” – water that had been fortified with deuterium, the heavy isotope of hydrogen. The deuterium can then be tracked as it travels through the body to calculate how much water is being used.

Researchers identified a number of different environmental and lifestyle factors that affect an individual’s water turnover. “We found that age, gender, height, physical activity level, occupation, athletic status, pregnancy, living altitude, air temperature, humidity, and socioeconomic status determine a person’s water turnover,” Yamada said.

Water turnover was highest in men aged 20 to 30, but water turnover remained at its peak in women aged 20 to 55 years. “I think it’s because of the gender differences in aging,” Yamada said. “Many researchers point out that men have greater muscle mass and physical capacity at their peak, but have a greater rate of decline in muscle mass and physical capacity during aging.”

Using this data, the researchers developed their equation to predict an individual’s water consumption, taking all of these factors into account:

water turnover = [861.9 × physical activity level] + [37.34 × fat-free mass in kg] + [4.288 × humidity] + [699.7 × athletic status] + [105.0 × human development index of country of residence] + [0.5140 × altitude in meters] – [0.3625 × age²] + [29.42 × age in years] + [1.937 × temperature²] – [23.15 × temperature in Celsius] – 984.8

The equation was created to inform strategies for global water access and for planning for future water needs. But Pontzer said individuals could rely on more intuitive methods to measure water needs.

“The best way to track the amount of water you need to drink each day is to listen to your body,” he said. “If you’re thirsty, drink something, preferably water or another healthy beverage.”