Water creates more than half weight, and insufficient water intake is a risk factor for heat stroke, kidney disease and heart failure. And yet, for something so vital, it’s hard to quantify exactly how much we need each day.

Now, scientists from an international research team have developed an equation to predict how much water our bodies actually use, and therefore how much we really need to consume.

You may have heard of the 8×8 guideline: We should drink eight 8oz glasses of water every day. But how accurate is it?

The general recommendation is to drink eight glasses of water a day, but research shows that this is an overestimate for most people.
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“The eight-glass-a-day dogma overestimates the water needs of most people,” said Herman Pontzer, a professor at Duke University who worked on the study. Newsweek. “As far as we know, this is not based on any real evidence. It’s more of a marketing slogan that seems to be adopted. 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 drink 3.7 liters (125 ounces) of water per day and 2.7 liters (91 ounces) for women. However, these figures include water consumed with food, which makes up at least 20 percent of your daily water intake.

Our bodies lose water not only with urine, but also with sweat, feces, evaporation from the surface of the skin and water vapor in the breath. However, Yosuke Yamada of Kyoto University, who led the research, said the amount of water your body uses, called water turnover, is not the same as the amount you need to consume.

“Drinking water accounts for about half to 40 percent of water turnover,” he said Newsweek. “We use the water contained in food, and our body itself produces some water in the process of energy metabolism. If you multiply about 0.4 to your water turnover, you might be able to get an answer 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 men use about 4.3 liters of water a day on average, while women use about 3.4 liters. “But it’s not unusual for men’s and women’s water needs to differ by plus or minus 1 liter per day,” he said.

Yamada said this variability exists between and within individuals. “The fluctuations in the water cycle are incredibly large. The lower limit for adults is about 1.5 liters per day, and the upper limit is about 6 liters per day … Even in an individual, if the average air temperature is 30 degrees Celsius, the water turnover is 1.0 liters per day more than at 10 degrees Celsius.

“A one-size-fits-all approach is a big problem both between and within individuals,” he said.

Man and woman drinking water
The stock image shows steam drinking water. On average, men need more water than women.

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

Researchers have identified a number of different environmental and lifestyle factors that influence water turnover in individuals. “We found that age, gender, body size, physical activity level, occupation, sporting status, pregnancy, altitude, air temperature, humidity, and socioeconomic status determine a person’s water turnover,” Yamada said.

Water turnover was highest in men between the ages of 20 and 30, but women’s water turnover was highest between the ages of 20 and 55. “I think this is due to age-related gender differences,” Yamada said. “Many researchers indicate that men have more muscle mass and exercise capacity at their peak, but have a greater rate of decline in muscle mass and exercise capacity as they age.”

Using this data, the researchers developed an equation to predict a person’s water consumption by keeping all of the following factors in mind:

Water rotation = [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 developed to help develop global water access strategies and plan for future water needs. But Pontzer said people can rely on more intuitive ways to measure water needs.

“The best way to track the 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 drink.”

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