An international study has for the first time determined the influencing factors for the water turnover in our body.

Water is vital for us – without the precious water neither metabolic processes nor cells function properly. If the fluid content of our body drops too far, special sensors sound the alarm. Our brain then sends us an unmistakable signal: We are getting thirsty. Because our body constantly loses water through urine, sweat and other processes, we have to regularly replenish it with food and drink.

But how much water does our body need every day? The usual recommendations for water intake range from 1.5 to three liters per day, but are mostly based on only rough estimates and generalizations. An international team led by Yosuke Yamada from the Japanese National Research Institute for Health and Nutrition has now for the first time examined in detail the amount of water the human body actually processes, loses and absorbs every day.

To do this, the researchers had more than 5,600 people from 26 different countries compete for the test. For the experiment, each subject drank 100 milliliters of water in which the normal hydrogen atoms had been replaced with the heavy hydrogen isotope deuterium. This isotope labeling allows one to follow how the labeled water spreads and dilutes in the body. This in turn reveals how high the water content of the human body is.

Individual water needs, on the other hand, are derived from the change in isotope levels over time: “If you measure the rate at which a person sheds these stable isotopes through their urine over the course of a week, that tells you how much water their body is taking in replaced at that time,” explains co-author Dale Schoeller of the University of Wisconsin–Madison. The team determined this water throughput for each test person, taking into account a large number of individual and environmental influencing factors – from the climate and physical activity to age, gender and body mass index.

The evaluations revealed that our body’s need for water varies greatly. Adults therefore need between one and six liters per day. “There are even outliers with up to ten liters a day,” reports Schoeller. The biggest influencing factors are age, gender, body weight and fat percentage. Physical activity and fitness, as well as climate, geographical location and wealth also play a role, as the team determined.

Based on their analysis, the researchers even succeeded in quantifying all of these connections and summarizing them in one equation. “The formula predicts human water needs in relation to anthropometric, economic and environmental factors,” the scientists explain. With this equation it is now possible for the first time to calculate the water requirement of each person – and to cover at least a large part of the individual differences.

But what does that mean in concrete terms? According to the analyzes men need almost half a liter more water per day than women of the same age under the same conditions. The water requirement for a 20-year-old, moderately active man weighing 70 kilograms in Germany is around 3.2 liters. Under these circumstances, the body of a woman of the same age and roughly the same height uses around 2.7 liters of water per day.

“The differences between sexes and age groups primarily reflect differences in body fat percentage, because adipose tissue contains less water than muscles and other organs,” Yamada and his colleagues explain. This is one of the reasons why the water requirement is highest in adult men between the ages of 20 and 30 and then decreases with age. In women, on the other hand, it remains largely the same up to the age of about 50. Only during pregnancy does their daily water requirement increase by almost 0.7 liters.

Other individual physical factors can also be quantified: A trained athlete needs around one liter more water per day than a non-athlete – even if both are equally inactive on that day. However, physical exercise also increases the need for water acutely: with a 50 percent increase in our energy expenditure, we have to drink around one liter more. And body weight and height also play a role: If you gain 50 kilograms more, the water requirement increases by 0.7 liters because the body then has to supply more water-hungry tissue.

In addition, the water turnover of our body changes in the course of life. According to this, newborns have the highest water requirement: their fast metabolism means that they have to replace around 28 percent of their body fluids with water every day. After the first months of life, the daily water requirement gradually decreases and is on average nine percent of the body water content in young adults. In old age, it decreases even further: At the age of 80, we need around 0.7 liters of water less than at the age of 30.

How much water we need also depends on our environment. The connection with climate and geography is obvious: heat, humidity, sea level and latitude have a measurable influence on the daily water requirement, as the team reports. Accordingly, it is highest at the equator and lowest around the 50th degree of latitude – as here in Germany, for example.

What is surprising, however, is that a country’s economy and level of development also play an important role. “Even after taking all other factors into account, people in countries with a low development index have a 200 milliliter higher water requirement than people in highly developed countries under the same conditions,” report Yamada and his colleagues. Among other things, they attribute this to the fact that people in richer countries spend more time in air-conditioned rooms and therefore do not have to sweat as much even in the same climate.

Taken together, these results demonstrate that human water needs are influenced by a variety of physical and environmental factors. “Our study clearly illustrates that there is no one-size-fits-all when it comes to the daily amount to drink,” the researchers write. “The current recommendation to drink two liters of water a day is not supported by the objective data.” (Science, 2022; doi: 10.1126/science.abm8668)

What: Science

This article was written by Nadja Podbregar

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The original of this post “What determines how much we need to drink?” comes from scinexx.