Water is essential for our body to maintain metabolic processes, regulate blood pressure and control body temperature. But how much water do we need a day? A study now shows that no generally applicable value can be given for this. The researchers determined the water consumption of thousands of people worldwide and came up with a wide range of average values from one to six liters per day. The results are relevant, among other things, in order to be able to better assess future global water requirements in view of climate change.
Every day our body loses significant amounts of water. According to widespread average values, an adult person excretes around two to three liters a day through urine, breathing and skin - and has to replace the corresponding amount of liquid. We get some of it through our food and we drink the rest. The previous average values for water turnover are based primarily on reports from volunteers who logged their water and food consumption. Objective surveys, on the other hand, have so far been lacking. How much water our body actually uses every day was unknown.
Objective measurement
A team led by Yosuke Yamada from the Japanese National Institute for Health and Nutrition in Tokyo has now measured over 5,600 people from 26 countries worldwide how much water their bodies convert per day. The subjects were between eight days and 96 years old and lived under very different conditions. Some came from industrialized nations, spent most of the day at their desks, and had access to clean water whenever they wanted. Others lived in less developed countries, worked long hours in the fields and did not always have access to drinking water.
Instead of relying on potentially distorted self-reports on fluid intake, Yamada and his team used an objective measurement method to determine the body's water turnover: At the beginning of the experiment, they gave all test subjects 100 milliliters of water that they had enriched with the hydrogen isotope deuterium to drink . Since deuterium has a different atomic weight than normal hydrogen atoms, it can be identified in samples. "If you measure the rate at which a person urinates these stable isotopes over the course of a week, you can use the hydrogen isotope to determine how much water your body is replacing," explains co-author Dale Schoeller of the University of Wisconsin in madison
Wide spread averages
In addition, the researchers collected numerous other influencing factors, including age, gender, weight and athletic ability of the test subjects, environmental factors such as temperature, humidity and altitude, as well as the Human Development Index of the United Nations. It is a composite measure of life expectancy, schooling and economic factors in a country.
The result: The average values of different age groups were between one and six liters a day, with men between 20 and 30 years and women between 20 and 55 years using the most water. Newborns convert the largest proportion of their body water: they exchange around 28 percent of their total body water every day. According to the study's calculations, an average active male non-athlete aged 20 years old and weighing 70 kilograms living in a well-developed country with an average air temperature of 10 degrees Celsius uses around 3.2 liters of water per day . A 130-pound woman under similar conditions would lose and absorb about 2.7 liters of water per day.
Formula for water consumption
"But there are also outliers who use up to ten liters per day," says Schoeller. "The large fluctuations mean that the reference to an average value doesn't say much." In order to be able to estimate a person's actual water requirement more individually, the researchers developed a formula that weights various influencing factors. The level of physical activity and athleticism have the greatest influence, followed by gender, the Human Development Index and age. The following applies: the lower the Human Development Index of the home country, the more water a person uses per day. Reasons for this are, for example, higher average temperatures and hard physical work.
According to the researchers, the formula can help to estimate the water needs of the world population more precisely, especially in view of climate change and regional water shortages. "Currently, 2.2 billion people around the world do not have access to clean drinking water," the authors write. "Our results can help develop strategies for managing drinking water and water-fortified food as the world population grows and the climate changes."
Source: Yosuke Yamada (National Institute of Health and Nutrition, Tokyo, Japan) et al., Science, doi: 10.1126/science.abm8668