How many liters of water enter and leave our body every day? While this question sounds anecdotal, it will become less so as climate change depletes water resources. The answer partly lies in a formula developed by an international group of scientists, and depends particularly on age, altitude, temperature or even socio-economic status.
“Until now we did not know the factors that determine our water renewal, nor the relative importance of the different factors.”, explains Yosuke Yamada, first author of these new works published in Science. However, this renewal is essential since every day we lose water in the form of urine and faeces, sweat and respiratory water vapor. To compensate for these losses, water intake is essential mainly through food and drink, but also through water vapor in the breathed air or transcutaneous water absorption. The balance of these quantities of water absorbed and lost is the renewal of water expressed in liters per day.
Monitor the daily renewal of water in our body
Tracing the location of water in the human body is not such a difficult process. It is sufficient for the subjects to swallow 100 milliliters of water, of which 5% of the molecules are modified. Instead of classic H2O, this so-called “heavy” water is called D2O because its hydrogen atoms are replaced by an isotope called deuterium. The advantage is that these isotopes are traceable. Once ingested, heavy water will mix homogeneously with normal water, perfectly reflecting its natural renewal. This is exactly the method used by scientists on 5,604 volunteers aged 8 days to 96 years and living in 26 different countries. “We collected urine samples on the day of isotope intake and a week or two afterward. We then measured the ratio of stable isotopes in urine samples using mass spectrometry“, a method for separating the components of a sample based on their mass, describes Yosuke Yamada.
Age, gender, place of residence… And socio-economic status
Researchers infer the many factors that determine the body’s water renewal: age, gender, body size, physical activity level, occupation, athletic status, pregnancy, living altitude, air temperature, humidity, and socioeconomic status, Yosuke lists Yamada. It is the latter factor that finds the most “amazing“, where the other criteria “were predicted based on human physiology“. Thus, a young and corpulent man, who lives at low altitudes and in a hot country, will need more water to renew his losses, and this all the more if he lives in a low-income country. “People living in low-income countries are more likely to be exposed to room temperatures without air conditioning“, explains Yosuke Yamada, and are therefore more exposed to heat.
A formula that explains 47% of human water turnover
Based on these observations, here is the formula for calculating a person’s water renewal (in milliliters per day):
(Physical activity level x 1076) + (Weight in kg x 14.34) + (gender x 374.9) + (humidity in % x 5.823) + (athlete status x 1070) + (development index of the country x 104.6) + (altitude in meters x 0.4726) – (age squared x 0.3529) + (age in years x 24.78) + (temperature squared x 1.865) – (temperature in ° C x 19.66) – 713.1
Note: Gender equals 1 for men, 0 for women. The athlete status is 1 for athletes, 0 for others. The country’s development index is 0 for high-income countries, 1 for middle-income countries, and 2 for disadvantaged countries. As for the level of physical activity, it varies between values 1 and 3 and is calculated based on the daily energy expenditure (in megajoules).
EXAMPLES. “A 20-year-old man weighing 70kg, is not athletic, has an “athlete status” of 1.75, and lives in a high-income country, at an altitude of 0m, where the average air temperature is of 10° C and the relative humidity is 50%, it has an expected water exchange a 3.2 liters per day“, illustrate the authors in the publication. “In contrast, a 20-year-old athletic female, weighing 60 kg, with an “athlete status” of 2.5 and living in a high-income country, at an altitude of 2,000 m, where the average ambient temperature temperature is 30°C and the relative humidity is 90%, there will be a renewal of 6.8 liters per day“.
Although it incorporates many components, this formula only explains 47% of the variations in water renewal in the human body. “The remaining 53% are still unknown“, admits Yosuke Yamada. According to him, this high variability could result from an unusually high water consumption, which forces the body to reject as much and therefore increases the number of liters of water renewed by the same amount. Despite this uncertainty, he has filed a patent on this formula and hopes to find a solution on a smartphone or smartwatch”.This will give the amount of water we need on a given day and place“says the researcher.
An ambition that could prove worthwhile, as scientists predict that climate change and the increase in the world’s population will soon affect the availability of clean water in many regions of the globe. Currently, 2.2 billion people lack access to clean water. “Our results will provide important information for the development of strategies for the management of drinking water and water-fortified foods in the respective regions.”