VOL.214 APRIL 2026
THE APPEAL OF YOSHOKU: JAPANESE-STYLE WESTERN CUISINE (PART 2)
[SCIENCE & TECHNOLOGY The Next Generation of Young Leaders] Unlocking the Secrets of Body Water Turnover and Factors Causing Variation
Photo: SATO Kensuke
It is a well-known fact that the human body is composed of more than half water and that water is indispensable for sustaining life. It was unclear for a long time, however, exactly how much water is taken in and lost by people’s bodies each day. We interviewed Professor YAMADA Yosuke of Tohoku University Graduate School of Biomedical Engineering, who established a formula that can predict this water turnover rate in a global-scale study and revealed that it varies based on factors including climate, living environment, skeletal muscle mass, and age.
Humans typically take in fluids not only from drinking water, but also from food. Meanwhile, water loss occurs not only through perspiration, urination, and defecation, but also through constant evaporation from the skin. Precise measurement of these processes is difficult, and significant individual variation exists due to living environments and the type and amount of food consumed. Consequently, up until recently, quantification1 of body water turnover has been considered challenging. A research team led by Professor YAMADA unlocked secrets related to this issue. Focusing particularly on research into water metabolism in the human body, Professor YAMADA received the FY2024 JSPS Prize from the Japan Society for the Promotion of Science for his work on “Environmental and Lifestyle Factors Contributing to Water Turnover and Energy Expenditure.” (See more about the JSPS Prize in the info box.)
■The formula established by YAMADA and his team to predict water turnover
–0.3529×Age^2 +24.78×Age (y) +1.865×Temp.^2 –19.66×Temp. (°C) –713.1
Water turnover (mL/d) = 1076×PAL +14.34×Body weight (kg) +374.9×Sex +5.823×Humidity (%) +1070×Athlete status [0,1]+104.6×Human development index [0,1,2] +0.4726×Altitude (m)
–0.3529×Age^2 +24.78×Age (y) +1.865×Temp.^2 –19.66×Temp. (°C) –713.1
* The Human Development Index (HDI) measures a country’s level of development in terms of three factors — life expectancy, education, and standard of living — representing each with a value between 0 and 1. For the prediction formula here, countries are categorized as advanced (high HDI), intermediate, and developing (low HDI) based on this value, with the coefficients 0, 1, and 2 assigned respectively.
* Physical activity level (sedentary: 1.5, average: 1.75, high: 2.0), sex (female: 0, male: 1), athlete status (non-athlete: 0, athlete: 1), HDI (high [advanced]: 0, intermediate: 1, low [developing]: 2)
“Our research revealed that children have higher water turnover per unit of body water than adults, with approximately one-fourth of the water in a child’s body, versus one-tenth of an adult’s body water, lost daily. This lost water is replenished by ingested fluids, meaning that an adult’s body water is completely replaced with new water roughly every 10 days,” explains Professor YAMADA.
To measure water turnover rates, YAMADA and his team focused on hydrogen-2 (2H), a stable isotope2 of hydrogen.
When about 5 ml of 2H2O — water containing 2H — is ingested, 2H levels in the body temporarily rise slightly but return to baseline within months. Using isotope ratio mass spectrometry (IRMS)3 enabling the measurement of such slight changes with precision makes it possible to calculate daily water turnover based on how quickly 2H levels return to baseline. Using this method, YAMADA and his team measured water turnover in 5,604 individuals, both male and female, ranging from 8-day-old infants to 96-year-old seniors residing in 23 countries around the world.
“2H2O is completely harmless and naturally exists in body fluids in trace amounts. We periodically measure the decrease in 2H levels using IRMS after having subjects drink a small amount. As obtaining accurate values, however, requires researchers who are skilled in operating the equipment, there were limits to the amount of data individual researchers could collect. This constraint led to building a global network of researchers and compiling the data as a collaborative study. The investigation is ongoing and has expanded in scale, gathering data from approximately 13,000 people in more than 40 countries,” YAMADA tells us.
■Conceptual diagram illustrating the principle of water turnover calculation based on 2H2O stable isotope water ingestion
Diagram: From materials provided by Professor YAMADA
■The relationship between age and body water turnover
(Graph 1) Average values
(Graph 2) Individual values and average values
While Graph 1 shows that, on average, approximately 10% of total body water (TBW) is replaced as water turnover (WT) daily in adults, Graph 2 reveals very significant individual variation. Large-scale data collection and analysis have identified factors that contribute to individual differences in water metabolism, such as living environment, climate, and body build.
The research revealed that people living in hot, humid environments, in developing countries with inadequate sanitation, and in extremely cold places like the Arctic tend to have higher turnover. It also revealed that athletes, individuals with high muscle mass, those with high levels of physical activity, and pregnant women tend to have higher turnover as well. Collecting and analyzing this vast amount of data enabled the establishment of a formula predicting how various factors, such as living environment and physical characteristics, affect water metabolism.
“I believe an accurate understanding of water turnover can be beneficial for strategic planning regarding how much water and food national and local governments should secure to maintain human health during disasters or emergencies, as well as for constructing predictive models for water shortages caused by global climate change and population growth,” YAMADA shares.
He also hopes to conduct further in-depth research on water metabolism based on the concept that the proper circulation of bodily fluids, including blood, is a key factor in maintaining health.
The JSPS Prize
In order to raise the level of scientific research in Japan to the world’s highest standard, it is important to recognize and support young researchers with rich creativity and superlative research ability at an early stage in their careers. To sustain the zeal of such researchers while offering them support in advancing their work, the Japan Society for the Promotion of Science4 established the JSPS Prize in FY2004. For more details, see the website of the Japan Society for the Promotion of Science.
- 1. Expressing phenomena in numerical form and representing them with objective measures. The process makes it easier to perform comparison and analysis.
- 2. Variants of chemical elements that share the same atomic number as the ordinary element but differ in the number of neutrons, resulting in different atomic masses. While their scientific properties are nearly identical, slight differences in their mass make them useful for things like precision analysis. They possess no radioactivity and stably exist in nature.
- 3. An analytical technique or device (isotope ratio mass spectrometer) capable of extremely precise measurement of elements with low atomic numbers — in other words, low atomic weights — such as hydrogen, carbon, nitrogen, and oxygen.
- 4. Established in 1932 with an endowment from Emperor Showa, JSPS is today Japan’s only independent funding agency with a mission to advance science. JSPS carries out a wide variety of programs in pursuit of this mission. They include programs to fund scientific research, foster researchers, promote international scientific exchange, and reform university systems.
By FUKUDA Mitsuhiro
Photo: SATO Kensuke; YAMADA Yosuke
