Water sustains life, but until now, figuring out how much water people truly need each day has been mostly guesswork. Standard advice is often one-size-fits-all and does not reflect how different people live, work, and experience their surroundings. A large international study is helping to change that by showing how daily water use varies depending on factors such as climate, activity levels, age, and where people live. The findings, based on information from thousands of people around the world, offer a clearer and more practical guide for staying hydrated.
Tens of researchers from all over the world from organizations such as the National Institute of Health and Nutrition in Japan, the University of Aberdeen in the United Kingdom, and Duke University in the United States led by Professor John Speakman, Dr. Mary Henderson, Dr. Xueying Zhang and Professor Yosuke Yamada. Their work is featured in the journal Science.
The team measured how much water people’s bodies used daily by tracking a harmless type of hydrogen in drinking water. This technique, called isotope tracking, uses a special version of hydrogen to follow water as it moves through the body. This method gave them a highly accurate picture of water movement in the body during normal daily life. They discovered that water needs change significantly based on a person’s age, size, activity level, whether they are pregnant, and on external factors like temperature, humidity, and the country’s economic condition. For example, people who are more physically active or who live in hotter climates use more water. People in wealthier countries tend to use less water because they have more access to climate control like air conditioning, which reduces water loss through sweat.
One of the most surprising findings was how much water use can vary. Some people only needed about a liter a day, while others required more than ten. Young adults generally had the highest water use, especially men in their twenties and women between their twenties and fifties. “Water turnover,” which means the total amount of water moving in and out of the body each day, “was greatest in individuals aged 20 to 30 years in men, and from 20 to 55 years in women,” Professor Speakman explained. Water needs drop off with age, mostly because of changes in body composition and how active people are.
The study also showed how important local conditions are. “We found a significant curvilinear relationship between outdoor air temperature and water turnover,” said Professor Yamada. A curvilinear relationship means that the effect of temperature is not a straight line—both very hot and very cold weather increased how much water people used. People living high in the mountains or near the equator tended to need more water. Women who were pregnant or breastfeeding needed more water as well, because of the changes their bodies were undergoing and the water needed to produce milk.
Based on these findings, the team created two sets of easy-to-use calculations to estimate how much water a person might need each day. These formulas take into account weight, age, physical activity, humidity, altitude, and the country’s development level. Altitude refers to how high someone lives above sea level, which can influence how much water the body loses through breathing. For example, a young athletic man living in a hot, humid, and high-altitude area might need more than twice as much water as someone the same age living in a cooler, lower-altitude area who is not active. These tools help give more tailored water advice instead of general rules.
The bigger message from this study is that water needs are personal. The old idea that everyone should drink eight glasses of water a day does not match what science now shows. “One size does not fit all for drinking water guidelines,” Professor Speakman said, stressing the importance of updating public advice based on real evidence.
With the world facing more heatwaves, expanding urban areas, and growing populations, this kind of knowledge is more important than ever. The study offers a practical way for health experts and governments to plan for how much water people need. It also suggests that tracking how much water a person uses might give clues about their overall health, since it relates closely to how active they are and how much lean tissue they have in their body. Lean tissue refers to muscle and other parts of the body that are not fat and tend to hold more water.
Journal Reference
Yamada Y., Zhang X., Henderson M.E.T., Sagayama H., Pontzer H., et al. “Variation in human water turnover associated with environmental and lifestyle factors.” Science, 2022; 378(6622): 909–915. DOI: https://doi.org/10.1126/science.abm8668
About the Authors
Professor John Speakman is an internationally recognized expert in physiology and metabolism. Based at the University of Aberdeen and also affiliated with the Chinese Academy of Sciences, he has spent decades studying energy balance, obesity, and human adaptation. His research often bridges evolutionary biology and public health, using cutting-edge techniques like doubly labeled water to measure energy and water use in real-world settings. A prolific scientist with hundreds of publications, Speakman’s work has helped shape global understanding of how humans use energy and water across different environments, life stages, and activity levels.
Dr. Mary Henderson is a human biologist and senior researcher specializing in lifestyle, physical activity, and nutrition. Affiliated with the University of Roehampton in London, her research explores how daily habits and body composition affect overall health. She has contributed to large international studies that track energy and water use, helping to improve public guidelines on hydration and wellness. Henderson’s work brings attention to the diverse ways people’s environments and behaviors influence their health, with a focus on making scientific insights more accessible and actionable for the general public.
Dr. Xueying Zhang is a biomedical researcher working at the intersection of metabolism, environmental health, and public nutrition. She is affiliated with the Shenzhen Institutes of Advanced Technology and the University of Aberdeen. Her work focuses on large-scale studies of human energy use, physical activity, and hydration needs. With a strong background in data analysis and international collaboration, Zhang has played a key role in global efforts to create more accurate models of human water requirements. Her research supports better health policies by showing how personal and environmental factors shape our basic daily needs.
Professor Yosuke Yamada is a leading researcher at the National Institute of Health and Nutrition in Japan. His work focuses on human physiology, especially body composition, energy expenditure, and hydration. He uses advanced methods like stable isotope tracking to understand how much water people use and how that varies by age, lifestyle, and climate. Yamada has been central to creating global models that help public health officials set better guidelines for water and food needs. His contributions are widely respected for their accuracy and real-world relevance in improving how we understand human health.
