Weighing up water needs

More than two billion people do not have access to safe drinking water, most of them living in countries with a low HDI: an index that measures three key dimensions of human development: life expectancy, education and per capita income. Few studies have examined differences in water turnover across populations around the globe. Water turnover is defined as the total movement of water, from intake and loss, through the body, and is measured in litres per day. 

To address this knowledge gap, the team compared water turnover in 5,604 people (3,729 females and 1,875 males) ranging in age from eight days to 96 years from 26 countries using isotope-tracking methods.

They found that people living in low-HDI countries had a higher water turnover than those in middle- and high-HDI countries.

Using their large dataset, the team was able to formulate an equation to predict human water turnover in relation to factors including body size, lifestyle and environment. 

“We found that age, sex, body size, physical activity level, occupation, athletic status, pregnancy, living altitude, air temperature, humidity, and socio-economic status determine a person’s water turnover,” says co-lead corresponding author Yosuke Yamada of the National Institutes of Biomedical Innovation, Health and Nutrition, Tokyo, Japan. 

Yamada explains that the variation in water turnover is “incredibly large” — the low end for adults is around 1.5 litres per day and the upper end is around six litres per day. Outliers are in the 10 litres per day range. 

“Even for an individual, if the mean air temperature is 30°C, water turnover is one litre per day more than at 10°C. A one-size-fits-all approach is a big issue both between and within individuals. Environmental and lifestyle factors affect actual water requirements,” he says. 

Water turnover was found to be highest in men aged 20–30 years and in women aged 20–55 years. Athletes were found to have around one litre more water turnover than non-athletes. Analysis of 63 pregnant women showed that water turnover is significantly higher during the third trimester of pregnancy and lactation compared with pre-pregnancy. People living above the Arctic Circle had a higher water turnover than those living at –50° or +50° latitude.

Given that climate change and global population increases will affect safe water availability in many parts of the globe, Yamada says: “Our findings will provide important information to shape strategies for drinking water and water-enriched food management in their respective regions.”

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