A new study led by Prof. John Speakman from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences (CAS) and the University of Aberdeen shows that the recommended water intake of eight 8-oz glasses of water per day (around 2 L/day) is too high for our actual need in many situations. 

The work was published in Science on Nov. 24.

Water is essential for life. Without water humans can only survive a few days. Yet the exact requirements for water are difficult to measure objectively. Most previous work has depended on subjective questionnaires applied to relatively small numbers of people. 

In this study, the researchers and their collaborators measured water turnover, which is closely related to water requirements, using a stable isotope technique. They applied this more objective approach to 5,604 males and females, aged between eight days and 96 years, from 23 different countries. 

They found that water turnover was higher in hot and humid environments and at high altitudes, as well as among athletes, pregnant and breast-feeding women and individuals with high levels of physical activity. 

The biggest factor influencing water turnover, however, was energy expenditure. Hence the highest values were observed in males between the ages of 20 and 35, the group with the highest energy expenditure. Their water turnover averaged 4.2 L/day. This figure decreased with increasing age, averaging only 2.5 L/day in males in their 90s. Among women, the average water turnover at age 20–40 was 3.3 L/day, and also declined to around 2.5 L/d by the age of 90. 

Water turnover was also higher in developing countries. This is probably because in developed countries air conditioning and heating buffer individuals from exposure to environmental extremes that elevate water demand. 

"The main outcome of our study was a general equation for predicting water turnover. This will help countries anticipate their future water needs," said Prof. Speakman. 

It should be noted that water turnover is not equal to the requirement for drinking water. Even if a male in his twenties has a water turnover of, on average, 4.2 L/day, he does not need to drink 4.2 L of water each day. About 15% of this value reflects surface water exchange and water produced from metabolism. So the actual required water intake is about 3.6 L/day. Since most food also contains water, a substantial amount of water is provided just by eating. Because the water content of foods varies so much, though, working out the exact drinking water requirement is difficult. 

For a typical man in his twenties in the US or Europe, probably more than half of the 3.6 L of water needed each day comes from food, which means that the amount that should be consumed by drinking is around 1.5–1.8 L/day. For a woman in her twenties, it is probably about 1.3–1.4 L/day. Older people will generally require less than this, while living in a hot climate, greater physical activity and being pregnant or breast-feeding will increase this figure. 

"Figuring out how much water humans require is significant due to explosive population growth and growing climate change. Water turnover is related to many health parameters like physical activity, body fat percent, etc., making it a new potential biomarker for metabolic health," said Prof. ZHANG Xueying from SIAT, co-first author of the study.