A new study headed by Professor John Speakman from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences (CAS) and the University of Aberdeen has shown that the suggested water intake of eight 8-oz glasses of water daily (nearly 2 L/day) is too high for the real need in several situations.
The study was reported in the journal Science on November 24th, 2022.
Water is necessary for life. Without water, humans can survive for a few days. But the exact water needs are hard to be quantified objectively. The majority of the earlier work has been based on subjective questionnaires employed on comparatively small numbers of people.
In this study, the scientists and their collaborators quantified water turnover, which is closely related to water needs, with the help of a stable isotope method. They employed this highly objective method on 5,604 males and females, aged between eight days and 96 years, from 23 countries.
They discovered that water turnover was greater in hot and humid surroundings and at high altitudes, among athletes, pregnant and breastfeeding women, and individuals with high levels of physical activity.
However, the greatest factor impacting water turnover was energy expenditure. So, the highest values were noted in males between the ages of 20 and 35, the group with the highest energy requirement.
Their water turnover nearly middled 4.2 L/day. This figure reduced with age, averaging only 2.5 L/day in males in their 90s. Among the women, the average water turnover at age 20 to 40 was 3.3 L/day and dropped to around 2.5 L/d by the age of 90.
Also, water turnover was greater in developing countries. This is likely because, in developed countries, air conditioning and heating tend to shield individuals from exposure to environmental extremes that raise 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.
John Speakman, Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences
It must be observed that water turnover is not equal to the need for drinking water. Even if a male in his twenties has a water turnover of, normally, 4.2 L/day, drinking 4.2 L of water daily is unnecessary. Around 15% of this value reflects surface water exchange and water generated from metabolism.
Hence, nearly 3.6 L/day is the required water intake. As most food also contains water, a considerable amount is consumed by eating. Since there is a big variation in the water content of foods, working out the exact drinking water need is hard.
For a normal man in his twenties in Europe or the US, perhaps over half of the 3.6 L of water required each day is gained from food, implying that the amount consumed by drinking is nearly 1.5 to 1.8 L/day.
For a woman in her twenties, it is presumably around 1.3–1.4 L/day. Normally, older people will need below this, while living in a hot climate, greater physical activity and being pregnant or breastfeeding will raise 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.
Xueying Zhang, Study Co-First Author and Professor, Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences
Yamada, Y., et al. (2022) Variation in human water turnover associated with environmental and lifestyle factors. Science. doi.org/10.1126/science.abm8668.