How much water do we really need?

Without water humans can only survive for a few days, however the exact amount we actually need daily is difficult to measure objectively.

New research from the University of Aberdeen published in Science this week (November 25), shows the recommended water intake of eight glasses (around two litres) a day seldom matches our actual needs, and in many situations, is too high. 

Previous research in this area has depended on subjective questionnaires applied to relatively small numbers of people. Now scientists from the University of Aberdeen have collaborated with scientists from across the world to measure water turnover (which is closely related to water requirements) using a stable isotope technique. They applied this more objective approach to 5604 men and women, aged between 8 days and 96 years old, from 23 different countries. The data were compiled in a database hosted by the International Atomic Energy Agency. 

The technique involves people drinking a glass of water in which some of the hydrogen molecules have been replaced by a stable isotope of hydrogen called deuterium. Deuterium is found naturally in our bodies and is completely harmless. The rate of elimination of the extra deuterium from the glass of water tells us how quickly water in the body is turning over. 

The work showed that water turnover is 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 however was energy expenditure. The highest values were observed in males between the ages of 20 and 35, which is the group with the highest energy expenditure. This group’s water turnover averaged 4.2 litres per day. Thereafter, it decreased with increasing age, averaging only 2.5 litres per day in males in their 90s. Among women, the average water turnover at age 20 to 40 was 3.3 litres a day, and also declined to around 2.5 litres by the age of 90.  

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

Professor John Speakman from the University of Aberdeen School of Biological Sciences, explains: “Water turnover is not equal to the requirement for drinking water. Even if a male in his 20s has a water turnover of on average of 4.2 litres per day, he does not need to drink 4.2 litres 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 litres per day. Since most foods also contain water, a substantial amount of water is provided just by eating.  

“Because water contents of foods vary so much, working out the exact required drinking water is difficult. For a typical person in the US or Europe, probably more than half of the 3.6 litres of water comes from food, which means that the amount needed to be drunk is around 1.5 to 1.8 litres day. For a woman in her twenties, it is probably about 1.3 to 1.4 litres per day. Older people will generally require less than this, while hot climates, being pregnant or breast-feeding and greater physical activity will increase it.  

“This study shows that the common suggestion that we should all be drinking eight glasses of water (or around  two litres per day) is probably too high for most people in most situations and a ‘one size fits all policy’ for water intake is not supported by these data.” 

The research resulted in a general equation for predicting water turnover which can be used to anticipate the effects of future changes, for example in climate and population demography. This will help countries anticipate their future water needs.  

Professor Speakman continued: “Understanding the factors that drive our water turnover and the relative importance of different factors is a big step forwards in our ability to predict future water needs. This work was built on contributions of scientists from all over the world, and shows the key importance of international scientific co-operation to answer big scientific questions.” 

Dr. Yosuke Yamada, Section Head of National Institute of Biomedical Innovation, Health and Nutrition in Japan, who co-authored the paper said: “The equations we have generated to predict water turnover will be of great benefit in modelling global water requirements. This work could not have been accomplished without the international collaboration of over 90 researchers.”   

Former Aberdeen PhD student Xueying Zhang who was co-first author on the paper added: “Water is essential for human survival, humans can survive a couple of weeks without food but only three days without water. Working out how much water humans require is becoming of increasing importance because of the explosive population growth and growing climate change. Water turnover is related to many health parameters including physical activity and body fat percentage, making it a new potential biomarker for metabolic health.”