According to scientists, the recommendation to drink eight glasses of water a day is excessive for most people.
The suggestion is accepted wisdom and often appears in health guidance. The most rigorous study to date on water turnover shows that people have a wide range of water intakes. The research suggests that people only need 1.5 to 1.8 liters a day, lower than the two liters typically recommended.
Yosuke Yamada, a professor at the National Institute of Biomedical Innovation, Health and Nutrition in Japan, said the current recommendation is not supported scientifically at all. Most of the scientists are not sure where this recommendation came from. One issue is that previous estimates of water requirements have ignored the water content of food, which can contribute a large portion of our overall intake.
If you eat bread and bacon and eggs, you will not get much water from food, but if you eat meat, vegetables, fish, pasta and rice, you can get about 50% of your water needs from food, said Yamada.
The study, published in the journal Science, examined the water intake of 5,604 people aged between eight days and 96 years old from 23 countries. The researchers involved in drinking a glass of water in which some of the hydrogen molecules were replaced by a stable isotope of the deuterium element, which is found naturally in the human body and is harmless.
The study found that the measure varied widely based on age, gender, activity levels and surrounding, as well as the rate of elimination of the extra deuterium. People living in hot and humid climates and high altitudes as well as athletes and pregnant and breastfeeding women had higher turnover, meaning they need to drink more water.
Energy expenditure is the biggest factor in water turnover, with the highest values observed in men aged 20 to 35, with an average of 4.2 litres a day. This decreased with age, averaging 2.5 litres a day for men in their 90 s. Women aged 20 to 40 had an average turnover of 3.3 litres, which has declined to 2.5 litres by the age of 90. Athletes turn over about a litre more than non-athletes. Newborn babies turned over the largest proportion, replacing about 28% of the water in their bodies every day.
This study shows that the common suggestion that we should drink eight glasses of water or around two litres a 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 this data, said Professor John Speakman of the University of Aberdeen.
He said that it is a recommendation that many people ignore and follow what their body is telling them.
Clean drinking water is not free to produce, as the authors point out that drinking more water is unlikely to be harmful for health. There is a cost to drinking more than we need, even if it is not a health cost, said Speakman.
If 40 million adults in the UK were following the guidelines and they drank half a litre of clean water more than they need each day, that equates to 20 m litres of wasted water every day.