Drinking eight glasses of water a day is probably excessive for most people, and not supported scientifically at all, despite the suggestion having become accepted wisdom and often appearing in health guidance, say a team of Japanese researchers.
The latest work, however, the most rigorous study to date on water turnover, reveals that people have a wide range of water intakes, reports The Guardian. Many only require about 1.5 to 1.8 litres a day, lower than the two litres typically recommended, the research suggests.
“The current recommendation is not supported scientifically at all,” said Yosuke Yamada of the National Institute of Biomedical Innovation, Health and Nutrition in Japan, and one of the paper’s first authors. “Most of the scientists are not sure where this recommendation came from.”
One issue is that previous estimates of water requirements have tended to ignore the water content of food, which can contribute a substantial proportion of our overall intake.
“If you just 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, assessed the water intake of 5 604 people aged between eight days and 96-years-old from 23 countries. The research involved people drinking a glass of water in which some of the hydrogen atoms were replaced by deuterium, a stable isotope of hydrogen that is found naturally in the human body and is harmless.
The rate of elimination of the extra deuterium reveals how quickly water in the body is turned over, and the study found the measure varied widely depending on a person’s age, gender, activity levels and surrounding. Those living in hot and humid climates and at 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 90s. Women aged 20 to 40 had an average turnover of 3.3 litres, which 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 all be drinking 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, a co-author.
“I think it’s a recommendation that many people just ignore and follow what their body is telling them,” he said.
Although drinking more water than your body requires is unlikely to be harmful for health, clean drinking water is not free to produce, the authors point out. “There is a cost to drinking more than we need even if it’s not a health cost,” said Speakman.
“If 40m adults in the UK followed the guidelines and drank half a litre of clean water more than they needed each day that’s 20m litres of wasted water daily.”
Study details
Variation in human water turnover associated with environmental and lifestyle factors
Yosuke Yamada, Xueying Zhang, Mary Henderson, Hiroyuki Sagayama, International Atomic Energy Agency (IAEA) et al.
Published in Science on 24 November 2022
Abstract
Water is essential for survival, but one in three individuals worldwide (2.2 billion people) lacks access to safe drinking water. Water intake requirements largely reflect water turnover (WT), the water used by the body each day. We investigated the determinants of human WT in 5604 people from the ages of 8 days to 96 years from 23 countries using isotope-tracking (2H) methods. Age, body size, and composition were significantly associated with WT, as were physical activity, athletic status, pregnancy, socioeconomic status, and environmental characteristics (latitude, altitude, air temperature, and humidity). People who lived in countries with a low human development index (HDI) had higher WT than people in high-HDI countries. On the basis of this extensive dataset, we provide equations to predict human WT in relation to anthropometric, economic, and environmental factors.
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