Public awareness campaigns promoting a healthy lifestyle urge us to drink at least 2 litres/ 8 glasses of water a day. According to a recent study, it may be much more than we actually need.
Scientists at the University of Aberdeen published a study this week’s Science journal showing that humans actually only need about 1.3 to 1.8 litres a day, not 2. Professor John Speakman explained for BBC Radio: “The original estimate of 2 litres a day comes from a slight miscalculation. The water that we’d need to drink is the difference between the total water that we need to ingest and the amount that we get from our food. The way they estimated the amount from food was by asking people how much they eat. Because people under-report how much they eat, there’s a misestimate and so you overestimate the amount of water that’s needed.”
The new research, based on 5,604 people aged eight days to 96 years from 23 different countries, involved people drinking a glass of water in which some of the hydrogen molecules had been replaced with deuterium, a stable isotope of hydrogen that occurs naturally in the human body and is harmless. The rate at which excess deuterium is eliminated indicates how quickly water in the body is turning over. People living in hot and humid climates and at high altitudes, as well as athletes, pregnant and breastfeeding women had a higher turnover, meaning they need to drink more water.
“Even if a male in his 20s has a water turnover of on average 4.2 litres per day, he does not need to drink 4.2 litres of water each day,” professor John Speakman said. “About 15 per cent of this value reflects surface water exchange and water produced from metabolism. The actual required water intake is around 3.6 litres per day. Since most foods also contain water, a substantial amount of water is provided just by eating.”
“This study shows that the common suggestion that we should all be drinking eight glasses of water 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.”John Speakman
Drinking the recommended eight glasses of water is probably not harmful, but it is also not necessary in most cases, Speakman noted. “If people on average drink half a litre more than they need and you multiply that by 40 million adults in the UK, that means that we’re needlessly drinking and peeing 20 million litres of water that we have to supply. There is a cost in doing that.”