A new study shows that the often-recommended eight glasses of water per day may be too much.

The research conducted by the University of Aberdeen in Scotland shows a water intake of eight glasses (around two litres) a day seldom matches our actual needs, and in many situations, is too high.

According to the study published in the Science journal, around half of the daily intake of water comes from food, and therefore people only really need about 1.5 to 1.8 litres per day.

“The original estimate of two 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,” Prof John Speakman from the University of Aberdeen told the BBC.

“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.”

Researchers surveyed 5,604 people from 23 different countries, aged between eight days and 96 years.

People took a glass of water in which some of the hydrogen molecules were replaced by a stable isotope of deuterium, which is a harmless element found naturally in the human body.

The rate at which the extra deuterium is being eliminated shows how quickly water in the body is turning over and people with a higher water turnover usually need to drink more water.

The study found out that people living in hot and humid environments and at high altitudes, as well as athletes and pregnant and breastfeeding women had a high water turnover.

Additionally, energy expenditure was found to be the biggest factor in water turnover and as such, men aged 20-35 recorded the highest values of over an average of 4.2 litres per day.

This decreased with age, averaging 2.5 litres per day for men in their 90s.

On the other hand, women aged 20-40 averaged a turnover of 3.3 litres, which also declined to 2.5 litres by the age of 90.

However, scientists said water turnover is not exactly equal to the requirement for drinking water.

“Even if a male in his 20s has a water turnover 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,” said Prof. Speakman.

“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.

“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,” he added.

On the negative effects of drinking too much water, the professor said: “Clean drinking water isn’t free, 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.”

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