Water shock! Why eight glasses a day might be too much

DRINKING the recommended eight glasses of water a day may be too high for our actual needs, according to scientists.

Given around half of our daily intake of water comes from food, researchers estimate we only really need around 1.3 to 1.8 litres a day rather than the recommended two litres.

Previous studies depended on data from small samples of people, but more than 90 scientists across the world have now collaborated to measure water turnover using a different technique.

They surveyed 5,604 people aged between eight days and 96 years old from 23 countries. Participants drank a measured amount of water enriched with the element deuterium, which is found naturally in the human body and is completely harmless.

Experts then measured the rate at which the deuterium disappeared from the body, which revealed how quickly water in the system was turning over.

Analysis published in the journal Science revealed those living in hot and humid environments and at high altitudes, as well as athletes and women who are pregnant or breastfeeding, need more water as their turnover was higher.

Professor John Speakman, one of the authors from the University of Aberdeen, said 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 4.2 litres per day, he does not need to drink 4.2 litres of water each day,’ he 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. 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.’

Drinking the recommended eight glasses is probably not harmful, he added, but also not needed in most circumstances.

‘The calculations suggest that a typical middle-aged man might need to drink 1.6 to 1.8 litres per day and a typical woman of the same age about 1.3 to 1.4 litres,’ he said. ‘For people in their 80s that might fall to about 1.1 litres a day.

‘But it depends a lot on what types of food they eat. If they drink lots of soup, for example, they would need to drink less.’

The likes of tea, coffee, fizzy drinks and fruit juice also count towards the daily total, he said, but they also have other things in them which may not be healthy.

Co-author Dale Schoeller, from the University of Wisconsin-Madison, said: ‘The science has never supported the old eight glasses thing as an appropriate guideline.

‘But this work is the best we’ve done so far to measure how much water people actually consume on a daily basis – the turnover of water into and out of the body – and the major factors that drive water turnover.’

The Eatwell Guide, developed by public health experts, states that adults should drink six to eight cups or glasses of fluid per day. Water, lower-fat milk and sugar-free drinks, including tea and coffee, all count.

It is thought the first reference to the supposed need for eight glasses of water was made by the Food and Nutrition Board of America’s National Research Council in 1945.

The recommendation did note that people could also get water from certain foods, but this part appears to have been forgotten.

Previous studies have also relied heavily on volunteers to recall and self-report their water and food consumption.