Scientists say eight glasses of water a day may be too much
New research has found that the recommended eight glasses of water a day may be too much.
Scientists from the University of Aberdeen discovered the recommended intake of two litres of water a day was often more than people needed.
Given around half of the daily intake of water comes from food, scientists estimate people only really need about 1.5 to 1.8 litres per day.
findings were published in Science this week.
Previous research into water requirements used surveys applied to small samples of people.
Prof John Speakman from the University of Aberdeen told BBC Radio's Good Morning Scotland: "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.
"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."
But scientists have now collaborated across the world to measure exact water turnover using a stable isotope technique.
urveyed 5,604 people from 23 different countries and aged between eight days and 96 years old.
Research involved people drinking a glass of water in which some of the hydrogen molecules were replaced by a stable isotope of the element called deuterium. 
It is found naturally in the human body and is completely harmless.
rate of elimination of the extra deuterium shows how quickly water in the body is turning over.
People with a higher water turnover usually need to drink more water.
Research found that this included those living in hot and humid environments and at high altitudes, as well as athletes and pregnant and breastfeeding women.
Energy expenditure is the biggest factor in water turnover. The highest values were observed in men aged 20-35, who turned over an average of 4.2 litres per day.
reased with age, averaging 2.5 litres per day for men in their 90s.
Women aged 20-40 averaged a turnover of 3.3 litres, which also declined to 2.5 litres by the age of 90.
But water turnover is not exactly equal to the requirement for drinking water, Prof Speakman said.
He said: "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.
"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 said the research represented a big step forwards in predicting future water needs.
But there can be negatives in drinking too much water.
"Clean drinking water isn't free," Prof Speakman said.
"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."