Eight glasses of water a day? Where the rule comes from and what science says

Many people believe they must drink at least eight glasses of water a day, about two litres, and that coffee or tea do not count. Some carry large bottles marked for every hour of the day. Is it necessary?

Where the rule comes from

In 2002 the American physiologist Heinz Valtin of Dartmouth Medical School looked for the scientific evidence for the eight-glasses rule. He found no study supporting it for healthy adults in a temperate climate. He suggested that the rule may have come from a 1945 recommendation which said an adult needs about 2.5 litres of water a day, but added that most of this amount is found in food. The second sentence was forgotten.

What EFSA recommends

In 2010 the European Food Safety Authority set reference values for total water intake, including drinks of all kinds and water in food:

• About 2.0 litres a day for women.
• About 2.5 litres a day for men.

About 20 per cent of this water usually comes from food, such as fruit, vegetables, soups and yoghurt. The rest comes from drinks: water, tea, coffee, milk and others. Studies show that coffee and tea, in usual amounts, contribute to hydration, contrary to what is often said.

The body has a perfect alarm system for water. It is called thirst.

Thirst as a signal

In most healthy adults, thirst is a reliable signal. The body regulates its water very precisely through the kidneys and hormones. The colour of urine is a simple indicator: pale yellow usually shows adequate hydration.

When more care is needed

• In hot weather and during long physical activity, when a lot of water is lost through sweat.
• In older adults, because the sense of thirst weakens with age. It is worth drinking regularly, even without strong thirst.
• In young children, who do not always express thirst.
• During illnesses with fever, vomiting or diarrhoea.
• With some heart or kidney conditions, where the doctor may recommend limiting fluids. In these cases, follow your doctor's advice.

Drinking too much water, in very large amounts within a few hours, can also be dangerous. Balance matters.