Eight glasses of water a day: the rule a 2002 review could not find evidence for

Few health rules are as widely known as eight glasses of water a day, and few are as hard to trace. It turns up on water bottles, in office posters and in well-meaning advice from friends. Ask where it comes from and the answer is usually “doctors say so”, which is a description of how often it’s repeated rather than a source.

In 2002 Heinz Valtin, a kidney physiologist at Dartmouth, went looking for the source. His review, published in the American Journal of Physiology, is still the clearest account of what he found, and what he didn’t.

What the review looked for

Valtin searched for scientific studies showing that healthy adults living in a temperate climate need to drink at least eight 8-ounce glasses of water a day, roughly two litres, on top of whatever else they eat and drink. He found none. He also found no good evidence for the familiar companion claims: that coffee and tea don’t count, or that by the time you feel thirsty you’re already dangerously dehydrated.

That isn’t evidence that drinking water is bad, or that eight glasses is too much. It’s evidence that the specific rule was never tested before it became common knowledge.

The forgotten second sentence

The most likely origin Valtin identified was a 1945 recommendation from the Food and Nutrition Board in the United States. It suggested about 2.5 litres of water a day for adults, roughly one millilitre per calorie of food. The next sentence said that most of that quantity is contained in prepared foods.

The rule survived because its first sentence was memorable. The second sentence, the one that said most of the water comes from food, didn’t make it into the slogan.

What the later reference values say

In 2004 the National Academies set adequate intakes for total water: about 3.7 litres a day for men and 2.7 litres for women. Those figures count everything, including water, coffee, tea, milk, juice and the water inside food, which supplies roughly a fifth of the total. The same report noted that most healthy people meet their needs by drinking when thirsty and with meals.

Read carelessly, 2.7 litres looks like it supports eight glasses and then some. Read properly, it’s the total from every source. Take away what comes from food and the drinks part for many women lands close to eight glasses, but that includes the coffee, the tea and the soup, not only plain water.

What it doesn’t cover

None of this applies to everyone. People working hard in heat, endurance athletes, people who are ill with vomiting or diarrhoea, and older adults whose sense of thirst has dulled may need more, or need to drink more deliberately. People with heart failure or kidney disease may be told to drink less. For them the right amount comes from their doctor, not from a slogan or from this magazine.

And the opposite risk is real, if uncommon. Drinking very large amounts of water in a short time, as some endurance athletes have done, can dilute the sodium in the blood dangerously.

What we’d actually do

Drink when you’re thirsty and with meals. Count tea, coffee and milk as part of your fluid, and remember that fruit, vegetables, soup and other water-rich foods count too. A rough check that most people can use: pale-yellow urine through the day usually means you’re getting enough.

If carrying a bottle and counting glasses helps you drink more than you otherwise would, and you tend to forget, keep doing it. Just don’t treat eight glasses as a medical requirement or worry on the days you have six. The evidence never said you had to.