Myth: You Need Eight Glasses of Water a Day

Somewhere in every office is a bottle with the hours marked on its side, eight o'clock at the top and six at the bottom, so that its owner can see at a glance whether the day's water is on schedule, and somewhere in every gym is a sign that says the body is sixty percent water and asks whether you have had your eight glasses. The rule is one of the most widely known pieces of health advice in the world: eight glasses of water a day, roughly two litres, drunk as water, on top of whatever else is eaten or drunk, and more in the heat. It is in the magazines, on the bottle labels and in the mouths of people who have never read a nutrition paper, and the question this article asks, on a site about water, is where it came from and whether it is true.

Looking for the source

In 2002 a kidney physiologist at Dartmouth, Heinz Valtin, who had spent his career on how the body handles water, set out to find the origin of the rule, and reported that he could not. There was no study. There was no agency that had issued it. The nearest thing he found was a sentence in a 1945 report of the Food and Nutrition Board of the American National Research Council, which said that a suitable allowance of water for an adult was about two and a half litres a day, and then, in the next sentence, that most of this quantity is contained in prepared foods. Two and a half litres is about eight glasses of 300 millilitres. Somewhere in the decades that followed, Valtin concluded, the first sentence survived and the second was lost, and a total that included the water in bread, fruit, soup and coffee became a target for plain water on top of everything, which is a different thing by about a litre and a half. He found no evidence that healthy adults in a temperate climate needed the rule, and some that the kidneys of a person who followed it were simply passing the extra through.

What is actually recommended

The food agencies have set figures since, and they look like the 1945 board's. The American Institute of Medicine in 2004 put the adequate intake of total water at about 3.7 litres a day for men and 2.7 for women, which are higher than Europe's because they were set from what healthy Americans were observed to drink rather than from need; the European Food Safety Authority in 2010 put it at 2.5 and 2.0. Both counted all sources, and both said that about a fifth of it arrives in food, more in a diet of fruit, vegetables and soup and less in one of bread and cheese, and that the rest can come from any drink. Coffee and tea count: the idea that caffeine dehydrates, which was the usual reason for excluding them, does not survive measurement, because the mild diuretic effect of the caffeine is smaller than the water in the cup. Milk counts, juice counts, and so does beer, within the limits that have nothing to do with water. Neither agency says eight glasses of water. Neither says plain water at all.

Measuring it

In 2022 a group led from Japan published, in Science, the first large measurement of how much water people actually turn over, using an isotope method in which a person drinks water with a known amount of a heavy form of hydrogen and the rate at which it is diluted out of the body over the following weeks gives the water that passed through. They measured 5,604 people in twenty three countries, from eight days old to ninety six years, and found a range from about one litre a day to six. The number depended on body size, on physical activity, on the temperature and humidity where the person lived, on altitude and on age: a large active man in a hot country turned over six litres, an elderly woman in a cool one turned over under two, and the average for a man of twenty to forty was about four litres, of which, again, a share came from food and a share was made inside the body by its own chemistry. The paper's conclusion was the one Valtin had reached from the other direction: there is no single figure for everyone, and the usual one is too high for most of the people who follow it and too low for some who do not.

When it matters

There are people who need a rule, and the rule they need is not eight glasses. The old lose the sense of thirst, which is why dehydration is a routine cause of confusion and admission in the elderly and why care homes count the cups; a person with a fever, diarrhoea or vomiting loses water faster than thirst reports, which is why the cholera article on this site is about a drink; a person with kidney stones is told to drink more, because dilute urine makes fewer; and a worker or an athlete in heat can lose a litre an hour in sweat and needs to drink to a schedule, with salt, because thirst lags the loss. At the other end, the rule has killed. Marathon runners who drank at every station, as the advice of the 1990s told them to, arrived at the finish with blood sodium diluted below the level the brain tolerates, and a study of the Boston Marathon in 2005 found that thirteen percent of the runners it tested had it, and a few, in that race and others, died of it. Hyponatraemia from overdrinking has also killed a woman in a radio contest to see who could drink the most water and soldiers made to drink on a schedule in training. Too much water is a rarer problem than too little and a real one, and the eight glasses rule has no number for the top.

Who sold the rule

The rule survived without a source because it had a sponsor. The bottled water industry grew from a curiosity to a business of a hundred billion dollars a year between 1980 and 2020, and its marketing leaned, through those decades, on the idea of a daily quota and of a population walking around dehydrated without knowing it: the phrase by the time you are thirsty it is too late, which has no basis in the physiology, came out of the sports drink advertising of the 1990s and passed into the magazines as fact. The bottled water article on this site has the industry's water; its health claims are the same campaign. The eight glasses rule was convenient to it because it made water a product to be consumed on schedule rather than a need met from a tap when the body asked, and the bottle with the hours on its side is the rule's monument. None of which makes drinking water bad for anyone. It makes the quota a sale rather than a science.

Thirst

The reason no rule is needed for most people is that the body has one. The concentration of the blood is measured continuously by cells in the brain, and a rise of one or two percent, which is a few hundred millilitres short, produces thirst and, at the same time, a hormone that tells the kidneys to hold water back, so that the urine darkens and the person reaches for a glass. A fall in concentration does the reverse. The system has kept every animal with a backbone within a narrow band of hydration for four hundred million years, and in a healthy adult with water available it works to within a few hundred millilitres, which is why the people who drink to a schedule and the people who drink when thirsty end up, on measurement, in the same place, with the first group passing more urine. Pale urine is the sign most doctors give, and it is a better one than a count of glasses.

Where this site stands

This site is about water, and the question of whether people drink enough of it has a bearing on the two litres a day that cities plan for, on the bottled water industry, whose marketing has leaned on the eight glasses for forty years, and on the plain fact, which the cholera and the Day Zero articles describe, that for a share of the world's people the question is whether there is any safe water to drink at all rather than whether they have reached a quota. In a city with a tap that runs, the two litres a person a day that a body needs are the smallest item in the household's water, under one percent of what goes down the drains, and the myth of the eight glasses is a myth about health rather than about supply. It costs nothing to believe, except the bottle on the desk.

Verdict

False, as a rule for everyone. The number came from a 1945 sentence whose second half was lost, the food agencies count all drinks and all food toward a total that most people reach without trying, and the measurement of thousands of people shows a range from one litre to six that no single figure can serve. Thirst does the job in a healthy adult. The old, the ill, and the athlete in the heat need a plan, and the plan is not eight glasses.

Two and a half litres in 1945, most of it in the food, and a bottle with the hours on its side.