No, you don't have to drink 8 glasses of water a day

Urine osmolality is rarely used by clinicians as a means to find out if a child is dehydrated or not.

It can be found in fruits, vegetables and other beverages and there is no proof that consuming more water has any health benefits

If there is one health myth that will not die, it is this: You should drink eight glasses of water a day.

It is just not true. There is no science behind it. And yet, we are inundated with news media reports warning that dehydration is dangerous and common.

These reports work up a fear that otherwise healthy adults and children are walking around dehydrated, and that dehydration has reached epidemic proportions.

Let us put these claims under scrutiny. I was a co-author of a paper back in 2007 in The BMJ on medical myths. The first myth was that people should drink at least eight 240ml glasses of water a day.

This paper got more media attention, including The New York Times', than other research I had done. But it made no difference.

Water is present in fruits and vegetables. It is in juice, beer, and even in tea and coffee. Before anyone writes to me to say coffee is going to dehydrate you, research shows that is not true either.

Two years later, I co-wrote a book on medical myths that once again debunked the idea that we need eight glasses of water a day. I thought it would persuade people to stop worrying.

But I was wrong. Many believe the source of this myth was a 1945 Food and Nutrition Board recommendation that said people need about 2.5 litres of water a day. But most people ignored a sentence that read: "Most of this quantity is contained in prepared foods."

Water is present in fruits and vegetables. It is in juice, beer, and even in tea and coffee. Before anyone writes to me to say coffee is going to dehydrate you, research shows that is not true either.

Although I recommend water as the best drink to consume, it is not your only source of hydration. You do not have to consume all the water you need through drinks. You also do not need to worry so much about never feeling thirsty.

The human body is finely tuned to signal you to drink long before you are actually dehydrated.

Contrary to many stories you may hear, there is no real scientific proof that, for otherwise healthy people, drinking extra water has any health benefits.

For instance, reviews have failed to find there is any evidence that drinking more water keeps skin hydrated and makes it look healthier. It is true that some retrospective cohort studies have found increased water to be associated with better outcomes, but these are subject to the usual epidemiological problems, such as an inability to prove causation. Moreover, they defined "high" water consumption at fewer than eight glasses.

Prospective studies fail to find benefits in kidney function or all-cause mortality when healthy people increase their fluid intake. Randomised controlled trials fail to find benefits as well, with the exception of specific cases - for example, preventing the recurrence of some kinds of kidney stones.

Real dehydration, when your body has lost a significant amount of water because of illness, excessive sweating or an inability to drink, is a serious issue. But people with clinical dehydration almost always have symptoms of some sort.

A significant number of advertisers and news media reports are trying to convince you otherwise. The number of people who carry around water each day seems to be increasing every year. Bottled water sales continue to grow.

In a recent study in the American Journal Of Public Health, researchers used data from the National Health and Nutrition Examination Survey from 2009 to 2012 to examine 4,134 children aged six to 19.

Specifically, they calculated their mean urine osmolality, which is a measure of urine concentration.

The higher the value, the more concentrated the urine. They found that more than half of the children had a urine osmolality of 800 mOsm/kg or higher.

They also found that children who drank eight ounces (236.5ml) or more of water a day had, on average, a urine osmolality about 8 mOsm less than those who did not.

So if you define "dehydration" as a urine osmolality of 800 mOsm/kg or higher, the findings of this study are really concerning. This article did.

The problem is that most clinicians do not. I am a paediatrician and I can tell you that I have rarely used urine osmolality as the means by which I decide if a child is dehydrated. When I asked colleagues, none thought 800 mOsm/kg was the value at which they would be concerned. And in a Web search, most sources I found thought values up to 1,200 mOsm/kg were still in the physiologically normal range and that children varied more than adults. None declared that 800 mOsm/kg was where we would consider a child to be dehydrated.

In other words, there is very little reason to believe that children who have a spot-urine measurement of 800 mOsm/kg should be worried.

In fact, back in 2002, a study was published in the Journal Of Paediatrics, one that was more exploratory in nature than a look for dehydration, and it found that boys in Germany had an average urine osmolality of 844 mOsm/kg. One paragraph in the paper recounted a huge number of studies from all over the world that found average urine mOsm/kg in children ranged from 392 mOsm/kg in Kenya to 964 in Sweden.

That has not stopped more recent studies from continuing to use the 800 mOsm/kg standard to declare huge numbers of children to be dehydrated. A 2012 study in the Annals Of Nutrition And Metabolism used it to declare that almost two-thirds of French children were not getting enough water.

Another in the journal Public Health Nutrition used it to declare that almost two-thirds of children in Los Angeles and New York City were not getting enough water. The first study was funded by Nestle Waters and the second by Nestec, a Nestle subsidiary.

It is possible that there are children who need to be better hydrated. But at some point, we are at risk of calling an ordinary healthy condition a disease.

When two-thirds of healthy children, year after year, are found to have a laboratory value that someone is labelling "abnormal", it may be the definition and not their health that is off.

None of this has slowed the tidal push for more water. It has been part of Mrs Michelle Obama's "Drink Up" campaign. In 2013, Mr Sam Kass, then a White House nutritional policy adviser, declared: "Forty per cent of Americans drink less than half of the recommended amount of water daily."

There is no formal recommendation for a daily amount of water people need. That amount differs by what a person eats, where he lives, how big he is and what he does.

But as people in this country live longer than before, and have arguably freer access to beverages than at almost any time in human history, it is just not true that we are all dehydrated.


•Aaron E. Carroll is a professor of paediatrics at Indiana University School of Medicine in the United States.

A version of this article appeared in the print edition of The Straits Times on August 28, 2015, with the headline 'No, you don't have to drink 8 glasses of water a day'. Print Edition | Subscribe