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How much water should you drink every day? Not as much as you've been told. This summer, like many summers before it, has seen a rash of articles warning us not only that is dehydration is dangerous, but that it is also ubiquitous. Real dehydration, when your body has lost a significant amount of water because of illness, excessive exercise or sweating, or an inability to drink, is a serious issue. But people with clinical dehydration almost always have symptoms of some sort.

These reports are worried about something completely different. They fear that otherwise healthy adults and children are walking around dehydrated, so many that dehydration has reached epidemic proportions. Under scrutiny, however, this assertion fails to hold water. That's the topic of this week's Healthcare Triage.

This was based on a piece Aaron wrote for the Upshot a while ago. If you want to read more, links can be found there: http://www.nytimes.com/2015/08/25/upshot/no-you-do-not-have-to-drink-8-glasses-of-water-a-day.html

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Aaron Carroll: This summer, like many summers before it, has seen a rash of articles warning us not only is dehydration dangerous, but that it's also ubiquitous.

Real dehydration -- when your body has lost a significant amount of water because of illness, excessive exercise or sweating, or an inability to drink -- is a serious issue. But people with clinical dehydration almost always have symptoms of some sort. The reports you read are almost always worried about something completely different. They fear that otherwise healthy adults and children are walking around dehydrated. So many, that dehydration has reached epidemic proportions. Under scrutiny however, this assertion fails to hold water. That's the topic of this week's Healthcare Triage.

-- Healthcare Triage intro theme music plays --

We, as a society, have been obsessed about not getting enough water for decades. I co-authored a paper back in 2007 in the BMJ on medical myths that lead off with the idea that people should drink at least eight glasses of water a day. This paper got more media attention than pretty much any other research I've ever done. It made no difference.

When, two years later, we published a book on medical myths that once again exposed the idea that we need eight glasses of water to be untrue, I thought that would convince people to stop worrying. I was wrong again.

Many people believe that the idea for this myth began with a 1945 Food and Nutrition Board recommendation that said that people need about two and a half liters of water a day. But they ignored the sentence that followed closely behind. It read, "Most of this quantity is contained in prepared foods."

Water is present in fruits and vegetables. It's in juice. It's in beer. It's even in tea and coffee. And before any of you start commenting that coffee is going to dehydrate you, research shows that's not true either.

While I recommended water as the best beverage to consume back in our episode on food recommendations, it's certainly not your only source of hydration. You don't have to consume all the water you need through drinks. You also don't need to worry so much about never feeling thirsty. The human body is finely tuned to signal you to drink long before you're actually dehydrated.

Contrary to many stories you may hear, there's no real scientific proof that, for otherwise healthy people, drinking extra water has any health benefits. For instance, reviews have failed to find that there's any evidence that drinking more water keeps skin hydrated, looking healthier, or wrinkle free. It is true that some retrospective cohort studies have found increased water consumption to be associated with better outcomes, but these are all subject to the usual epidemiological issues, such as an inability to prove causation. Moreover, they define "high water consumption" at far fewer than eight glasses. Prospective studies fail to find benefits in kidney function or all-cause mortality when healthy people increased their fluid intake. Randomized controlled trials failed to find benefits as well, with the exception of specific cases, like in preventing the recurrence of some kinds of kidney stones.

A significant number of advertisers and media stories are trying to convince you otherwise. The number of people who carry around water each day seems to be larger every year. Bottled water sales continue to increase dramatically.

This summer's rash of stories was inspired by 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 4134 kids, age 6 to 19. Specifically, they calculated the kids' mean urine osmolality, which is a measure of how concentrated your urine is. The higher the value, the more concentrated your urine. They found that more than half of children had a urine osmolality of 800 milliosmoles per kilogram or higher. They also found that children who drank eight ounces or more of water a day had, on average, a urine osmolality of about 8 mOsm/kg less than those who didn't.

So if you define dehydration by having a urine osmolality of 800 mOsm/kg or higher, the findings of this study are really concerning. This article did that. The problem is that most clinicians don't. I'm a pediatrician, and I can tell you that I have rarely -- if ever -- used urine osmolality as the means by which I decide if a child is dehydrated. When I ask colleagues, none thought 800 mOsm/kg was a value at which they'd be concerned.

I did a quick search of the internet, and most sources I found thought that values up to 1200 mOsm/kg were still in the physiologically normal range, and that kids varied more than adults. None declared that 800 mOsm/kg is where we'd consider kids dehydrated. In other words, there's very little reason to believe that children who have a spot urine osmolality of 800 mOsm/kg should be worried. In fact, back in 2002, a study was published in the Journal of Pediatrics, one that was more exploratory than looking for dehydration, and it found that boys in Germany had an average urine osmolality of 844 mOsm/kg. The third to last paragraph in the paper recounted a huge number of studies from all over the world finding average urine mOsm/kg in children ranging from 392 in Kenya to 964 in Sweden.

This hasn't stopped more recent studies from continuing to use the 800 mOsm/kg definition 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 aren't getting enough water. Another in the journal Public Health Nutrition used it to declare that almost two thirds of children in Los Angeles weren't getting enough water. Both those studies, by the way, were funded by Nestlé Water or their subsidiaries.

It is totally possible that there are children who could be better hydrated, but at some point we're at risk for calling an ordinary health state a disease. And we made an episode on that. When two thirds of healthy children are found, year after year, to have a laboratory value that you're labelling "abnormal," it may be the definition, and not their health, which is off.

None of this has slowed the tidal push for more water. It's even been the thrust of Michelle Obama's Drink Up campaign. As a spokesperson declared (and I'm quoting), "Forty percent of Americans drink less than half the recommended amount of water daily." There is no formal recommendation for a daily amount of water people need. That amount obviously differs by what people eat, where they live, how big they are, and what they're doing.

But as people in this country live longer than ever before, and have arguably freer access to beverages than at almost any time in human history, it's odd that we keep hearing that we're all dehydrated.

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Healthcare Triage is supported in part by viewers like you through Patreon.com, a service that allows you to support the show through a monthly donation. Your support helps us to make this bigger and better. We'd especially like to thank our Research Associate, Cameron Alexander, and our first-ever Surgeon Admiral, Sam. Thanks, Cameron. Thanks, Sam. More information can be found at Patreon.com/healthcaretriage.