Health's unkillable urban legend: "You must drink 8 glasses of water/day"

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Well, maybe you don’t, but I sure as hell do.

Personal weirdness aside, I’m a little confused by the argument being made here. Not to say that the myth isn’t a myth - I’m sure it is. What confuses me is the idea that if the average german boy generates a certain reading that that reading doesn’t indicate mild dehydration. It’s actually totally plausible that the average german boy is mildly dehydrated on the average day. Now, if the reading is saying they need to be hospitalized, then sure, that’s BS, but it’s entirely plausible (not saying it’s true, just that it could be true) that we, on average, would benefit from getting more fluids. That might mean drinking more water, it might mean eating more high-water-content food.


I usually drink 2 litres a day but not because I’m told to, because I’m thirsty and if I don’t drink it I get temporary rough dry skin on my hands. Maybe there’s something (else) wrong with me.


You’re saying that drinking water helps prevent dry skin? That doesn’t sound right to me.

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Meh, does for me. My fingers get dry and catch on certain fabrics if I run them along it. There at least seems to be a correlation. And in the words of Randall Munroe, correlation doesn’t imply causation, but it does waggle its eyebrows suggestively and gesture furtively while mouthing ‘look over there’.


I probably wasn’t drinking enough water when I had my kidney stones, though. I try to down at least a couple of pints a day nowadays, more on warm days. Kidney stones aren’t worth trying to mythbust water myths.


So, what you’re saying is that the human body is incapable of finding its own setpoint about how much water to carry? We have to push extra fluid consciously, overcoming our bodies’ own signals, in order to remain healthy? The average must be defined as ‘ill’?

I have a certain amount of anecdotal knowledge about how much is needed. As an example, a week ago, I went through no less than nine litres of water in a day. I was carrying a backpack many km in quite hot weather, and needed every drop of that. And that sort of extreme is the only circumstance in which I try to hydrate proactively: drink extra now, because I know I’ll be needing it by the time my stomach actually absorbs it. Otherwise, there’s a pretty reliable indicator for when I need to drink: I get thirsty. (Also, on a day that sweaty, I need to up my sodium intake. I normally try to watch the salt, but once I’m up to 4 litres or more water a day, I’m losing sodium faster than my normal diet will replace it.)

What’s particularly puzzling about the myth is that there’s really a lot of magical thinking associated with it. If I arrange chicken, and rice, and vegetables nicely on a plate, and have a glass of water beside them, that water “counts.” But if I use exactly the same substances and prepare a soup instead, suddenly that’s “water in food” and doesn’t count toward the magical “eight glasses.” It’s all going to the same place!


Uhm, right. Although I’d describe that as ‘an abnormal physiologic condition relieved by excess water’ rather than necessarily ‘the result of dehydration’. Most of us go through our lives drinking when we’re thirsty, rather than pushing fluid, and still never suffer from kidney stones.

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I try to regularly drink water all through the day on a routine, because I totally don’t even notice when I’m actually thirsty unless I am insanely thirsty. For example. Just reflecting on it now, I realize that I am, in fact, thirsty, and haven’t had anything to drink today. If I hadn’t actually focused on it, I would have happily gone another 2 hours till lunch time without a drink, most likely. That reminds me that I need to set up the reminder on my phone again to actually think about whether or not I’m thirsty. Yes, I’m weird.


I feel like that’s an excessively hostile interpretation of what I said. I said it is plausible (in absence of other knowledge) that an entire population would be healthier if they hydrated more. Our bodies send us lots of signals and lots of people ignore them or aren’t used to listening to them. Especially when it comes to children - I know a child who ended up hospitalized for dehydration just because they wouldn’t drink anything for who knows what reason.

When people are tired they get upset and then rationalize that they are upset for real reasons instead of realizing they should go to bed early. When people are dehydrated they get headaches - what percentage of the population is taking Tylenol or Advil every day?

Defining the average as ill is stupid. But defining it as healthy seems pretty stupid too.


Without access to your advanced internet diagnostic equipment, though, the difference between “abnormal physiologic condition” and “result of dehydration” is moot. I can’t even tell how you think I’m supposed to draw the distinction, or why I would need to.


Well, the NHS disagrees. From

To avoid getting kidney stones, make sure you drink plenty of water each day so that you don’t become dehydrated.

I think the author’s point was that if everybody seems to have a reading that’s above a completely-arbitrary line on a test, then it’s simply more likely that the test is faulty than everybody’s unhealthy.

His point was that the threshold of 800 mOsm/kg was made without any scientific validity behind it. Given that, combined with the fact that the majority of people tested on it are above the threshold while apparently suffering no ill effects, it simply seems that a poor threshold was chosen.

It’s possible, sure, that the vast majority of people are walking around dehydrated without realizing it, but for me I’d say the only reason this possibility “feels” like it might be right, in the absence of evidence that something is harming us, is the general sense that “oh, everybody’s unhealthy in the modern world.” But since the article mentioned several times that they’ve found no measurable benefits at all to drinking more water, this just seems unlikely.

The average person may be dehydrated, but then why wouldn’t giving us more water cause some measurable improvement?

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I’m with Captain Haddock as regards water. Give me booze or tea any day.


The article specifically mentions drinking more water to prevent the recurrence of kidney stones.

Although, contrary to what @knelmes quoted, I haven’t seen any evidence that says it’s caused by dehydration per se, instead of simply being that people who are prone to kidney stones have improved outcomes if they drink more.


Yeah. It’s very rare that I drink anything that isn’t coffee or beer.


What’s happening here is that I’m being a ridiculous pedant and saying that one facet of an otherwise sound case is flawed. The fact that the threshold was set without any scientific reason for setting it there is a good reason to question it. If studies are failing to show that there are any ill health effects associated with that threshold then we have a good reason to discard it completely. I’m only questioning the validity of one part of the argument, and that is appealing to the average.

I don’t think that everybody is unhealthy in the modern world, I think that everybody is extremely healthy in the modern world compared to all historical times. So we know for a fact that at many point in history the average was unacceptably unhealthy (by our standards) and that there was some action that, if taken up by the population at large, would improve health.

I know there is some evidence to suggest we could reduce heart disease by adopting the cultural practice of everyone having afternoon naps. That may or may not be true, but we’d know that based on arguments about the actual health effects of doing so, not an appeal to current averages which would necessarily miss the benefits of things we currently aren’t doing.


Don’t believe the NHS? How about The British Association of Urological Surgeons? From

Poor fluid intake combined with a low-roughage, high protein diet containing a lot of refined sugar increases the risk of forming stones

Or the Mayo Clinic?

Uric acid stones can form in people who don’t drink enough fluids or who lose too much fluid, those who eat a high-protein diet, and those who have gout. Certain genetic factors also may increase your risk of uric acid stones

I get what you’re saying, and I’m all for afternoon naps (except then I can’t sleep at night), but I think the difference is that the article is describing a clinical condition, and saying that “if we use this threshold, then we define the majority of people as having this condition.” That’s different than describing some benefit that might happen if we all decided to do X.

If I found that overdosing on vitamin B12 made everyone a foot taller, it would still be wrong of me to describe everyone below 6’6" as having dwarfism, and saying that it’s just because they’re not getting enough vitamin B12. I think, absent other information, we should generally err on saying that the majority of people represent the healthy status quo, not some clinical condition, even if there may be benefits do doing something that the majority of people don’t do.


Those still don’t describe dehydration, which is a clinical condition, which is what we keep saying. Everything those recommendations say is consistent with the notion that people who are prone to kidney stones can have better outcomes by drinking more water. That’s different from saying that people who get dehydrated are prone to kidney stones.

(They also cite no sources.)

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