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Why Do We Think We Need 8 Glasses of Water a Day? · So That's Why

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Welcome to the, so That's Why podcast, where we unpack the
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science behind everyday questions.
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I'm Jen.
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I'm Chris.
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And I'm Matt.
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Right.
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So today we're looking at something that's been bugging me for ages.
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Why do we think we need eight glasses of water daily?
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I can't tell you how many times I've been told this, by literally everyone
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from my mom to fitness influencers to that one coworker who carries
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around a huge bottle of water.
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Why are you pointing at me?
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I just said, I dunno.
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I am definitely not that coworker.
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Here's what's mental.
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We're basically the only animals that have convinced ourselves we need to
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force feed ourselves water on a schedule.
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You don't see wild animals setting a phone alarm to remind themselves to
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drink, and they seem to be doing fine.
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Yep.
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It's totally true.
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Yeah.
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Great observation.
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So this whole kind of eight glasses a day thing is, it's a
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rule that's come so deeply embedded in, um, everything that we do.
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It's almost like over in our body's natural programming.
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So hydration is of course really important, but we've got this
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sophisticated system to manage it for us.
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It's called thirst.
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Thirst.
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I'll obey my thirst, said a well-known drinks company who we won't get into.
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Other drinks are available, but there's got to be some science behind this, right?
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Yeah.
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So here's where it gets interesting.
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Well, depending on your definition of interest.
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Uh, so the original eight glasses a day thing actually comes
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from a misread recommendation.
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From a report all the way back in 1945, which suggested we've gotta take
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in 2.5 liters of total fluid per day.
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But that's not just from drinking water, so that means fluids from
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food, from coffee, tea, even beer.
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But somewhere along the way that kind of 2.5 liters of total fluid as turned into
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eight glasses of just water on its own.
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So what you're saying is all this time we've been glued to our water
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bottles because of a misunderstanding.
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Yep.
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Exactly.
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Now.
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It goes without saying good hydration is of course, like vitally important.
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Mm. But so to balance, so there was a, a piece of research on 5,600 people in,
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I think 23 countries, and they used this clever technique called isotope tracking.
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To reveal something really interesting about what your body actually needs.
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And lemme guess, right?
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It's not eight glasses, is it?
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Correct?
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It is probably not eight glasses.
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And actually when we force ourselves to over hydrate, we're putting
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unnecessary strain on our bodies.
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For example, our kidneys have gotta work overtime processing all the extra water.
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So what does the research actually show?
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So that study I referred to, it shows that.
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Like young adult males actually need about 1.5 to 1.8 liters of drinking water a day.
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That's roughly six or seven glasses.
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And for females, a little bit less, like 1.3 to 1.4 liters.
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Okay?
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Okay.
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1.3, 1.5. I dunno what that is.
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So how much have we been over hydrating?
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So for many people, as much as 30 to 40% more than needed every day.
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And the thing is like over hydration isn't good.
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So, as Joe mentioned, your kidneys are working harder to eliminate
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this excess water and it can disrupt your electrolyte balance.
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And you kind of train yourself to ignore your thirst instincts.
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So this explains all those times I felt like a failure 'cause I couldn't
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quite finish my daily water quota.
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Plus there's the guilt isn't there around.
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Around that.
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And like, I've got friends.
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I'm not saying it's me.
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It's not me.
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Uh, maybe, uh, you feel guilty about having a cup of tea instead
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of, you know, a glass of water.
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I tell you, the guilt is real.
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The guilt is real.
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And that's, and this guilt is made based on that misinterpretation of
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recommendations from all those years ago.
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Because realistically, a lot of us believe our intake is too low.
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But actually in many cases, we're probably perfectly hydrated.
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Okay.
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So how did you get from a recommendation that made perfect sense?
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To hear,
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so dare I say it.
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Go on,
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go on.
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Delay it.
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There could be commercial interests from, uh, the rather large bottled water
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industry, which kind of exploded on the scene in the seventies and eighties,
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and that might have helped spread the word about this eight glasses per day.
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So originally the US Foods and Nutrition Board recommended about
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two and a half liters of fluid daily.
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Yeah, but that's the thing.
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This included water from every source.
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So not just drinking a glass of water.
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It's water from food and from all beverages.
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And that includes coffee, tea, soft drinks, even alcoholic drinks.
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And that's all the part that got lost in this kind of eight glasses per day.
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So it's like a, a massive.
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Sort of multi-decade game of Chinese whispers then,
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but surely there's gotta be some scientific backing
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for a higher water intake.
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Yeah.
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So again, as I said earlier, there's no question that like
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good hydration's essential mm-hmm.
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Um, it's really a case of like questioning what are those
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quantities that get banded around.
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And there was, um, there's a well-known like kidney specialist called Dr.
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Hines Valton, and he spent years trying to prove, find proof of
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this eight glasses per day thing.
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And do you know what his conclusion was?
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He said, basically, I can't find any evidence that every person must drink
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at least eight glasses a day, or they're gonna, you know, spontaneous combust.
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So a kidney expert couldn't find evidence for what we've all been told is essential.
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Exactly right.
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But tell us more about the isotope research.
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Yeah, so isotope tracking, it's a bit like giving water
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molecules like little name tags.
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So that we can follow 'em around the body.
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And so what happened?
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These participants would drink water containing this special chemical marker
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that I'm not gonna try and pronounce.
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And, um, researcher would be able to track how quickly your
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body processes that water.
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And what did they find?
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Unsurprisingly, they found loads of variation.
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But to put this into perspective, so some people need only
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one liter of water per day.
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Some people need 10 liters or more.
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So there could be variations of like a thousand percent.
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And it, it varies based on age, based on activity, level on climate, on body size.
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So everyone's thinking they need eight, exactly.
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Eight glasses of water.
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It's like everyone trying to wear the same shoes, same size shoes.
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Yeah, no, perfect analogy really.
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And, um, because your body manages hydration through
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sensors in, in your brain.
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And it pretty much tastes blood for salt.
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That sounds really weird, doesn't it?
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But it, it's true.
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And if the salt, if the blood is too salty, that triggers
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your thirst sensations.
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If it's too watery, it sends signals to your kidneys to
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eliminate that excess fluid.
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It tastes blood.
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Love this.
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I've just with a nice chii, right?
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I'd rather not think about
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that.
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Yeah.
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To say it.
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So I, I. Maybe a better reference then is, is what I'm hearing is
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our bodies have like got a built-in sort of gauge, like a petrol gauge.
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Exactly, yeah.
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And it's course a pretty sophisticated one that's been refined
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over what, thousands of years.
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See I just assumed that by the time I feel thirsty I'm already dehydrated.
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Yeah.
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So there is this kind of thirst is too late thing and, but this idea
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comes from, uh, studies on athletes in extreme conditions or for elderly
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people where they've got impaired.
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Um, mechanisms.
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So for most healthy people in normal conditions, thirst is a perfectly
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fine and excellent indicator.
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So what you're saying is we've pathologized perhaps
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a normal body function.
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Yeah, in many cases.
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So if you think about it, you know, your body detects when you need food.
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It detects when you need to sleep, when you need to go to the bathroom.
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So why would water detection be uniquely unreliable?
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Yeah.
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Yeah, good point.
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What about coffee?
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I've heard that that's dehydrating.
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So great news for coffee fans.
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Although coffee gets, its like reputation for dehydrating, it's
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normally actually totally fine and, uh, can actually contribute to hydration
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because even though, um, the caffeine is known to be, have a kind of mild
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diuretic effect, the water content, the coffee more than compensates for it.
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So a morning coffee.
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Counts towards hydration, which is, let me tell you, this is good
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news for my coffee addicted son.
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He loves coffee.
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Yeah.
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So the key is normal amounts.
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So sorry to burst any, uh, coffee lovers bubbles, but don't go over doing it.
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But yeah, for most people, normal amounts of tea, coffee, even beer,
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they all count towards your hydration.
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Beer
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count.
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The
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tea is, yeah,
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beer counts.
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This is awesome.
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As much better than coffee, let me tell you.
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Uh, but obviously, I mean, you know, I can't have a beer
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at 10 o'clock in the morning.
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So let's get practical.
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What does this look like for different people?
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So if we start by taking, uh, a typical everyday office worker,
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so males need roughly kinda 1.8 liters of total fluid a day.
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And for females a little bit less, maybe 1.3 liters.
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But again, that includes your morning coffee, your afternoon tea, and
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water that comes from your food.
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So if I think about my day, if I have two cups of tea, a smoothie
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at lunch maybe, and then like three to four glasses of water.
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Throughout the afternoon and evening, I'm sorted
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most likely.
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Yeah.
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Uh, but it always depends on the circumstances.
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So if we were to contrast that office worker with, let's say a
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construction worker based in a very hot and sunny climate, their fluid
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ranges are, are gonna be much higher.
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Fluid needs are higher.
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So it could range from 3.5 to six liters per day.
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So much more than that office worker would need.
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Yeah.
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Yeah.
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Which is fair, right?
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It's like the guy laying tarmac in.
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40 degree heat needs more than someone whose biggest challenge
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is, may be going to the well to the label printer on the desk in there.
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Right?
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So we talked about variation.
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What about older adults?
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How do I need to change as we get older?
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Yeah.
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So aging's fun, right?
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Um, so people over 65 have diminished thirst sensation and also their.
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They've got less total body water content, so they do need to pay
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more attention to hydration and their kind of hydration strategy.
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Mm-hmm.
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So do genetics affect it then?
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They certainly do.
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Yeah.
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So genetics affect hydration needs, but personality traits can too.
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So people with higher anxiety tend to have higher water turnover, which
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could be linked to the fact that you've got increased breathing rate.
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Mm-hmm.
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Increased heart rate.
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Mm-hmm.
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So some people then will stress drink a bit like.
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Maybe I, maybe not me.
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Somebody out stress eats chocolate.
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You know,
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me, me, you.
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No, exactly.
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And the other thing to bear in mind is life stage.
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Uh, life stages, sorry.
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So, life stages change.
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Hydration needs two, but maybe not as much as we sometimes think.
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So pregnant women, for example, would need maybe 300 mil of water,
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milliliters of water more per day.
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Someone breastfeeding might need.
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700 mil more per day.
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So you do need more, but maybe not to the level that people think.
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So there's this tendency for pregnant women or breastfeeding women to think
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they've got a guzzle, you know, from sunrise to sunset to get that hydration.
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Okay.
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So what about kids then?
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Because I mean, it was a while ago, IGI grunt you, but I remember chasing
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my kids around with, you know, water bottles trying to get them to drink.
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Oh yeah,
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yeah.
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So children are brilliant at kind of self-regulating.
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So when they're thirsty, you'll be the first to know.
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Um, but interestingly, again, depending on what your class is, interesting, milk
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is actually more hydrating than water.
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Due to its electrolyte content.
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So baby and infant formulas and milks do a very good job.
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Wait, milk is more hydrating than water?
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Yep.
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Because milk can retain fluid about 50% better than water because of
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its sodium and protein content.
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Wow, I did not know that.
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There you go.
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Fact, every
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day's, every a school
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day.
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It is.
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Exactly.
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So what about electrolyte drinks?
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I use them from time to time, and I definitely feel better when I do.
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Yeah.
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So for most people doing everyday activities, uh, plain water's
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enough, it, it does the job.
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But electrolytes can give you that kind of extra edge, especially if you
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are like sweating a lot, exercising intently or working in hot conditions.
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But even for some people doing kind of light activities, some find it,
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these kind of electrolyte things help them 'cause it makes them feel more
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refreshed or a bit more energized.
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Mm.
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Yeah.
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I'm like you, Jen.
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Every morning I have, you know, the electrolyte tablets in my water bottle.
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So I'm a big user.
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I guess my question is, do I need an intervention on this?
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I think we all know you need an intervention.
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Yeah.
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Maybe
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not for
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this.
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That's not for this.
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Uh, yeah, but no, not necessarily.
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Um, your kidneys might not need all that sodium that you're taking
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from these electrolyte drinks, but it won't do you any harm.
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So if you think of it as it's given you hydration, a little upgrade.
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So sometimes it's about performance, but sometimes it's just about how you feel.
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So if we're moving away from the strict eight glasses rule, how do we know
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if we're actually drinking enough?
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So I'm gonna take this in a slightly weird direction now, if you don't mind
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that doesn't like you, Chris.
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I know.
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Let's talk about urine.
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That's not a line I ever thought.
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I'd say.
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Basically, your urine is the best indicator of hydration.
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So pale, yellow, or clear.
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Colored URI means you're well hydrated, dark yellow or amber
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means you need more fluids.
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So no apps needed.
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It is as simple as it sounds.
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Just take a look at that color.
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Well, what about as well as looking at the color?
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I guess the thing which I'm more aware of is how often I pee these.
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It's an old man bladder syndrome I feel.
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Um, but what about frequency?
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How often should I pee?
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Yeah, so on average, every two to four hours is seen as reasonable, but.
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We all have that friend who's got, you know, the blood, the size of a pea.
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Mm-hmm.
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And the key point is, um, consistency.
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So there is loads of variation, but yeah, as long as you've
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got consistency, that's fine.
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Okay.
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So if we want to change then, uh, drinking water habits, what are some of the common
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mistakes that people seem to be making?
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So often when people think I'm drinking way too much, they'll.
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Cut out completely.
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So, you know, a common mistake is swinging between this over hydration
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to under hydration, just out of, you know, reaction or rebellion.
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Um, re if you have been someone like forcing yourself to drink constantly,
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your thirst signals might be a bit dulled.
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So, if you are gonna change these habits, maybe give yourself a
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week or two to recalibrate rather than just ditching those big water
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bottles all together at once.
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So drink when thirsty, watch the urine color.
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Adjust your kind of hydration intake for things like hot weather, exercise,
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air conditioning, but trust your thirst.
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And we're saying all fluids count, right?
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Coffee, soup, tea.
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Exactly.
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And alcohol too.
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But again, just to do the captain sensible thing.
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Everything in moderation.
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Okay.
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Thanks dad.
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Um, so what about, um, what about fluids in food then?
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Does that count
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it?
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Totally.
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And.
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This almost comes back to the whole point of why the eight glasses per
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day thing's not always accurate because we get loads from food.
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It could be 20 or 30% of your daily fluid intake comes from your food.
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And if, if you think of some of the biggest examples, like cucumbers,
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96% water, tomatoes, 95% water, oranges, I think 85, 80 6% water.
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So what about in special circumstances like illness?
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You know, should we adjust our habits?
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If my kids are ill, I feel like all I say to them is, you know, make
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sure you're drinking, keep drinking.
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But I think that's just, 'cause that was what was said to me as I was growing up.
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Yeah, that message is still correct though, because fever increases the
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knees by roughly 200 milliliters of per day per degree above
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your normal body's temperature.
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Um, so yeah, it's really important during fever and of course the other famous
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times to increase our, like, air travel.
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We know air travel dehydrates you due to the cabin pressure, but again,
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we're talking maybe your normal thirst level plus one extra glass, two
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extra glasses, not doubling intake.
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Well, it's, this is interesting, isn't it?
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I, I'm not sure, uh, if this is good news for Yeti.
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Uh, I seem to spend a fortune with them.
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Uh, on these water bottles,
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but it is such a great reminder and it actually makes me curious about what
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else we might start seeing more clearly.
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Yeah.
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Like when did drinking water become so complicated that we
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needed apps and hourly reminders?
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I know, I mean, there are apps for everything now, right?
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Mm-hmm.
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And let's, let's be honest, there are very good roles for these kind of apps.
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Mm-hmm.
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But we've got to remember to trust our body too.
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Because actually this over management of everything, it
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can interfere with good health.
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'cause if you are constantly interrupting your work for, or sleep
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for bathroom breaks or you know you're feeling bloated because of
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over hydration or you're spending all that mental energy tracking absolutely
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everything, it affects your stress, your productivity, and your wellbeing.
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Yeah, there's definitely been times when I've made myself anxious
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about drinking enough 'cause I've not drunk enough in the day.
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So then I forced myself to drink before bed.
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So then I've woken up in the night and to disturb my sleep.
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And really it was probably unnecessary.
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Yeah.
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And we know that, um, poor sleep affects your mood, which affects your decision
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making, which affects your energy.
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So I guess optimizing one thing over here can undermine a whole
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bunch of stuff over here, right?
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Mm-hmm.
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The body's ability to maintain its natural balance, which is known as homeostasis,
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is remarkable because like your kidneys, your brain, your hormonal systems,
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uh, they all work together to keep you hydrated without any conscious effort.
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So what we're saying is proper hydration is about trusting our bodies.
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Exactly.
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So sometimes the simplest approach is the most effective.
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Your thirst mechanism is designed by nature to tell you when you need to drink.
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Well, our ancestors managed to stay hydrated by doing something.
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Revolution revolutionary, then drinking when they were thirsty.
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Exactly.
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So there you go.
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So that's why the eight cholesterol exists and why you don't have
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to necessarily worry about it.
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Your body knows how to manage hydration.
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Trust your thirst.
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Check you are in color.
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Save that mental energy for something more pressing.
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Drink when you're thirsty and trust your body.
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It's actually smarter than you think.
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And that makes a lot of sense.
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So that's it for this week.
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I've learned so much about hydration and if this has changed, how you are
18:15
thinking about drinking water also.
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Make sure you hit that subscribe button, as we are gonna be unpacking the science
18:21
behind everyday questions each week.
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And tell us in the comments if there's anything you'd like us to explore.
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Thanks so much for geeking out with us, but for now, that's it from me.
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That's it from me,
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and that's it from me.
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Bye.