Water, water, everywhere, but how much should I drink?

Giant Water Mug

Giant Water Mug in its Natural Habitat

Pop quiz: How much water should we drink each day?

You already know the answer, right?  Eight glasses, I hear you say.  Eight eight-ounce glasses, in fact, I hear you say.

I know.  We all know.  It’s been drilled into our heads since…  since…  since time immemorial, right?

So much so that a couple years ago I bought this mug you see to the right.  This really big mug.  The idea was to find a mug for use at work that would hold my daily water requirement – that magic 64 ounces that we all need.  And I even found one at my local Super-Mega-Hyper-Mart™ that fit the bill perfectly.  Trust me, this sucker is big.  I’ll admit, it’s hard to appreciate the mug’s presence and stature without reference objects, but if I had put anything too close to it, said object would be irretrievably trapped in the mug’s gravity well.

But that’s not all — not only is GargantuMug big, it’s informative, too.  It mentions the 64-ounce guideline, tells you what things could go wrong with you if you get dehydrated, tells you to drink this much in a day, and even gets silly now and then.  In upside down text, it reads, “If you can read this, I need more water!”

So clever!  So useful!  And yet…

Maybe not so useful…

An article in Scientific American from last June sheds some light on the story.  Physician Heinz Valtin, MD, from Dartmouth Medical School, found no evidence of any science indicating a reason to drink that much water, and also saw no evidence in the general population of their being chronically dehydrated to the point of needing to drink 3½ gallons of water each week.  He did locate what is believed to be the origin of the “8×8 rule”, and it’s interesting, to say the least (emphasis mine):

Valtin thinks the notion may have started when the Food and Nutrition Board of the National Research Council recommended approximately “1 milliliter of water for each calorie of food,” which would amount to roughly two to two-and-a-half quarts per day (60 to 80 ounces). Although in its next sentence, the Board stated “most of this quantity is contained in prepared foods,” that last sentence may have been missed, so that the recommendation was erroneously interpreted as how much water one should drink each day.

To quote Mr. Spock..  “fascinating”.  A misinterpretation of a recommendation back in 1945 has resulted in the commonly-held belief that we all need roughly a six-pack of water every day.

It goes even further.  Of course, we also all know that the 64 ounces have to be actual water, right?  That soft drinks — especially caffeinated soft drinks — don’t count, right?

Think again.

Finally, strong evidence now indicates that not all of the prescribed fluid need be in the form of water. Careful peer-reviewed experiments have shown that caffeinated drinks should indeed count toward the daily fluid intake in the vast majority of persons. To a lesser extent, the same probably can be said for dilute alcoholic beverages, such as beer, if taken in moderation.

And really, when it comes down to it, the whole “caffeinated drink doesn’t count because it’s a diuretic” idea doesn’t really pass the sniff test.  Think about it.  The idea was that the diuretic effect of the caffeine it made the drink not count.  Would a 12-ounce can of caffeinated Coke really make you urinate an extra 12 ounces?  That seems unlikely (Of course, that’s easy to say now, given that I’ve just quoted the results of experiments backing up that point).

Basically, I see two takeaways from this.

One is about the water – if you’re thirsty, drink something. If you’re not, don’t. If you like water, drink it. If you’d rather drink something else, that’s good too. Just stop stressing about it!

And the other has to do with science in general. I’ve had people say that science is too rigid, and once its mind is made up (science has a mind?), it doesn’t change. But honestly, nothing could be farther from the truth. When supplied with evidence, science changes, and that’s an important point to teach our children. T-Rex models look different now. Pluto is no longer a planet. There’s water on the moon. The coelacanth is not extinct. There’s life on Earth in ridiculously hostile environments. And most people don’t need to drink 64 ounces of water every day.

When science is viewed as a process, not a book of facts, it becomes alive and exciting. Let your kids in on the amazing way our knowledge changes thanks to science.

..Rob T. is going to scratch all the incorrect information off of his GargantuMug – and then only drink about 3 ounces out of it!


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9 Responses to Water, water, everywhere, but how much should I drink?

  1. failmom says:

    I have a fitness instructor who reminds us everyday, “Once you’re thirsty, you’re already dehydrated.” I get that he’s trying to encourage people to drink water while they’re exercising, and that’s a good thing, but this statement just seems so wrong. Why would our bodies tell us to drink water after it’s already too late? I’ve been trying to find a good study to show that thirst isn’t always an indicator of dehydration, but I’m having trouble finding one. What do you think about the thirst=dehydration meme?

    • The Nerd says:

      The strongest supporters of this meme that I’ve found are the “organics only” crowd with a subset of “homeopathy works!” Those sorts don’t always pay attention to scientific consensus (especially the ones who believe in the Big Science conspiracies).

      • Rob T says:

        In the Dartmouth article linked to at the bottom of my post, there were these bullets, also from Dr. Valtin:

        * Thirst Is Too Late. It is often stated that by the time people are thirsty, they are already dehydrated. On the contrary, thirst begins when the concentration of blood (an accurate indicator of our state of hydration) has risen by less than two percent, whereas most experts would define dehydration as beginning when that concentration has risen by at least five percent.

        * Dark Urine Means Dehydration. At normal urinary volume and color, the concentration of the blood is within the normal range and nowhere near the values that are seen in meaningful dehydration. Therefore, the warning that dark urine reflects dehydration is alarmist and false in most instances.

      • failmom says:

        Oh, that will teach me to read all the references thoroughly!

        The Nerd – I hear the thirst=dehydration meme most often from fitness instructors/personal trainers/nutritionists, which leads me to believe that it’s being propagated in PE-related programs (I forget what they’re calling PE these days). Of course, there is a quite a bit of overlap between the jock crowd and “Big Farma/Pharma is out to get me!” crowd. I think it’s probably just one of those myths that will not die.

  2. the number of glasses of water to be drunk may varies according to season.during summer we have to drink more water

  3. [...] in Daily life, Food, Health, Science at 10:50 am by LeisureGuy Fascinating post at The Skeptic Dad: Pop quiz: How much water should we drink each [...]

  4. Nick says:

    Brilliant analogy in one of the posts above regarding the fitness instructors comment;

    I will remember that comment when my body tells my brain to tell my cognitive persona that I am hungry. “It’s to late to eat” that’s a great diet and weight loss strategy !

  5. Allison says:

    When I was pregnant, there was a serious threat of preterm labor. So, I was told to drink not 64, but 128 ounces per day. I tried – for several weeks. By the end of the day I was miserable – bloated, sloshing, by abdomen distended by fluid. The thought of drinking another sip of water made me want to vomit – literally, sometimes I would gag. Now, I’ve never been a water drinker, so drinking that much was unthinkable. After several weeks of the pregnancy water torture, I gave up, and threw my gargantumug out the window. I still tried to remain well hydrated, and I ended up drinking about a liter and a half a day of water (mostly in seltzer form). I was much happier, with no untoward effects. There’s no one set caloric need for everyone – so why should we think there’s a set water limit? I’m a scientist, and I’m all about evidence based medicine, but sometimes you really do just have to listen to your own body.

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