The Frontal Cortex


The history of science is littered with surprising discoveries that forever changed our conception of the unvierse and ourselves. The earth is a sphere, even though it appears flat. Life has no designer, even though it looks designed. But this may be the most surprising discovery yet, a fact that seems to undermine one of the basic truisms of my morning routine:

It was long thought that caffeinated beverages were diuretics, but studies reviewed last year found that people who consumed drinks with up to 550 milligrams of caffeine produced no more urine than when drinking fluids free of caffeine. Above 575 milligrams, the drug was a diuretic.

So even a Starbucks grande, with 330 milligrams of caffeine, will not send you to a bathroom any sooner than if you drank 16 ounces of pure water. Drinks containing usual doses of caffeine are hydrating and, like water, contribute to the body’s daily water needs.

Can that be? The last time I drank a large Starbucks coffee I had the misfortune of sitting in a window seat on a transcontinental flight. Let’s just say that I was yearning for a catheter. The real question, of course, is this: If coffee isn’t a potent diuretic, then why does it seem like one?


  1. #1 Mozglubov
    August 5, 2008

    I don’t drink coffee, but tea certainly seems like one to me. However, that might just be because one tends to drink hot drinks in larger quantities than other beverages like juice. Either that or perhaps there is another chemical in coffee and/or tea that is a diuretic even in low quantities. After all, the study tested caffeine rather than the entire mixture that is coffee and tea.

  2. #2 who cares
    August 5, 2008

    Dangit. Life does NOT look designed.
    I’m an engineer (A.K.A someone who designs stuff).
    If someone would deliver the equivalent sloppy work (that a living thing is) in my field, you’d get fired and possibly get your diploma stripped from you.

  3. #3 Rachael
    August 5, 2008

    I don’t often gulp down a large glass of water (I sip water), but I do gulp down large glasses of iced coffee every morning so I can leave on time. It probably depends on the person, but for me, coffee is consumed rapidly and water is not.

    Also… is it possible that coffee increases the urge to urinate without increasing the quantity?

  4. #4 Becca
    August 5, 2008

    I thought that caffeine, or possibly something in the mixture that is coffee (but applies to a lesser degree to tea), is an irritate to the bladder or kidneys, thus increasing the urge.

  5. #5 Justin
    August 5, 2008

    Brewed coffee is ballparked at 22mg per fluid ounce, yeilding around 440mg for a *venti* (large) coffee at Starbucks. This is approaching the 550mg threshold cited in the link before coffee becomes a diuretic. But note that…

    (1) The kidneys aren’t going to wait for the 551mg mark and say, “now its time to let it all go.” There is likely a gradient starting way below 550mg.

    (2) This is biology. People are different. The 550mg number is likely derived from some statistical average across multiple people. The NYT article doesn’t mention the standard deviation of this number.

    (3) Like all drugs, the total consumption is less important than the consumption per unit time. The 550mg mark is likely over the course of some arbitrary “typical” amount of time to drink a cup of coffee. Perhaps an hour? Chugging a 20oz coffee fifteen minutes before a flight will likely be different than if it was drunk over the course of a morning.

    (4) Going to the March newsletter of the Center for Science in the Public Interest (which the NYT cites), they cite 8/8 studies which show no diuretic effects for drinks with 45 to 225mg of caffeine. In the 240mg to 55mg range, however, the story is not as clear, with 9 of 13 studies showing no diuretic effects (see numbers 1 & 2 above).

    There’s probably an even less consistent story if we were to look directly at the published papers (analyzed in Exercise & Sport Sciences Reviews. 35(3):135-140, July 2007). Follow the news to its source. Don’t just swallow the sound bites.

  6. #6 MM
    August 5, 2008

    My understanding also is that caffeine can irritate the bladder, increasing the urge to urinate even though the bladder may not be completely full.

  7. #7 HP
    August 5, 2008

    If coffee isn’t a potent diuretic, then why does it seem like one?

    I’m gonna buck the trend of the previous commentators and suggest that the perception of coffee as a diuretic is confirmation bias.

    Assume a normal day with normal coffee consumption and normal access to toilets/urinals/trees-on-a-slightly-downhill-slope. How many times a day do you get up to urinate? With what urgency? How much volume? I can’t speak for you, but I have absolutely no idea how often or how much I piss. Probably more often than I’d guess, since stopping to take a leak is such a mundane occurrence.

    But imagine that you find yourself stranded without access to a place to relieve yourself. Now you’re acutely aware of your body’s needs, and since “everybody knows” coffee is a diuretic, that’s the first thing you think of.

    You could test this by avoiding all caffeine in the morning and then getting stuck in a traffic jam for two hours, but who wants to do that?

  8. #8 Rachael
    August 5, 2008

    Oh, another thought: what about caffeine tolerance? I don’t know what percentage of the population drinks coffee regularly… but when you do, it stops being effective. To do a real study, they’d have to start with caffeine-free individuals or talk about change from baseline

  9. #9 Peggy
    August 5, 2008

    Well, I know from experience that 8 ounces of strong brewed coffee will cause me to need to pee much sooner and more urgently than 8 ounces of water. I wonder, though, if that effect is at least partially on bladder function (stimulation?) rather than extra urine production.

  10. #10 Shannon Murphy
    August 5, 2008

    Not to be gross, but:

    Coffee never makes me pee but it does make me poop rather…enthusiastically. And (okay, now I’m being gross) the poop is like wetter and more liquid-y than it is when I haven’t had coffee. Would that count as a diuretic effect even though you’re not peeing the liquid out? I don’t know much about physiology but it seems relevant.


  11. #11 Alan
    August 5, 2008

    “So even a Starbucks grande, with 330 milligrams of caffeine, will not send you to a bathroom any sooner than if you drank 16 ounces of pure water.”

    This is implicitly assuming you drink your grande without added sucrose or lactose; that you drink it at the same rate as you would water, and yes, that the water you imbibe is served at the same temperature as your morning brew–all the while controlling for time of consumption. And what of circadian rhythms affecting metabolic rate (such as higher cortisol levels in the morning when caffeinated beverages are typically consumed?)

    On another level, what is the psychological effect of knowing you’re consuming a ‘grande’ instead of just a 16oz ‘glass of water’? Might it be that the size name itself primes the brain at some unconscious level that we’ve just drunk a substantial, even large, amount of something and that a bathroom break is the appropriate behavior?

  12. #12 Brad A. Greenberg
    August 5, 2008

    I just can’t imagine this is true, for the same reason you cite, Jonah. We’ve all been there, and it’s not a place you end up after drinking a tall glass of OJ.

  13. #13 kel
    August 5, 2008

    I’m new to this blog, so I am curious as to your evidence regarding the very direct statement that “life has no designer, even though it looks designed.” I was surprised to see this.

  14. #14 jon
    August 6, 2008

    Wow, you make one tiny nod (mostly just for comic effect) towards the *fairly* universally accepted notion that organisms develop by evolution, and whaddya got? A random vigilant creationist who doesn’t even read the blog demanding (albeit politely) that someone personally explains the evidence for evolution – evidence which I am sure he/she already *knows* does not exist. Oh well.


    I refuse to believe that coffee doesn’t have some effect on provoking urination, and I will cling to this belief even more strongly than the most ardent creationist. And does the temperature of the beverage perhaps have anything to do with the rate at which it is successively .. uhh .. dealt with? So many unanswered questions in life.

  15. #15 outeast
    August 6, 2008

    I’ve long been puzzled by the caffeine=diuretic claim, since my father is still with us and AFAIK he drinks nothing other than vast quantities of caffeinated tea throughout the day (pace the Kinks) and the odd beer or two of an evening.

    I personally don’t find coffee particularly diuretic; tea neither. The effect which Shannon mentions as associated with the former sounds familar, though.

  16. #16 shelby
    August 20, 2008

    As an avid admirer of your clear and thoughtful writing, I was very surprised to see this: “Life has no designer, even though it looks designed.”
    Do I believe in a designer? Well, I have never seen one, but then again, I have never not seen one. I can’t think of any scientific proof one way or the other. It is truly a matter of belief. I was surprised and amused to see the above statement next to something so scientifically factual as “The earth is a sphere.”

  17. #17 shelby
    August 20, 2008

    After reading all the comments, I realized that I should probably clarify that I don’t believe the world was created in six days and is only seven thousand years old.

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