You’re feeling dehydrated. Your mouth is dry. You’re tired. You can feel a headache coming on. So you pour yourself a cup of coffee.
Whoa there, stop the simulation, you’re thinking. It was probably coffee that got you into this mess in the first place. If you’re feeling dehydrated, you need a tall glass of water. Nothing else will do.
Everybody knows coffee dehydrates you. For a practised critical thinker though, the only mystery more alluring than the thing nobody knows is the thing everybody knows. Let’s look a little closer. A 250ml mug of black coffee contains, for all intents and purposes, 250ml of pure H20[1]. How is it possible that drinking all this water dehydrates us?
This is a great question to ask. This is exactly the kind of thing that should make you narrow your eyes and wonder if you’re missing something. But, to be clear, asking this question isn’t the whole investigation. Just because something seems contradictory doesn’t mean it’s necessarily wrong. Remember the other grandpas.
So what’s in those dissolved coffee solids that could completely negate the hydration effects of a quarter-litre of water? Well, caffeine obviously. And, to a much lesser extent, the related chemical theophylline. These are the two most commonly cited diuretics in coffee. A diuretic, for those who’ve heard the word a thousand times without ever knowing the exact meaning, is anything that increases the production of urine. Everyone knows that drinking coffee makes you need to pee. And peeing quite obviously dehydrates you. It’s literally water leaving your body.
Except, of course, that drinking 250ml of water would also make you have to pee. The body maintains a delicate fluid balance, and water that comes in must eventually go out. So is water also a diuretic? Technically, yes. But when we talk about something having a diuretic effect, we’re usually talking about it promoting urine production in excess of the body’s usual fluid regulation. Water is the baseline.
When we drink a 250ml cup of coffee, we’re hydrating ourselves with 250ml of water. Some of that water will be used to replace fluids lost through breathing, sweating, and other basic biological realities of being a human animal. The rest will head to our kidneys, pick up some trash, and then come out as urine. In order for coffee to have a net dehydrating effect, the diuretic action would need to be strong enough to make us pee so much extra that there’s nothing left for replacing lost fluids.
So how strong is the diuretic effect of caffeine and theophylline? To answer that, we’ll turn to the 1928 study, “Tolerance and Cross-Tolerance in the Human Subject to the Diuretic Effect of Caffeine, Theobromine and Theophylline.”[2].
Or, rather, let’s not turn to this hundred-year-old study because it had a sample size of just three participants, and its results have been invalidated again[3] and again[4] and again[5] by larger and more rigorous studies and reviews. Yet, this 1928 paper remains the ultimate source for the common claim that coffee dehydrates you. This adage has been repeated so many times, in newspapers, on television, in Internet articles, in doctor’s offices, and in casual conversation (over coffee!) that it has become an accepted truth. Science, meanwhile, marches on. But it’s very difficult to dislodge an idea that’s already taken hold. Beliefs are sticky.
Caffeine is indeed a diuretic[6]. Injected in high doses to rabbits, to rats, to humans, caffeine will reliably cause excess urine production. At lower doses, such as those found in a cup or two of coffee, the diuretic effects of caffeine are… zero. Not measurable. Even among people who drink five to ten cups of coffee a day, the mild diuretic effect of caffeine has been shown to have no measurable effect on overall fluid balance. You might pee a bit more, but all that water you’re drinking more than makes up for it[7].
Coffee, because it’s almost entirely water, hydrates you just as well as water does. And if you think that first cup or two of coffee is making you run to the bathroom, well, it is. But only because you just drank a bunch of water.
As far as critically examining our deeply held beliefs go, this one is admittedly relatively low stakes. But that’s exactly what makes this sort of exercise particularly valuable for honing our tools of critical thought. Do you feel a need to fact check what I’ve just shared with you? To see if I’ve made a fundamental error? Good. Or do you feel a combative indignation at having an ingrained belief challenged? Or an immediate elation at now knowing something others don’t? Or an eagerness to believe because you wanted permission to drink more coffee anyway? Keep an eye on that. Consider how much more dangerous those instincts will be when it’s a topic that really matters.
[1]. Depending on strength and brewing method, a mug of coffee will generally be between 1% to 2% dissolved solids by weight. You know, the coffee part. Even if these solids fully displaced an equivalent amount of water, that 250ml mug of coffee would still contain 245ml to 248ml of H20. But that’s not how solutions work. Adding that soluble weight mostly increases the density of the solution, not the volume. Do note that espresso can be up to 15% dissolved solids by volume. If you’re drinking 250ml of espresso at a time though, addressing your problems may be outside the scope of this article.
[2]. Eddy, Nathan B. and Ardrey W. Downs. “Tolerance and Cross-Tolerance in the Human Subject to the Diuretic Effect of Caffeine, Theobromine and Theophylline.” The Journal of Pharmacology and Experimental Therapeutics (1928)
[3]. Maughan, Ronald J., and James Griffin. “Caffeine ingestion and fluid balance: a review.” Journal of Human Nutrition and Dietetics 16, no. 6 (2003).
[4]. Armstrong, Lawrence E.; Casa, Douglas J.; Maresh, Carl M.; Ganio, Matthew S.. “Caffeine, Fluid-Electrolyte Balance, Temperature Regulation, and Exercise-Heat Tolerance.” Exercise and Sport Sciences Reviews 35(3):p 135-140, July 2007.
[5]. Seal, Adam D., Costas N. Bardis, Anna Gavrieli, Petros Grigorakis, J. D. Adams, Giannis Arnaoutis, Mary Yannakoulia, and Stavros A. Kavouras. “Coffee with high but not low caffeine content augments fluid and electrolyte excretion at rest.” Frontiers in nutrition 4 (2017).
[6]. Theophylline too, although much less so, and it’s present in such tiny amounts in coffee as to be functionally irrelevant.
[7]. It’s worth noting that consuming that much caffeine might be bad for you in other ways. But it’s not dehydrating you.