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11 January 2011

Stuff I Used to Believe: Caffeine is a Diuretic

Well, it is. Sometimes. For some people. In high doses.

A diuretic is a chemical substance that increases the rate of urination and thus dehydrating the subject. There is apparently a useful distinction to be made between diuresis and aquaresis (increasing water excretion without the attendant loss of electrolytes), however the information that I could find on the subject was scarce.

It's common knowledge that caffeine causes dehydration, however several recent studies have questioned this bit of popular wisdom. From the Wiki:

Caffeine has diuretic properties when administered in sufficient doses to subjects that do not have a tolerance for it. Regular users, however, develop a strong tolerance to this effect, and studies have generally failed to support the common notion that ordinary consumption of caffeinated beverages contributes significantly to dehydration.

And here are the references from the entry:
The Claim: Caffeine Causes Dehydration (The New York Times)
Fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption. (PubMed)
Caffeine ingestion and fluid balance: a review. (PubMed)
Caffeine, Fluid-Electrolyte Balance, Temperature Regulation, and Exercise-Heat Tolerance (Exercise and Sport Sciences Reviews)

About a year ago, one of our employees asked me why coffee was said to cause dehydration. I replied that caffeine was a diuretic, and recommended that he look it up. He did, and to my surprise noted that recent research had questioned this assertion.

So it seems to me that if you consume enough caffeine to be worried about its diuretic effect, you don't need to worry about its diuretic effect.

You do, however, have to worry about the other effects of caffeine. I am under doctor's orders (and dietitian's orders, according to my lovely wife) to reduce my coffee intake from roughly two litres each day to no more than two cups each day, as I suffer from fairly aggressive gastroesophageal reflux which is apparently exacerbated by caffeine consumption. Bollocks.


  1. I wonder how long it takes to become tolerant, or if that's different for each person. I've been drinking tea regularly for two to three years, and days when I drink it I often need to make emergency bathroom trips. I asked a few of my friends and it's the same case with them.

  2. I don't know. I'd be cautious about relying upon self-report, though; there could be some confirmation bias going on.

    In fact, I feel the same way when I drink lots of coffee or tea—but what I sometimes forget is that when I'm drinking lots of coffee or tea I'm consuming a greater volume of liquid than when I don't. More liquid in, more liquid out. Also, the key may be the "days when [you] drink it". If you're not doing so every day, maybe you won't have the same tolerance.

    I don't know much about the dose-response relationship, so perhaps the type of tea that you're drinking (white or green, perhaps) has little caffeine compared the the medium (black tea) or high (coffee) concentrations of caffeine you'd find in other beverages, and the amount of caffeine could be sufficient for a mild diuretic effect but insufficient to develop significant tolerance.

    The point is, I frankly have no idea. :D

  3. Caffeine does, however, increase gastric motility in the colon