THE CLAIM: Coffee is a diuretic (meaning, it dehydrates you)
The notion that you have to drink an extra glass of water for every cup of coffee you drink is one of the most persistent urban legends in nutrition. We’re talking Sasquatch territory here. The origin of this pseudo-myth comes from early studies — dating as far back as 1928 — suggesting that giving people a single dose of caffeine (typically as a pill) can trigger a small increase in urine output (read: they pee more). That might make it sound like caffeine does, in fact, cause dehydration, but there is more to the story.
If chronic coffee consumption really did cause us to become dehydrated, then a lot of people would be end up hospitalized
First, the studies that demonstrated some type of dehydration-like effect typically used relatively large doses of caffeine (500 mg or more), or about the equivalent of drinking 3.5 cups of coffee, or 10 cups of tea, all at once. Second, any diuretic effect seems to be most pronounced in those who don’t consume coffee regularly. Intuitively, this makes sense: After all, if chronic coffee consumption really did cause us to become dehydrated, then an awful lot of people would be end up hospitalized, or dead, due to dehydration every day.
There is a third important consideration with caffeine research, and that is that relatively few studies use real coffee; most use caffeine, which is just one component of coffee. We know that coffee is a fantastically complex beverage (it contains dietary minerals such as magnesium, as well as numerous antioxidants and numerous active compounds, all of which can have an effect on the body. And its properties change quite significantly with the way it is prepared — for example, whether it is filtered or unfiltered).
If you only drink coffee occasionally, then you might want to add an extra glass of water on the days you do grab a cup
For that reason, a recent study, published in the journal PLoS ONE, is an important contribution. In this study, researchers examined the effect of drinking a moderate amount of coffee on 52 healthy men who were regular coffee drinkers at the start of the study. During the study, the men were given 200 mL (roughly 7 ounces) of either plain water or coffee, four times per day, while keeping the rest of their eating and drinking habits constant. After three days of this routine, the researchers found no difference in any measurement related to hydration, including urine output or total body water (considered a gold standard indicator of hydration status).
The take-home message from this study? If you are a regular coffee or tea drinker, don’t sweat the hydration/dehydration issue. On the other hand, if you’re new to coffee, or you only drink it occasionally as a pick-me-up, then you might want to add an extra glass of water or two on the days you do grab a cuppa. Even then, however, there is some debate: not every study has found a diuretic effect associated when introducing caffeine to non-users, and a recent study found a caffeine dose of 5 mg per kilogram body weight per day (equivalent to about 350 mg, or a bit more than two standard cups of coffee per day) had no effect on total body water in low caffeine users (those who were accustomed to taking in less than 100 mg of caffeine per day).
Even if we assume that caffeine does dehydrate you slightly, however, the diuretic effect should wear off within about four days: that’s the amount of time it takes to become habituated to caffeine, on average, if you start taking it daily. That’s also the amount of time it takes to de-habituate to caffeine, so if you kicked your coffee habit while on vacation last week, only to reignite your deep and personal relationship with your favourite barista once you returned to work this week, you might want to grab an extra glass of water for the first couple of days, just to be sure.
FACT OR FALLACY: Fallacy. Mostly.
— Jennifer Sygo, MSc., RD, is a registered dietitian and sports nutritionist at Cleveland Clinic Canada. Visit her on the Web at jennifersygo.com and send your comments and nutrition-related questions to her at email@example.com.