"Coffee dehydrates you.”
Yeah, and gum stays in your stomach for seven years if you swallow it.
I can’t overstate the number of times I’ve heard someone say that coffee dehydrates you.
Generally, it’s said when someone (i.e. you) is getting ready to drink some coffee.
For whatever reason, people love to repeat facts like this—or more accurately, things they heard repeated somewhere, that they think are facts.
Next time you hear someone say this, ask them where they heard it.
Bet they can’t remember.
Now, I’m not saying that you should drink coffee in place of water. (Do as I say, not as I do.)
However, coffee does not dehydrate you.
At least if you go by sources that are generally pretty reliable—like dictionaries, the Mayo Clinic, et cetera.
If you tend to give higher credibility to things that somebody heard someone else say that somebody else said that they heard sometime somewhere… I’m probably not going to change your mind with this.
First, let’s make sure we’re both on the same page about what dehydration means. According to Oxford’s dictionary definition, dehydrate in this context means to “cause (a person or their body) to lose a large amount of water.” When people lose more fluid than they’re ingesting, it can be very dangerous.
Now, let’s look at what experts have to say (bolding emphasis mine):
“Drinking caffeine-containing beverages as part of a normal lifestyle doesn't cause fluid loss in excess of the volume ingested. While caffeinated drinks may have a mild diuretic effect — meaning that they may cause the need to urinate — they don't appear to increase the risk of dehydration.” — Katherine Zeratsky, R.D., L.D., Mayo Clinic
Or, check out the results of a study that looked for a correlation between coffee consumption and dehydration—based on the commonly repeated suggestion that “coffee dehydrates you”—by analyzing the participants’ blood, urine, and plasma:
“These data suggest that coffee, when consumed in moderation by caffeine habituated males provides similar hydrating qualities to water.”
Source: No Evidence of Dehydration with Moderate Daily Coffee Intake: A Counterbalanced Cross-Over Study in a Free-Living Population (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3886980/)
If you drink coffee, your body isn’t going to retain exactly as much water as if you drank the same amount of water, but you’re not going to lose fluid either.
A more accurate thing to say would be that coffee is not the absolute most optimal thing on earth to drink if the only thing you’re concerned about is hydration.
But it’s far from being dehydrating.
The idea that coffee dehydrates you is basically claiming that if you were, say, stranded in the Mojave desert and you stumbled across an abandoned pot of Eight o’Clock Coffee, you’d be better off drinking nothing rather than drinking some.
That’s patently absurd.
The reasons you wouldn’t want to drink it are 1. germs and 2. it’s Eight o’Clock Coffee. But if you were in actual danger of dehydration, drinking a cup of coffee—no matter how lacking in flavor—would hydrate you.
A cup of coffee is usually about 98% water, and the diuretic effects from the caffeine don’t offset the hydration.
In a medically reviewed article at Everyday Health, Registered Dietician Nutritionist Kelly Kennedy recommends that for every 8oz of coffee you drink, you count a little less than 7oz of them as water.
Similarly, according to WebMD’s answer to What Counts as Water? “Coffee and tea also count in your tally. Many used to believe that they were dehydrating, but that myth has been debunked.”
Drinking coffee isn’t like drinking alcohol, or sea water—both of which do dehydrate you.
Personally, I think people probably mixed up the facts about sea water and alcohol with coffee, way back in the old days, and that was the genesis of the coffee/caffeine dehydration myth.
Coffee is hydrating.
It does not dehydrate you.
But it’s not as fun to tell other people that when they’re about to drink it, I guess.
TL;DR, The myth that coffee dehydrates you is a myth.