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AUDIE CORNISH, HOST:

It's ALL THINGS CONSIDERED, from NPR News. I'm Audie Cornish.

MELISSA BLOCK, HOST:

And I'm Melissa Block. For lots of us, the darkness and colder temperatures of winter make us rely a little more on a favorite pick-me-up: coffee.

Now, for all its benefits, coffee does have the reputation of sending us on multiple trips to the bathroom because of a diuretic effect. But as NPR's Allison Aubrey reports, a new study finds the beverage may actually help keep us hydrated.

ALLISON AUBREY, BYLINE: Like a lot of people, Chuck Moran has a serious coffee habit.

CHUCK MORAN: This is a grande, dark-roast coffee.

AUBREY: It was 4 o'clock in the afternoon when I caught up with Moran in a coffee shop at D.C.'s Union Station, where he was waiting to catch a train.

MORAN: I'm in the middle of a really long journey right now. I kind of want to stay awake until I get home, so I figured I'd have a cup of coffee. (Laughter)

AUBREY: Moran knows from experience that coffee does help him push through fatigue, and he's not somebody who gets the coffee jitters. But still, he's always thought, like most of us, that coffee can lead to dehydration.

DOUGLAS CASA: One of the great wives' tales has always been, you know, caffeine is a diuretic and, you know, people - they're always assuming that caffeine is bad for them in terms of the hydration.

AUBREY: That's Douglas Casa, of the University of Connecticut, who has studied the effects of caffeine in athletes. He says while it is true that the caffeine in coffee can elicit a diuretic effect, prompting the body to flush out fluids, what new research has been proving is that in many people, the effect is actually quite small. And in some of us, it's not detectable at all.

CASA: One of the benefits of drinking coffee is you are getting a lot of water while you're drinking that, so that certainly does enhance the hydration process.

AUBREY: The most recent evidence comes from a new study done at the University of Birmingham, in the U.K. Sophie Killer is the lead author.

SOPHIE KILLER: It's well understood that if you drink coffee habitually, you can develop a tolerance to some of these potential diuretic effects of caffeine.

AUBREY: This has been shown in performance athletes. But Killer wanted to know if it held true for everyday coffee drinkers.

KILLER: We felt there was really a gap, and that no one had specifically answered the question.

AUBREY: So Killer recruited 50 men to volunteer for the study, all of whom had been in the habit of drinking coffee every day. The volunteers had to spend three days drinking coffee as their main source of fluid. Then they went off caffeine, and spent another three days drinking an equal amount of water. When Killer tested their fluid levels, surprisingly, she found virtually no difference. The coffee and the water had the same effect.

CASA: So in habitual coffee drinkers, caffeine really had absolutely no influence on hydration status.

AUBREY: Now, Douglas Casa says it's possible that in the uninitiated - those who don't usually consume caffeine - coffee may have a diuretic effect. But for those of us who drink it daily, it looks as if the old wives' tale is just that, a tale. When I told Chuck Moran that water and coffee likely had the same effect on his hydration, it took him a moment to process it.

MORAN: Interesting.

AUBREY: Does that surprise you?

MORAN: I mean, there's water in it but yes, it is surprising.

AUBREY: So with his grande in hand, he headed out to catch his train.

Allison Aubrey, NPR News.

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