Despite caffeine's many benefits, there's a belief out there that a daily coffee habit can cause dehydration.
So is it true? Not according to the findings of a new study.
Researchers at the University of Birmingham in the U.K. studied the fluid levels of 50 men who had a habit of consuming about three to six cups of coffee each day.
With this kind of moderate coffee consumption, the authors conclude that "coffee ... provides similar hydrating qualities to water."
To compare the hydrating effects of coffee directly with water, each participant completed two phases of the study. In one phase, they drank coffee as their main source of hydration. In the other phase, the participants went off coffee and drank equal amounts of water.
"Caffeine really had absolutely no influence on hydration status," concludes Douglas Casa of the University of Connecticut, a researcher we asked to review the paper. In other words, the coffee didn't prompt the body to pee (or flush out) more fluid.
"It's well understood that if you drink coffee habitually you can develop a tolerance to the potential diuretic effects of coffee," says study author Sophie Killer, a doctoral researcher at Birmingham.
Prior research has already chipped away at the dehydrating myth. For instance, one study found that caffeine didn't hinder hydration among athletes who consumed caffeinated beverages to rehydrate throughout practices in the heat.
This new study adds to the evidence, that among everyday coffee drinkers — say, those of us who routinely sip a cup of Joe or two at the office — the old wives' tale that coffee will lead to dehydration is really just that: a tale.
But it would be an overreach to interpret these findings as coffee is the same as water when it comes to overall health. Too much coffee can lead to jitters. And most experts agree that water or sports drinks are best for rehydrating after fluids have been depleted.
Bottom line: A daily coffee habit won't lead to dehydration. But it's best to limit caffeine to moderate levels to steer clear of jitters and interruptions to sleep.
And, oh, I should mention another new study that links caffeine to enhanced memory.
The research, published in Nature Neuroscience, evaluated the effects of placebo tablets compared with 200 milligrams of caffeine tablets (about as much caffeine as a large cup of coffee) on the memories of participants who were shown a series of images.
One day after the initial viewing of the images, the participants who had been given the caffeine tablets were more likely to distinguish between the images they had already viewed and new, similar images that had been added to the mix.
"We've always known that caffeine has cognitive enhancing effects, but its particular effects on strengthening memories and making them resistant to forgetting has never been examined in detail in humans," wrote Michael Yassa of Johns Hopkins University in a release about the study.
"We report for the first time a specific effect of caffeine on reducing forgetting over 24 hours," wrote Yassa.
There are certainly a lot of questions yet to be answered. For instance, caffeine may help to consolidate memories in the short term. But is it useful in helping us retrieve memories? This study suggests not.
As we've reported, caffeine-containing nectar has been shown to improve the memories of honeybees, so stay tuned — there may be more buzz about the effects of caffeine on memory in research still to come.
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.
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. Transcript provided by NPR, Copyright NPR.