It’s been a “fitness fact” forever — if you want to hydrate, cut back on caffeinated drinks. After all, caffeine is a diuretic, right? Goodbye, sweet cup of morning Joe.
Well, scratch that old, outdated advice, says Carrie Ruxton, Ph.D., RD, a nutritionist based in the United Kingdom who has studied the effects of caffeine (from tea) on the body.
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“Caffeine alone does have a diuretic effect, since it stimulates kidney receptors to increase urine output,” Ruxton explains. In high doses (200 to 500 milligrams, which is more than you would get from a typical 10-ounce cup of coffee) caffeine in pill form can have a diuretic effect, causing you to lose a little extra fluid every time you urinate, contributing to dehydration.
But most caffeinated drinks such as tea have enough water in them to make up for this minor fluid loss. “In a study [in men] published in 2011 in the British Journal of Nutrition, we found that up to six mugs of tea per day produced similar effects as water when hydration markers in blood and urine were measured,” Ruxton says. Another study published January 2014 in the journal PLOS One, found no difference in hydration on days when people drank four cups of coffee compared to days when they drank the same amount of water. In other words, drinking caffeinated beverages did not cause the drastic dehydration that was previously thought to occur.
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However, if you regularly drink caffeinated beverages — or use caffeine in other ways — you’ll be less affected than those who never consume the stuff.
Ultimately, it’s the caffeine-to-water ratio that determines whether a beverage will dehydrate you, says Ruxton. “An espresso — high caffeine, small volume — is more likely to dehydrate than a large mug of tea with milk — low caffeine, large volume,” she says. “However, caffeinated drinks are mostly water, which offsets the effect of caffeine.”