This short article by Peter Weber was published by The Week on April 7, 2014.
We’ve all heard the warning: Even though coffee is at least 98.5 percent water, that cup of joe you’re drinking is a diuretic that will dehydrate you if you don’t chase it down with a cup of H20. Well, good news, javaheads — and dedicated tea drinkers, too: Unless you’re mainlining the caffeine, coffee and tea are probably about as hydrating for you as an equal amount of water, according to Claudia Hammond at the BBC’s Medical Myths blog.
Most of the research underpinning the coffee-as-diuretic idea focuses on the role caffeine plays, and “one of the most frequently mentioned studies was conducted way back in 1928 with a sample of just three people,” says Hammond. Here’s her theory on why people have believed in diuretic effect for so long:
Although we might notice needing the loo more when we’ve been drinking coffee, the mistake is basing our observations on a comparison with the time we’ve drunk nothing, not with a similar amount of water. If you chose a glass of water instead of a cup of tea, you’d probably see the same effect. [BBC]
That doesn’t mean you should necessarily drink eight cups of coffee a day (any more than you need to drink eight cups of water), but you probably shouldn’t fret too much about that daily pot of coffee dehydrating you, either.