Humankind downs some 1.6 billion cups of coffee a day, making for some mean dehydration, right? That's the conventional wisdom, but it just might not be true, explains Claudia Hammond for the BBC. She digs into the existing research on caffeine—most dehydration-related studies zero in on that particular element of the drink, largely because it's known to stymie the absorption of sodium, which then needs to be flushed out. And it turns out one of the most touted studies is from 1928, and involved a participant pool of ... three. They had their urine measured after drinking four cups of coffee daily (and sometimes tea), cutting those drinks off for two months. They then returned to caffeine, drinking caffeine-infused water, and their urine volume rose, indicating dehydration. But what have more recent studies found?
A review of 10 studies by a UConn researcher found no damning conclusions: Hammond writes that 80% of the comparisons involving caffeinated water showed no difference in urine volume. Another study found a 41% increase in volume, but the study participants had refrained from drinking coffee before the study, so Hammond points out that the finding isn't necessarily applicable to regular coffee drinkers. But a January study out of the UK's Birmingham University may be: Participants habitually drank three to six cups daily, reports NPR, and drank coffee in the study's phase one and then an equal amount of water in phase two. They had their urine volume and kidney function analyzed. The bottom line: no sign of excess dehydration. As the study author explained, "It's well understood that if you drink coffee habitually you can develop a tolerance to the potential diuretic effects of coffee."