An important thing to remember about science is that, like a good soap opera, it's never really finished. No issue is really settled as long as there are more people capable of asking more questions and doing more research. This is a good thing. It's what keeps us from accepting things as true without testing them against new ideas, data or technologies.
Thankfully, someone thought to challenge the supposedly unhealthy aspects of coffee. It's been a long held understanding that caffeine, like alcohol, causes dehydration. Since staying well watered is key to good health (though scientists even disagree on the specifics of that), there's been a lot of concern over the average person's use/abuse of coffee. Especially because the U.S. is one of the coffee-drinkingest countries in the world, consuming an annual 4.2 kg of coffee per person, more than three times the global average.
It turns out the whole idea is, for now anyway, bunk. A research team at the University of Birmingham in the U.K. has found that coffee actually has the same hydrating qualities as water. As NPR points out, these findings support previous research that caffeine didn't prevent athletes from re-hydrating normally after a workout.
The Birmingham study involved 50 men going through two phases of observation, one in which they used mostly coffee as their source of hydration (which, ugh) and then another in which they drank water like normal humans. They found no significant change in the subjects' hydration levels. Which means that despite common knowledge, coffee doesn't actually force you to pee (note: it does make you poop ).
The results only hold up for moderate consumption, so as of now it's ill-advised to down eight 8-ounce glasses of coffee a day, but surely someone will come along and do a study on that soon enough.
This news comes at the same time as new research suggesting caffeine actually boosts memory. Scientists had already established that it improves the memory of bees, so it's about time someone figured out how to apply that to humans.