We've all grown accustomed to the idea that coffee dehydrates you. A study from as early as 1928 suggested that caffeine was a diuretic because caffeinated beverages increased urination. But more recent research finds there's little evidence supporting this claim. While drinking coffee might cause you to make more trips to the bathroom, it isn't actually making you more dehydrated.
More and more research is finding the answer to the question Does coffee dehydrate you? to be a certain "no." A 2014 study, aptly titled "No Evidence of Dehydration With Moderate Daily Coffee Intake," explored this exact question, "directly comparing the effects of coffee consumption against water ingestion across a range of validated hydration assessment techniques." What researchers found was coffee, consumed in moderation, actually provided similar hydrating qualities to water. "Caffeine really had absolutely no influence on hydration status," Douglas Casa, professor in the department of kinesiology at the University of Connecticut, explained to NPR when asked to review the paper.
"The fact that we don't have hospital emergency rooms filled [with patients] because they [drank] caffeinated beverages is clear evidence," Lawrence Armstrong, a colleague of Casa and professor and director of the Human Performance Laboratory, tells LiveScience. "If there were negative health effects, [they] certainly would have been identified. Though coffee isn't dehydrating, it is possible to drink too much. Consuming more than 500 mg of caffeine daily (there's approximately 95 mg in one cup) could lead to caffeine intoxication—with symptoms such as tremors, rapid heartbeat, and diarrhea (which does cause dehydration).
Next up: Find out what happened to one editor's skin when she gave up coffee.