The received wisdom is that coffee dehydrates you. Fact. Period. No arguments. But a new study shows that, actually, in moderation, coffee is no less hydrating than water.
A team of researchers from Loughborough University, UK, were interested in accurately establishing the effects of coffee drinking on people's daily balance of fluids. In fact, they actually set out to establish whether regular intake of coffee resulted in chronic low-level dehydration. The results, however, were surprising.
Sophie Killer and her colleagues ran experiments on 50 participants. Each one was was required to drink four mugs — each 200ml — of either black coffee or water per day for three days, then switch for three days. Using measurements of body mass, total body water, and blood and urine analysis, the team kept a close eye on hydration in each subject.
Weirdly, there was no significant difference between those who were told to drink coffee and those who drank water, in either order — both were equally hydrated at all stages. In fact, the researchers have stuck their necks out and said that, when consumed in moderation, coffee "has similar hydrating qualities to water."
While it's well-known that caffeine is a diuretic, that doesn't necessarily mean that coffee is dehydrating, because it provides fluid as well as driving some of it out of the body. Speaking to The Conversation, Killer explained that "numerous studies have documented the body's ability to develop a tolerance to caffeine's acute diuretic effects."
In fact, that tolerance is quick to build — it takes four to five days — and means that regular coffee drinkers shouldn't need to pee any more than those that rule caffeine out of their diets. In turn, clearly, that means that a moderate consumption of coffee — of four cups or less — isn't necessarily dehydrating at all. Take that, received wisdom. [PLOS ONE via The Conversation]