Unraveling one of our most persistent nutrition myths, researchers at the University of Birmingham School of Sport and Exercise have determined that drinking coffee and other caffeinated drinks does not dehydrate you.

Conventional wisdom advises against all forms of caffeine during flu season, heat waves, and other situations where proper hydration is essential to staying on point. Coffee, they say, is a diuretic that drives fluids out of the body. However, that’s usually where the wisdom ends.

Annoyed by this type of two-dimensional advice? So is exercise metabolism and sports performance doctoral candidate Sophie Killer — so much so that she and her colleagues decided to debunk it once and for all by conducting one of the first scientific studies on the subject. "Despite a lack of scientific evidence, it is a common belief that coffee consumption can lead to dehydration and should be avoided, or reduced, in order to maintain a healthy fluid balance,” she said in a press release. “Our research aimed to establish if regular coffee consumption, under normal living conditions, is detrimental to the drinker's hydration status."

For the study, the team enrolled 50 healthy men reporting a habitual coffee consumption of about three to six cups of joe a day. The participants were then split into two groups for a three-day trial. In addition to a fixed food and fluid intake, each subject drank either four cups of coffee or an identical amount of water. At the end of the experiment, Killer and colleagues used samples from both groups to assess biological factors known to influence total body water (TBW) — a standard measurement of hydration.

The result? Four cups of coffee appeared to have the same effect on a participant’s overall hydration profile as four cups of water. "We found that consumption of a moderate intake of coffee, four cups per day, in regular coffee drinking males, caused no significant differences across a wide range of hydration indicators compared to the consumption of equal amounts of water," Killer explained. "We conclude that advice provided in the public health domain, regarding coffee and dehydration, should be updated to reflect these findings."

Coffee and Dehydration

The clear and immediate results naturally raise a number of questions. Why do so many people swear by the dehydrating capacity of coffee? And why do people assume that a mild diuretic component like caffeine instantly turns a substance into a fluid-sapping cocktail?

It would appear, however, that the answer is pretty simple: dehydration is a pain to test. While it’s easy to quantify the effect of caffeine pills and other isolated substances, investigating coffee-drinking itself requires the researcher to keep tabs on each subjects’ individual habits. In an email to Medical Daily, Sophie Killer described the many obstacles that have so far allowed the myth to thrive.

“Hydration studies in a ‘free-living environment’ (i.e., where participants do not have to stay confined to the laboratory) are notoriously difficult to performance with any validity,” she wrote. “Ensuring participants are hydrated over an extended period (whereby you are providing all food and fluids) is another tricky one, as everyone requires different daily fluid intakes and thus there are no set guidelines for how much an individual should drink to stay hydrated.”

But this did not deter Killer and colleagues, who in the spirit of good scholarship decided to spend months solving these problems. With the results, we may finally be able to retire the already outdated link between coffee and dehydration.

Source: Killer SC, Blannin AK, Jeukendrup AE. “No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population.” PLoS One. 2014.