How well do different drinks hydrate you?

Not all drinks take the same amount of time to reach your bladder. This fact, a team of researchers write in a recent American Journal of Clinical Nutrition article, “is of real clinical and practical benefit in situations in which free access to fluids is limited or when frequent breaks for urination are not desirable.”

Sounds a lot like a long run, doesn’t it?

As it turns out, there are a lot of factors that affect how quickly you pee out a given drink. For example, drinks with lots of calories or electrolytes tend to be retained in the body for longer, while diuretics like caffeine and alcohol speed things along. How much you drink at a time also has an influence. And despite all the folk-wisdom out there, there’s very little actual data comparing different drinks.

To address this gap, researchers led by the University of Loughborough’s Ron Maughan, along with colleagues from Bangor University and the University of Stirling, recruited 72 subjects to test 13 different drinks (each subject tested water plus three other drinks). At each testing session, the subjects drank a litre of the chosen beverage, then collected all their urine for the next four hours.

The result is a “beverage hydration index” that compares how much of the drink was retained after two hours compared to a litre of water. (They used two hours because most of the urine had been passed by then, and it represents a more realistic time interval between drinks than four hours.)

So, without further ado, here’s the data:

A higher bar indicates more fluid retained. The dashed line represents twice the coefficient of variation away from water, meaning you can be relatively confident it’s a real effect above that line.

So milk stays in the body, presumably because of its calorie or potassium content. The same is true (barely) for orange juice. Oral rehydration solution stays in, thanks to its calories and sodium.

The biggest surprise? Neither beer nor coffee nor tea seem to produce significantly more urine, though coffee is marginally the lowest value on the index. One possibility is that the diuretic effects of alcohol in beer are counterbalanced by its retention-promoting calories. That’s supported by a previous study that compared regular and alcohol-free beer and found that regular beer triggered 12 percent more urine. Larger doses of caffeine or alcohol might get things flowing more noticeably, the researchers point out.

Does this have anything to do with running? Probably not - but don’t tell me you’ve never wondered about these questions.