SubEx

Myth check

Does coffee dehydrate you? A litre of it, tested against water

In one small trial, a litre of coffee with 212 mg of caffeine left young men passing no more urine over four hours than a litre of water. Larger doses were not tested.

Caffeine

Coffee is often said not to count as a drink, because caffeine makes you pee. In one trial, 17 young men drank a litre of black coffee with 212 mg of caffeine and passed no more urine over four hours than after still water, and tea and cola did no worse. The trial could only catch a gap of 220 ml or more, and the men were fully hydrated and fasted. At that dose the water in the cup still counted; above about 300 mg at once, the researchers expect a measurable effect.

The findings behind this

1–
Physical & bodyCoffee did not make you lose more fluid than plain water

In one trial, young men drank a litre of black coffee with 212 mg of caffeine and passed no more urine over four hours than on water. Tea and cola, with less caffeine, did no worse.

★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.With 17 men on coffee, the trial was sized to catch a difference of 220 ml of urine or more, so a smaller loss could have gone unseen. Larger doses were not tested, and the researchers note a measurable effect on urine is likely once a single dose passes about 300 mg of caffeine. The men were fully hydrated and had fasted overnight. The litre went down in half an hour, not as cups spread across a day.Click to keep this openBoost energyCaffeine

More on this

All stories →