Coffee 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.
What the study found
Coffee with 212 mg of caffeine held on to no less fluid than water
The test was litre for litre, coffee against still water. In one trial, 17 young men passed no more urine in four hours after a litre of black coffee with 212 mg of caffeine, nor after tea or cola, lower in caffeine. At these doses the caffeine did not cancel the water it came in.
The caveat · Too few men to rule out a small loss
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.