In my case, any water lost to caffeine-bladder effects during exercise is trivial compared to water lost through sweat glands.

So heavily sweating athletes are probably a noisy data source.

One hydration study followed 59 healthy male volunteers for 11 days, using varying levels of caffeine. The researchers found no evidence of dehydration. “These findings question the widely accepted notion that caffeine consumption acts chronically as a diuretic,” the scientists concluded.

While this finding will seem counterintuitive to many coffee drinkers, especially commuters who have suffered through bladder-bursting traffic jams, Ganio said the science bears it out. Twelve ounces of coffee or 12 ounces of water will have about the same effect.

The claim is that coffee is indistinguishable from water and challenges the notion that caffeine is a diuretic at all, which is a much, much stronger statement than that caffeine consumption is a net-neutral on hydration levels because drinkers tend to offset the diuretic effects with just enough additional liquid volume.

My bodily functions definitely respond differently to a large (cup and a half) coffee than to a large water, so I find the claim highly suspect.

Excess salt is an absolute dehydrating agent. The kidneys filter it out and dilute it to a safe concentration.

Caffeine is usually consumed with more than enough water to offset absolute dehydration. If by “no evidence of dehydration” they mean caffeine-takers don’t end up dehydrated, this might be what they’re talking about.