Originally Posted by YK
Caffeine has a known pharmacologic diuretic effect. Many believe it is dose dependent and only larger doses do it. I actually think it is true based on personal experience of exposure to high doses. Second aspect is that individual diuretic response may be dependent on person's overall exposure to caffeine. Having said that, a potential diuretic would be the last thing I'd take before expected dehydration of a tennis match. Caffeine, as many above said, provides zero metabolic fuel value.
I agree that caffeine has no metabolic value as a fuel for energy, but as a stimulant that helps you stay mentally alert, it's quite effective. Personally, I don't notice a diuretic response from it. Of course, I drink plenty of water during a typical tennis match.
In the past, I've used a few pre-workout powders that have lots of caffeine in them (among many other ingredients), most notably NO-Xplode and Assault. I actually liked the results on the tennis court. They didn't make me jittery or anything, and I was looking out for that. The only negative side-effect was that if I took them in the late afternoon or evening, I couldn't get any sleep that night. I don't take them anymore, but I can see why they might have some value for tennis players.