Why do you pee a lot when you drink water but only pee once or twice when you drink Soda?

My urologist says Diet Soda irritates the prostate, causing more frequent irritation in men -- although I haven't seen scientific support for this. My personal experience is that this is definitely true. What I'm not sure is whether this is limited to dark-colored diet soda, and whether some other types of diet soda (such as diet tonic) may not have this effect.