One of my guys (Tanner) was diagnosed with IBD a couple of summers ago. IBD is the diagnosis that is given when nothing else really shows up. The only way it can be positively diagnosed is if the vet takes bioposies from the GI tract. That was how I found out about Tanner.

I eventually moved to making my own cat food so there are no dyes or preservatives in the food and he has been fine ever since then. (I buy a pre-mixed foundation from the web and then add the rest of the stuff)