Not actually "food", but I am a huge pusher of probiotics - I recommend http://www.omegaalpha.ca/.../Probiotic_8_Plus_trade/

I foster cats and kittens, and most of them come to me with horrible digestion from eating garbage, having parasites, etc. I mix the recommended dose of probiotic (and a pinch of prebiotics - Fructo-Oligo-Saccharides aka FOS can be found cheap at any supplement store) into their wet food. It takes a while for the gut bacteria to properly repopulate, but eventually they move to great digestion. And eat less while getting more out of their food. The stuff is recommended for dogs, too.

CHG