I posted to this forum a couple months ago, about our one kitty having persistant diarrhea, plus the vet said he needed to go on a diet (said my big boy was a bit overweight).
They tested him for everything known to man, and all bacteria, parasites, etc were all negative.
So the vet just said lets try a different brand of cat food, maybe the diarrhea is just a food intolerance.
Now my kitties have always been on just plain dry Iams, which I've read on this forum can really put on the pounds.
So we tried them on Ichanuba Low Residue dry, and lo and behold, everything so far is fixed. The diarrhea is gone, and both my kitties are slimming down. I know that Ichanuba is also made by Iams, but its a different formulation.
Its also a big more expensive (I pay about $15 for a 6 pound bag), and you have to have an Rx from the vet to even buy it at Petco or one of the big pet stores.
But I'm sold on how well its working for my little cat family!