I have an 8 yr old, obese British shorthair cat that is sensitive to many catfood brands. He was often constipated and had bright red blood in his stool that smelled awful. The vet(s) told me that he had food allergies and recommended Hills Science diet or Eukanuba wt loss diet. I had him on both diets for quite sometime, both left him severely constipated and produced even more blood. I finally tried Nutro Wt Management diet. He has been on it for over a year with no problems and normal stool, but has only recently began having diarhea that smells putrid. He does not receive any human food of any kind. His vets told me to switch back to Hills science diet, but I am reluctant to as it made him so ill. I am interested in using the Royal Canin brand cat food but I would like to know other people’s experience with it before trying it out. Please help.