I never had much luck with hypoallergenic food. The problem is, knowing what a cat is intolerant of/allergic to. There are are a few possibilities, but often, just feeding a food that doesn't contain wheat often does the trick. Obviously there are other possibilities. You could try James Wellbeloved, but it still may contain something that causes a reaction. Apparently, with hypoallergenci food, any meat is processed so much that it can't be recognised in the body as any particular type of protein.
So basically, an elimination diet is best, but can be a long haul - but if you do find out what is causing the problem and just eliminate that from the diet, there are a lot more food options available.
With Alice (No longer with us) she had terrible, bloody diarrhea. After countless vet trips and her refusing to eat various brands of hypoallergenic food (RC, Hill's, Purina etc) I decided to start eliminating various things in her diet. I started with wheat and just fed her Purina One sensitive and cereal-free wet food (bog standard Felix senior) almost overnight she got better. She was still having the odd episode and then I realised that she was also having issues with anything that contained dairy. It wasn't the lactose, it was milk proteins - so even a lick of cream or butter (she loved both) would start her off.
Just one more thing, more recently, we adopted a stray (Domino) and he has some skin issues. After a couple of tests, the vet suggested doing full allergy testing. He said it was around £500, but it would tell us exactly what was causing the problem. He told me that insurance would probably cover it, but at that stage, We hadn't got round to insuring him. I switched him to Purina one sensitive and cereal-free and his skin cleared up. He still has patches of dry skin on his back, but I think that is more down to him being a big cat and not being able to groom properly - the red, sore patches have all gone.
