skin allergy food and putting on lots of weight

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Rescue cat we've had two and a half years. His skin was red raw after a while from food allergy. Difficult to tell from what as we bought all sorts of random food. Vet put him on Hills z/d ultra and his skin has been fantastic BUT he continues to stack on weight and I think he really is now Too Big. We feed him NOTHING else. Honestly, not deluded. He might find a tiny bit of grated cheese we drop on the floor, or some other tiny morsel of table detritus but that's it (other than he is visiting an unkown neighbour and eating there, in which case he is not allergic to that food thank goodness as skin stays great)

Any advice on what to change food to?

Raw. Start with a single protein such as chicken in order to work out any allergies and slowly add other proteins over time.

It is also very possible that someone else might be feeding him. I put a tag on my cat saying 'medical condition, do not feed' in the hope of deterring people. She is perfectly healthy of course

Raw you say? Interesting... Expensive though...

I actually find it cheaper than any commercial diet. It doesn't have to be expensive!

Er - how much are you giving him? And how active is he?

Do you actually measure the z-d for the weight of cat you want to have.