Hi,
You're right, a "sudden" change in diet isn't ideal when there’s a specific clinical condition involved, especially since cats can be quite resistant to new things.
Since your cat has never had grains in his diet, introducing them now is a major shock to his metabolism and gut flora, which won't be able to digest the new food properly. The body adapts to what it’s fed; if an animal doesn’t consume much starch (which is found in grains), it produces fewer of the enzymes needed to digest it. It’s a bit like people with lactose intolerance: the less milk they drink, the harder it is for them to digest it.
To try and avoid this, make sure you follow a proper food transition when switching from the old food to the new one. This usually takes about a week: give 1/4 of the new food and 3/4 of the old for two days, then half-and-half for another two days, and so on until you’re only feeding the new food. You’ll need to be very patient and take it slowly during this transition—even if it means extending it—because your cat's metabolism is not at all used to this type of diet.
I’m not sure what food you’ve tried or exactly what your cat’s kidney issues require, but in theory, a diet with high-quality, highly digestible proteins is your best chance at finding a compromise between your cat's health needs and what his system can actually handle.
Best of luck.
All the best,
Quitterie
