If your cat has chronic diarrhea — and your vet has ruled out medical conditions such as kidney or liver disease, and doesn’t suspect an allergy — you may just try switching to another cat food. It may not matter which one. “One study found that cats with chronic diarrhea often improve with diet change — it doesn’t matter what you change them to,” says Dr. Heinze.
If that helps, it likely wasn’t an allergy but an issue with another property of the cat food such as the type of fiber or digestibility. If you’re worried that your cat has a sensitivity to a specific diet, it’s fine to change the diet once and see if that helps, she says. “Choose one with the fewest ingredients possible. The more ingredients, the more complicated it gets to actually know what’s really working or not working.”
However, what you don’t want to do is keep trying diets without having a plan with your veterinarian for how and why you are doing it — to avoid specific ingredients, to increase digestibility, to reduce fat, etc. Without a well-thought out plan, you may just complicate matters and prolong the time it takes to get a real diagnosis.