I have discussed this with Technical Services veterinarians from both Hills Pet Nutrition and Royal Canin. Here is their answer, and my 40 years of experience working with my clients supports this.

In theory, a sudden switch from one food to another can trigger intestinal upset, usually evidenced by diarrhea. In actual practice, though, this is pretty uncommon. People switch their pets all the time and rarely is there any problem.

Imagine yourself eating only one exact meal three times daily for two years (e.g. a chicken sandwich with green beans and plain yogurt), then suddenly switching to a steak dinner; contrast this to our varied diet, constantly presenting our GI tract with different stresses.

Just like our nutrition needs varied ingredients, any dog or cat food (commercial or home recipe) will be deficient in specific places in the nutrient profile. By changing foods regularly, those deficiencies will average out.

On the other hand, if your pet has a specific medical condition that is being supported by a veterinary therapeutic diet, don’t switch unless advised to do so. The nutrient profile being fed is more important than any deficiencies.