It’s a good idea to avoid feeding your dog or cat any fruit, especially apples.
The main reason for this is because it causes diarrhea in dogs and cats once the vast majority of the sugar from the apple is rinsed from its skin. The amount of sugar in an apple varies depending on how much time has elapsed since it was picked, but the average quantity falls somewhere between 8-12%. In their natural habitat cats can survive eating birds (predominately small, juvenile songbirds) who eat insects (that are mostly plant matter). Keep in mind that commercially grown tomatoes contain around 7% of their weight in sugar, which means limiting tomato intake to one serving per day.