What happens if I keep feeding kitten food to an adult cat?

The extra calories and fat relative to an adult's maintenance needs are a common and avoidable contributor to feline overweight and obesity once growth has finished.

How do I know if my cat has finished growing?

Most cats reach close to adult size by around 12 months, though some larger breeds take longer. Tracking growth against expected patterns and checking body condition score are more reliable than age alone.

Should I transition the food gradually?

Yes -- mix increasing proportions of the new food with the old over about a week to reduce the chance of digestive upset.

Kitten Food vs. Adult Cat Food: When to Make the Switch | PetCalculator

Kitten food is formulated for a much faster growth rate than adult maintenance food. Here's how the formulas differ and when most cats are ready to transition.