The PlexiDor small accommodates cats up to 24 pounds, so most kitties will be able to use it even if they gain some weight. But be aware those extra pounds can be a health hazard to your pet, so you may want to consult your veterinarian and find a way to help kitty get back in shape.
Look at the recommendations for the cat food you’re using and make sure you’re not overfeeding. Giving an appropriate amount of food and treats will ensure your cat doesn’t gain any more weight. Your veterinarian may also recommend using another type of food to help your cat lose weight.
Cats should not fast. The feline liver does not down-regulate during a fast and susceptible cats can become very ill after not eating for just a couple of days.
Interactive play that entices your cat to run and jump is great for building muscles and losing weight. Many cats love toys like DaBird, or to chase a laser pointer. As an added benefit, playing together will strengthen your bond.