Cats are naturally thin, lithe animals, but being too skinny is as unhealthy for your cat as being too fat. Cats can lose too much weight due to illness, injury or just finicky eating habits. You can't hold your cat down and force it to eat, but encouraging your underweight cat to gain weight is not as difficult as it might first appear.
Take the cat to your veterinarian for a thorough physical exam. Underweight cats often have serious health problems such as feline diabetes or thyroid conditions, which will not improve without medication. Your vet will also worm your cat to remove harmful parasites.
Replace half of your cat's normal food ration with soft cat food. Soft cat food is easier to eat and has a stronger scent, which can help spark your cat's appetite.
Add a spoonful of canned tuna to your cat's food, and pour the tuna juice over the food to entice the cat to eat. Most cats enjoy tuna and will readily eat a tuna-infused meal.
Stir a teaspoonful of olive oil into your cat's food. Olive oil is high in fat and calories and will help your skinny cat add a few extra pounds.
Supplement your cat's daily food intake with a high calorie-paste. These pastes, such as Tomlyn and Nutri-Cal, are packed with calories and nutrients that can help add pounds. Hold the cat gently in one arm and squirt the recommended dose directly on its tongue. The paste is extremely sticky, so wipe the cat's face with a damp rag to keep it clean.
It may take your cat a few days to get used to a new diet. Mild digestive upset, including soft stools, are normal during this transition.
Don't feed your cat too many table scraps. While it may gain a little weight, it may refuse to eat its normal cat food.