How can I fatten up my cat?

First, choose a high-quality cat food with protein above 40% and meat among the first ingredients. Usually supplement with boiled chicken breast, fish, egg yolk, liver, and goat milk, and occasionally cat strips and canned food for extra nutrition.

Also deworm regularly, since many thin cats are infected with parasites that steal nutrition and make them thinner despite eating. Finally, consider spaying/neutering, because after it hormones drop, metabolism slows, and activity decreases, making weight gain easier.