If your dog or cat is underweight, it’s a good idea to schedule a veterinary consult—especially if your pet is older in age, very young (a growing puppy or kitten), is suddenly losing weight, or is showing signs of illness.
This will rule out any underlying medical condition that needs to be addressed, such as internal parasites, diabetes, hormonal conditions, dental issues that make it hard to eat, etc.
Also, your vet will confirm that your pet is truly underweight—interestingly, excessive weight in pets is so common in North America, that often a perfectly healthy, spry dog or cat will be mistakenly perceived as underweight!
If your pet is otherwise healthy, here are a few other things to consider about their food.
Growing pups and kitties need a food designed for their metabolic demands, not an adult pet food. This is also true for pregnant and lactating pets.
Check the feeding recommendations on the food’s packaging relative to your pet’s ideal weight, and increase if necessary—especially if your pet is underweight AND still hungry after meals.
If that’s the case, they may need more food than the recipe’s feeding recommendations specify. This will be true for very high-energy dogs—such as those that participate in sporting events.—
A good rule of thumb is to make changes gradually. So if you do decide to increase your pet’s food intake, do it slowly and watch your pet’s response. Otherwise it’s easy to go too far the other way, resulting in excessive weight gain.