What food should I feed my underweight 4-year-old cat to gain weight?

Taking in more calories is the only way to gain weight. Knowing the number of a pet's weight doesn't really tell us if they are healthy. Instead, we use body condition score to determine if a pet is under- or overweight. Here is a multi-part video on BCS for cats: https://www.youtube.com/watch?v=9yRvL8NcfHs

I suggest you take your cat to a veterinarian for a weigh-in and a wellness exam. Your vet can tell you if your cat is underweight. You can also use a body condition score chart to monitor her body condition as weight alone is not a reliable way to assess a cat's condition. If your vet says your cat is in good condition then you will not need to change food. Should you have to "fatten" her up you should use a complete cat food manufactured by a reputable company such as Iams, Hill's, Royal Canine, etc.. You can feed kitten food for a few weeks until she has the target condition specified by your vet. Here is a body condition chart: https://vet.osu.edu/vmc/sites/default/files/images/Body%20condition%20score%20chart%20cats_0.pdf

Thank you for submitting your question regarding Jastin. First, I would have her evaluated by her veterinarian to determine if she is in fact underweight or just small in stature. Some perfectly healthy cats are petite and do not grow to the size typical of the breed. However, there are numerous disease that can affect weight gain and growth. You want to ensure there is not a medical issue that needs addressing. In the meantime, ensure that she is eating kitten food as this will be higher in calories, fat and protein. It will aid in growth. Offer her canned food as well to supplement her dry kibble. This will help her to take in extra calories. I hope this information helps!

If there is a tag on her collar I would call her owners to see if perhaps she has any medical conditions which account for her small size. It could also be that she is not fully grown, so appears small. A very possible explanation as well is that she could have a parasite infection such as worms which are making weight gain difficult. If she has an owner I would not advise giving her any medications, even over the counter. As you do not know if her owners have also wormed her recently. I hope this was a helpful answer!

How lovely of you to take in Leo and look after him! I would be feeding him a high quality cat food, such as Royal Canin, Hill's Science diet, Purina ProPlan, or Iams. Feed him according to the package instructions on the food label. You can speak to your vet to see if they feel feeding him more than what the label recommends is appropriate, or if a prescription weight gain diet would be beneficial. I hope this helps!

Since she was missing for so long, an exam with your veterinarian would be recommended. Submit a stool sample to diagnose intestinal parasites. If she gets a clean bill of health, continue feeding a high quality canned diet until she is back to her normal weight.

It would be a good idea to rule out issues such as thyroid problems, internal parasites, metabolic disease, and organ issues, before switching Bootsie's foods, as just changing the foods may not be enough for her to keep the weight on. A complete exam from your vet along with bringing in a stool sample can help figure out if there is an underlying issue or if Bootsie just needs some extra calories.