LoreOfSerpents posted:

Suggestions:
1. Buy a baby scale.
2. Make sure he isn't stealing food. Never leave him unattended with access to the other cat's food. If you leave him unattended, I guarantee that he's stealing her food.
3. Be very specific how you measure his daily food. If you feed him multiple times throughout the day, or if someone else helps feed him, measure out his daily portion into a separate jar and make sure you only feed him from that jar. Don't just reach for the 1/4 scoop and try to eyeball amounts.

It doesn't matter if online sources are telling you that you're feeding him the right amount. If he's gaining weight like that, he isn't being fed appropriately for his situation, because you only have two levers: food and exercise. Very sedentary cats don't need to eat as much to maintain their weight. I have one very petite, hyperactive cat who weighs 8 pounds and eats more calories than my lethargic 15-pound monster. It makes no sense when you look at the numbers, but their weight is good this way.

A baby scale is pretty much mandatory for any serious pet weight loss at home. I use something like this one but you can find cheap baby scales through thrift stores, garage sales, and mom Facebook groups. Don't use an adult bathroom scale, because it won't be accurate enough. A baby scale will let you track your cat's weight and make adjustments week by week. My cats all change their food/exercise habits seasonally, so I end up having to modify their food portions a few times a year to maintain their weight.

My goal for my two overweight cats was to get them to lose 1/4 pound per month just by gradually scaling back their food. That's intentionally a pretty slow pace for weight loss because I wanted to take it easy on them. One cat was overweight at 17 pounds (goal: 15 pounds) and another cat was overweight at 13 pounds (goal: 10 pounds). It took a long time. Even now that their weight is under control, I weigh them every 1-2 weeks.

As for what you wanted to hear, about food recommendations... unfortunately, I think you should consult a different vet about that, if you don't trust your current vet's recommendation. Food allergies aren't very common in cats in most regions, and they usually manifest as skin/hair problems near the face, or (less often) as problems in the GI tract. Your situation is pretty specialized if your vet suspects a food allergy related to asthma.

I can offer some general info, though. Limited ingredient diets are supposed to be strictly followed for 2-3 months to see if symptoms improve, but the diet becomes unreliable if you have multiple cats who eat different diets but still groom each other/share the same water dishes/have different treats/etc. So if you want to know if a limited ingredient diet is helping, you have to monitor the results closely over that time period, and you probably need to switch all cats in the house. Also, although food allergies are relatively uncommon for cats, the most common food allergen is the protein source. Most cat owners who use a limited ingredient diet can swap in another food that has the same protein source, as long as that's the only protein source. The same rules apply, though, because you'll need to monitor over another 2-3 months to see if conditions improve/worsen, in case your cat is actually sensitive to something other than the protein source.