Truthfully, I think it is the type of food you are feeding that has caused her to gain weight. Yes, their metabolism can change after spaying, but if you are feeding a lot of carbs ... a cat can gain weight and they will eat-eat-eat if their food doesn't make them feel full, which is what I think carbs do?
I'm NOT 100% sure about that, but I do know cats need *meat* and not grains.
Our Squirrely-Jo was an 18# fat-cat.
I fed Iams free-choice and thought I was feeding a premium type food. I didn't learn until coming here to CatForum that it really wasn't all it was cracked up to be. It was still a McKittyCrack type of catfood. Tasty, and had all the correct nutritional requirements ... but just as a steady diet of McDonalds isn't healthy for people, neither was this food I was feeding with all of the carbs that felines don't need and can't digest.
About 2yrs ago I switched to Innova free-choice and included one large canned meal a day. S-Jo lost 1/3rd of her body weight in about a year. She went from a hernia-making 18# fluffy bowling ball of a cat, to a svelte 12# cat who would run, jump and play. She stopped parking herself in front of the food all day and eat-eat-eating and now she only eats enough to 'feed her machine' and she is much healthier and happier being a more active cat.
Check out these different brands and their first ingredients, I stopped at "natural flavors".
In particular, I shall also highlight just the first THREE, which are major ingredients w/in the products. Please take note of the actual *meat* in the different products. ('meal' is not considered a 'meat'
Avo-Derm Chick/Herr kitten
Chicken Meal, Ground Whole Corn, Chicken Fat,
Herring Meal, Corn Gluten Meal, Ground Whole Brown Rice, Ground Whole Wheat, Natural Flavor,
Avo-Derm Chick/Herr adult
Chicken Meal, Ground Whole Corn, Corn Gluten Meal,
Chicken Fat, Ground Whole Brown Rice, Ground Whole Wheat, Herring Meal,
Avo-Derm Chick/Herr indoor/hairball
Chicken Meal, Ground Whole Corn, Whole Ground Brown Rice,
Corn Gluten Meal, Chicken Fat, Tomato Pomace, Oatmeal, Herring Meal, Oat Bran,
Iams Orig OceanFish/Rice:
Chicken By-Product Meal, Corn Meal, Corn Grits,
Animal Fat, Ocean Fish, Brewers Rice, Dried Beet Pulp,
Turkey, Chicken Meal, Chicken,
Potatoes, Eggs, Barley, Chicken Fat, Rice, Flaxseed
Could you try to introduce your Old Girl to a dry food with better ingredients?
http://www.littlebigcat.com/?action=lib ... cannedfood
...best of luck...