You can calculate how much to feed on your own:
((IDEAL body weight in kg x 30)+70) x RER = kcal needed per DAY
RER = resting energy requirement.
1.8 for an active adult dog
1.4 for maintenance
1.2 for something else.
0.8 for WEIGHT LOSS
1.0 is what I use for pets almost at ideal weight or at ideal weight that have a tendancy to gain weight.
Now you can see why every manufacturer over estimates pet food quantities to feed. 1. they want to sell pet food, but 2. it's dependent on the pet and their ideal weight as opposed to their current weight.
Go online to find the kcal/cup or can/pouch in the foods you feed. Most dry foods have 4-500 kcal/cup. This is why the average ideal body weight cat of 8-10 lbs needs only 1/4 cup of food AM and PM.
Nutritionally one could grind up shoe leather, oil, tar and bones and get the same ingredient analysis as many pet foods. ie reading labels is not so helpful as a lot can be hid behind those labels. Pet food companies of all sizes may switch sources or content of pet food depending on what's cheapest and the calorie content may change or there may not be much research behind it. We as veterinarians may not always get much education on it - though some of us seek it out to recommend the best. While I do not like the fact that there is corn in many of the prescription pet foods I carry for my clients, the other benefits of the foods out weigh that and the fact that the low carb containing ones contain high fat contents which tends to be worse. Actually wet foods usually contain higher fat contents than dry foods. Wet will provide more hydration to your cat, but will make the teeth worse. The teeth in turn will shower bacteria in the body and since kidneys get 40% of the blood supply, it is easy to see why kidney problems are the leading cause of poor dental health which is very much related to diet if brushing is not done daily (I can't even find the time to do that on my pets and I know the benefits).
I recommend Royal Canin Rx diet: Calorie Control High Protein. Instead of the avg 400 kcal/cup like most foods including m/d (which was taken off the market and may soon be affected by more recall issues), it has only 212 kcal/cup. ie more can be fed to make the cat have more volume, but also protein makes the cat feel fuller as opposed to high fiber which just bulks up the stools. This diet comes in dry, canned, and pouch.
The OTC diets again contain high fat for the most part and likely may not have consistent kcal contents. You can find wide range of opinions on this topic.
Measure food - ie calories in alone can give results though it may be easier if more food can be fed within that calorie restriction and less crying for food happens if the pet feels fuller.
I would start with an ideal weight of 17 lbs if your cat is 28 lbs. Once your cat it getting close, change to an ideal weight of 12 -lbs. ANY improvement will help decrease the six times more likely your cat is to develop diabetes being overweight. Use the RER of 0.8 and once 12-14 lbs is reached, then you can use an RER of 1.0. Remember all sources of calories including treats and supplements contributes to the daily calorie intake.
This helped my own cat and helps lots of my patients. Let me know if you have further questions or if this doesn't make sense.
I agree with the above comments and would like to add my thoughts.
Have you had your vet check your cat for hypothyroidism? This is not uncommon in cats especially cats like yours that weigh 28 pounds! As a practicing veterinarian for many years I have seen this in a handful of cats with signs similar to yours.
Checking your cats thyroid status involves a simple blood test your vet can take and send to the lab. If the test is positive your vet will give you a product to correct this.
The thyroid gland controls the rate at which your cat's body functions. If her thyroid level is low, a condition called Hypothyroidism, then she would gain extra weight without really eating an excessive amount of food or having a ravenous appetite. Once her thyroid function is corrected, she would eat normally and loose the extra weight.
As an alternative example, if her thyroid levels were too high, a condition very common in older teen age cats, called Hyperthyroidism, despite having a ravenous appetite and always being hungry, she would eat excessively and loose weight.
I hope this is helpful.
Dr. Carol Osborne, DVM