All natural or not?

I'm at a loss on what to feed my cat! He is currently on Prescription Diet M/D for his weight. I have been researching and read that I should not feed him anything with "corn", "corn meal" or "by products". That all of these things can cause health problems. Is this true? Should I feed him an all natural product?

DO NOT PANIC (in large, friendly letters).
   
There is such a thing as too much information. Please do not over-react.
   
I'm not going to make any recommendations until you give me some details.
   
Let's look at the problem:
   
Exactly how overweight is your cat? Is he an average-sized-build cat that weights 20 lbs? 15 lbs? What are we talking here? Average-sized cats should weigh 10-12 lbs, depending on their bone structure. On those "how to tell if your cat is fat" charts (see link), where does your cat clock in?
   
Are there any underlying health problems that are causing the weight problem (Diabetes? Low thyroid function? When was his last full blood panel?), or is your cat just a fat cat who was overfed?
   
Is he happy with the food? Is he otherwise healthy & lively as can be expected for a fat cat?
   
Answer these questions, then we'll get into a discussion on feeding an overweight cat.
   
(And remember, virtually all cat food (& for that matter, much of processed people food) has "by-products" - which is a catch-all word that means very little at times - & corn in it.)
   

   
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Thank you for the reply.
   
Buddy is about 16lbs. He is very healthy.
   
From what I read (and you're right a little knowledge can be dangerous) ... they cannot digest corn and that all of these are "fillers". My main concern is that I want him to be healthy, eating healthy food.
   
I realize that you get what you pay for and I am willing to pay more for good food. I just don't know what I should be feeding him!
   
I also have 1 inside/outside cat that I recently adopted and he'll eat anything. Plus, (I know I'm the crazy cat lady) I have 3 kittens (4 mos) and 1 female cat that have adopted me. And their father stops by for breakfast every morning. So, needless to say, I'm feeding 7 cats!
   
I can't figure out the best thing to feed them all. Any suggestions would be much appreciated!