The relationship between the ailments that plague so many cats and nutrition could not be more clear to me now. But that sure as heck wasn't always the case. For years, I fed dry food (with canned food a 'treat') and put enormous faith in various prescription diets that vets recommended to me. I fed cats that way for many years, never making the link between the diseases that they struggled with that cut their lives shorter than I could bear and the wholly inappropriate diet I was feeding them.
It was only when, out of desperation, I began investigating other approaches to dealing with disease -- thanks to my amazing and very brave cat, Duke, and his six years of diarrhea and intestinal upset -- that the light bulb lit up brightly for me. Since then, I've come to learn that while changing a sick cat's diet isn't going to necessarily set everything right, until the diet is right nothing can ever truly heal. Diet is the brick and mortar of physical health in cats. There is no single change you can make to your cat's life that will have a more conspicuous impact on health.
I've put together some information here on inflammatory bowel disease (IBD), diabetes, and obesity just for starters. If you're dealing with any of these issues, please surf around, read, and see if there's anything here you find helpful. Please note too that with each passing year, more and more courageous people are publishing articles, papers, and books that succinctly lay out the relationship between a cat's nutritional requirements and health. My reading page has some of those, so if you want to dig deeper, check out some of those links to get an idea of what else is out there.
If your cat is facing any urinary tract issues, I strongly recommend a careful read of Dr. Lisa Pierson's web page on the subject.
I shudder now remembering those years of shoveling that awful dry, grain-filled cereal into my carnivores. But the good news is that there is now ample information available at our fingertips for shifting the tide back in a sane direction when it comes to feeding these magnificent creatures.
Read. Learn. Feed well.