I have researched cat and dog foods for years. I have come to some conclusions, a good deal of it common sense.
The cat is of the highest order of carnivores. That means they have to have meat in order to stay healthy. When a cat eats it's prey, they eat the whole thing, guts, brains, eyeballs, hair, bone and pre-digested stomach contents. Everybody say Yuk!
The cat with it's God given wisdom and instincts know exactly what to eat to stay healthy. A farm cat that hunts is very healthy. They don't have all the diseases that our companion cats have. It all boils down to nutrition.
First of all, you have to remember that even though the package always says "complete" nutrition, it is only complete for what is known today. We don't know everything about nutrition, so we don't know what constitutes "complete" nutrition for our cats. We can only try to feed the best food we can, as naturally as possible.
Being carnivores, the cat doesn't need carbohydrate, and fiber in more than tiny amounts. A cat should not have grain. They do not attack stalks of grain in the farmers field! It is safe to say that most of the cat food on the market today is GRAIN based. What, primarily does Purina do? They are a CEREAL co. Where do you think the by products from their cereal mfg company goes? Why do you think they got into the pet food industry in the first place? So they would have some place to dump their by-products for gain, instead of throwing them out. By the way, they make terrific rabbit feed, and chicken feed. Be-cause these animals thrive on a grain based diet.
The first thing you have to do is to learn how to read labels. If the package says, chicken, corn gluten meal, rice, rice polishings, even though the first ingredient is meat, this is a grain based product. They get very tricky!
The meat is 65% water to begin with. The next ingredients are fractionated parts of the grain, which is bad because it is not a whole food. The grain is already dry, so that food is practically all grain.
I never feed gluten meal of any kind. After the Bruhaha over soybean they got real clever, and started using gluten meals to take it's place. Gluten, is the protein part of the grain, which causes people who are allergic to wheat, etc, all the trouble. So think what it does to a total carnivore! A cat cannot handle any protein other than from meat. It stresses the immune system and compromises the health of the cat. They will try to get along with it, but eventually they will break down, especially breeding animals who are under stress.
By products is something that has gotten a lot of bad press. Some companies even advertise that their products contain NO by products, as if it were a good thing! It is not a good thing. In the wild, the cat eats ALL of the parts including the by products. The SOURCE of the by products is the thing you should be worrying about. That they are clean and free from toxic junk, and that they are human grade from an inspected plant. Why are by products necessary? Because there is nutrition in by products that cannot be gotton anywhere else, i.e. muscle meat.
The best way to feed a cat is a raw diet. If it is not practical to do that, then some raw meat everyday should be given. How would you like to eat nothing fresh for your whole life? Only eat junk food and out of a can? The enzymes from raw food are essential for health.
Virtually, all petfood mfg plants today are USDA inspected. It comes under FDA regulations. They are in BIG trouble if they use inferior feedstuff. Most in fact, is human grade, and all is tested for toxic chemicals, etc.