The more I read and learn about cat food composition, the more confused I've become!
I've just spent some time researching the canned food products that are available at a chain store such as Petsmart--and based on what I've found out, there are no truly 'good' foods there (the ones that have been mentioned on this site--Solid Gold, Innova, etc-- are not sold there). My kittens like the Nutro pouches, but in checking the ingredients, there's no by products, but there is wheat gluten. Other canned foods-- Science Diet, Iams (some of them, others looked pretty good, IMO, and I may check them out further), ProPlan, Fancy Feast, Nutro cans) all had at least some of these less desirable ingredients.
We've also fed some Fancy Feast varieties--all of them have byproducts in them, as well as wheat gluten, but some of them have more of those ingredients than others. Not necessarily much 'worse' than other canned brands that you can find, based on my observations. The nitrites (spelling?) don't particularly bother me. I think they probably should--but in taking a personal reality check, I realize that I do feed my children hot dogs on occasion, and the fact that they have this ingredient hasn't stopped me from doing that. They will eat hot dogs, and I don't think they're such a terrible thing in the context of a well-balanced diet.
I've come to the conclusion that I need to really take a step back and stop worrying so much about what my cats are eating. I've come to realize that I don't worry this much about what my young children eat, and that has really made me stop and think. I'd like to feed the cats a decent diet that we can all feel good about, that I don't have to travel far and wide to find. I guess I just can't believe that every canned variety sold at a place like Petsmart is bad or is junk food for cats. Many of these are supposed to be 'premium' foods. I have to believe that there are a number of foods out there, even among the more 'commercial' or well-known foods (for lack of a better way to say this) that cats can eat and thrive as a result.
Just a few thoughts--thanks for all who answered my question. I have learned so much from this site.