So I always try to steer clear of any cat foods or dog foods for that matter that have any type of byproduct in them because I feel they are not beneficial for our pets. There was an article on here and I had a hard time getting back to it that said that some byproducts are okay, but the problem is you don't know where they've come from. I wanted to share this link to the dog food advisor site which explains more thoroughly about byproducts and why you need to be careful, especially with your cats...
The Truth About Animal By-Products in Dog Food
The Truth About Animal By-Products in Dog Food