Here is some info. for you -
Regarding plant proteins:
"Animal proteins generally have a more balanced amino acid profile, with a greater proportion of essential amino acids, and better digestibility than plant proteins."
- BSAVA Manual of Companion Animal Nutrition & Feeding
"This protein source (corn gluten meal) is not as digestible as high quality animal protein ingredients, but its protein is often more available than some of the poorer quality animal proteins."
-Canine and Feline Nutrition,Case,Carey, Hirakawa
http://www.ncbi.nlm.nih.gov/entrez/quer ... s=12224854
" Like most legumes, soybeans contain several complex carbohydrates and simple sugars that cannot be digested by enzymes of the small intestine. These carbohydrates pass undigested into the large intestine, where bacterial fermentation results in the production of intestinal gas and flatulence."
-Canine and Feline Nutrition,Case,Carey, Hirakawa
Regarding chicken/poultry by-products:
"Poultry by-product refers to the clean parts of carcasses of slaughtered poultry, which may contain bone, heads, feet and viscera. Because of the inclusion of heads and feet, poultry by-product meal may be lower in nutritional value than fresh poultry meal."
- Canine and Feline Nutrition,Case,Carey, Hirakawa
Here are some links:
http://www.littlebigcat.com/index.php?a ... w&item=004
http://cats.about.com/cs/catfood/a/tipsforchoosing.htm
http://www.maxshouse.com/feline_nutrition.htm
http://www.mihog.org/diet/foodingred.phtml