Regarding dog and cat food, you have to remember that wolves and wild cats heat whole animals including stomachs and intestines. Therefore, the kinds of food the animals were eating become part of their nutritional needs. What do deer, antelope and bison eat; what do rabbits, birds, and mice eat. Often it is grains, grass and seeds.
Well i have special knowledge of the prey animals. they are grazers and designed to extract the max nutrients out,of low energy foods like grass hays and often have guts more like cows and other ruminants to do so. but they have been designed to work best on this type of food intake. their guts are more delicate than humans/ominvores straight shot through difestive systems.