I feed my dog grain free food. But dogs in the wild will eat a more omnivorous diet and can survive on it. Cats cannot subsist on the same sort of diet, they are what is properly termed an obligate carnivore.
http://en.wikipedia.org/wiki/Carnivore#Obligate_carnivores
“Can survive” does not mean “is best for the animal”
Their bodies render them anatomically and physiologically carnivores.
Studies on wolves highlight that even wolves will avoid herbivorous contents of their prey’s stomach:
http://rawfed.com/myths/stomachcontents.html