In Meathooked: The History and Science of Our 2.5-Million-Year Obsession with Meat, science writer Marta Zaraska does a great job of exposing these claims as myths.
“Vegetarian animals ranging from gorillas to water deer, she reports, have bigger, sharper canines than we do; our canines aren’t specially meant for processing meat. What we lack dentally is more important, in fact, than what we have. Gently open a (calm) dog’s jaw, and there at the back will be the carnassial teeth, “blade-like and sharp and perfect for slicing meat.” Lions and tigers, racoons and house cats — all carnivores — have them too. We don’t. All the high-quality amino acid proteins we require are readily available in plants, Zaraska says, listing soy, buckwheat, quinoa and potatoes as examples.”
“Neal Barnard of the Physicians Committee for Responsible Medicine even notes that when people switch from meat-eating to plant-eating, their intake of vitamins and other nutrients improves.”