The carnivores' livers secrete a far greater amount of bile into the gut to deal with their high-fat meat diet. Much strong hydrochloric acid in stomach to digest tough animal muscle, bone, etc. Reproduced with permission. The gorge of a cat, for instance, will rise at the smell of a mouse or a piece of raw flesh, but not at the aroma of fruit. If a man can take delight in pouncing upon a bird, tear its still living body apart with his teeth, sucking the warm blood, one might infer that Nature had provided him with carnivorous instinct, but the very thought of doing such a thing makes him shudder.
Some scientists disagree with the above analysis.
They assert that people's natural diet is omnivorous, based on both flesh and vegetarian foods. They point to the many years that our ancestors have eaten meat and the fact that primates, the animals whose systems are closest to ours, have been observed to eat meat. Certainly people have eaten meat for at least thousands of years. Just as an automobile will travel on a fuel which is not most suitable to it, people can live on a diet that is not ideal. The issue is not what people eat now and have eaten in the past, but the diet that is healthiest for people and is most consistent with our anatomy, physiology, and instincts.