That brings up a good point, I've always been under the impression that humans do best when eating a broad range of foods and that eating all meat or all non meat was not as healthy. While it appears the vegetarian can with a little work can make sure they get all the proteins they need. However the reverse is not true, you can't get all the nutrients you need by eating all meat. So how do the pure carnivores do it?
The inuit used to eat only meat and fish.
That was quite fine, because all parts of the animal body contain different nutrients and vitamins. And this differs per species as well.
So the reverse is also true.