Whether we can digest meat or not has no relevance to whether we *should* kill animals to do it -- that is an ethical question. And since meat is not necessary at all, the answer to that question is obviously "no". Your argument that vitamins aren't natural is also flawed. Just because something isn't natural, doesn't mean it isn't [i] beneficial[/i] -- if you were consistent with your logic, you would be against all forms of modern medicine. Would you turn down vaccines, antibiotics and life saving treatments because they aren't "natural"? I hope not. We don't know whether lentils can suffer, but we have no good reason to believe they can. They have no brains for consciousness and no nervous systems to feel pain. Animals, on the other hand, clearly have both these things. It is the obvious rational decision to eat a plant instead of an animal.