I find that there is no ground for ethics which is outside of the human condition; that there is no one perfect ideal or set of ethics.
Rather, out of our condition we build ideals. And our ideals we build as best we can based on what we know and what works for forming a world in which we would like to live.
An explicit and primary feature in my ethics is to minimize suffering and cruelty.
As to the eating of animals:
Insofar as one can minimize or eliminate harming animals in order to meet dietary requirements, that person is worthy of praise. Some people, owing to both their circumstances and their particular genetic constitution are able to eliminate animal products. Others are not, and we are not justified in asking them to harm their own health in order to be in keeping with our ideal; for, this in itself increases suffering.
Look at circumstances. Assume one is in an environment in which sufficient plant supply is not available; and if this person does not get nutrition, she will harm or destroy her body. One can imagine one being in an airplane crash, and finding only turkey sandwiches to eat, etc. Or one can imagine a people who live far from farm land, say, in the far north of Canada or Siberia, and the only way to meet their human requirements is to eat the flesh which is a feature of the environment.
It follows from this that ethics is contingent on environment, and we are not justified in making universal a single recipe for ethical eating, except to say that we respect that eating which minimizes suffering and cruelty.
Look at constitution. I remember clearly a passage in Thoreau’s Walden in which he mocks the farmer who says that a man must eat meat in order to get enough protein, and he does this as he is following a massive and powerful ox as it plows his field. The grass the ox eats provides plenty for the ox to make its protein.
But people are not oxen; we are chemically distinct. That is, our metabolic processes are slightly different owing to evolution, wherefrom we have become omnivores. We have more need of some chemical compounds, and less of others, than does the ox. Therefore, there will be different dietary demands.
Nor are all human constitutions alike. Some humans can adopt either a vegan or vegetarian diet with little difficulty, and perhaps with great advantage. There are others who do get sick. Take for example the Dali Lama, who had to give up his vegetarianism because his body grew ill, and turned yellow. I do not fault him; and I do not condemn his choice to eat meat.
I gave up my vegetarianism on similar grounds. On the advice of a doctor, I allowed meat into my diet, and this helped me to heal my body.
Still, I would prefer that I ate less meat, for both health and ethical reasons. So, I make that an ideal.
The key to making one’s ideal more compassionate is to recognize the contingent features of ideals. Both circumstance and constitution are important aspects to take into consideration; and if we don’t, we wind up being rather pope-like: looking morally like silly old men with silly hats who are virgins teaching about sex.