Many years ago, I recall someone writing that people should not eat ANYTHING that animals are, or have had, a part in producing. Well, that would eliminate a TON of plant-based foods, too, as insect pollination is necessary for many of them.

I knew a guy who was obsessively vegetarian (vegan?) and was so by choice, not because of any food allergies or intolerances, but because he thought eating any animal products, even a little, is really bad. He wasn't any healthier than anyone else, and tended to get sick a lot, and always seemed to injure himself doing minor things.