Back before vegetarianism got popular enough that you could easily find vegetarian restaurants in cities and vegan options in menus, I had a friend who was raised vegetarian (maybe vegan? I don’t think back then I recognized a difference). We were in high school, so we all hung out together, which often meant McDonalds. He loved the smell of burgers, so he tried one once… and it made him violently ill. We figured out that his stomach literally did not have the biome necessary to digest meat, or that much meat, anyway. Last time he ever tried it AFAIK; he was a bit wistful about it, because he said he thought it was delicious. I’ve since lost track of him, but last I heard he was still vegetarian. The world has only gotten easier for vegans: there are so many products readily available almost everywhere now.