Vegan is almost an anagram of vagina!
When I was studying to get the bit of paper I have, that most folks would find ironically unbeleivable, I went on a vegan diet for 6 months to do blood tests and stuff, I was ill as fuck. Cholesterol was down to really 'healthy 'levels but B12, iron, zinc and calcium were fucked as fucked can be. B12 deficiency is a cunt because it can make you totally Radio rental. The Vegan society recommends eating foods fortified with B12, but as 99% of those use B12 from critters, cos that's where B12 comes from, they would be really upset if you told them that - so fucking tell them.
To be compassionate and caring, bear in mind when a vegan tells you they are a vegan, which they will do, they are probably suffering from B12 deficiency stark raving mental syndrome, so just tell them to fuck off and eat a fucking steak.
The big probelm with all these food nazis telling other people to eat what they do is that everyone has a different genetic makeup, a very small minority of people can be healthy on a vegan diet, some folks can eat gluten without aggro, I can't, but I don't go round telling the world and its dog to stop eating glutenny yummy bread. Some people will die if they eat peanuts, celery, eggs, garlic, dairy stuff etc. Some people say they have an allergy but really they are just a bit sensitive. Some people obviously have a B12 deficiency when they're not even vegan, because they still think Brexit is a good idea.