I have recently been asked by a friend this question. This was a loaded question, because I know he does not like the idea of being a vegetarian. He also mentioned that all MDs suggest a "balanced diet" that should include "everything".
Since the question was quite sudden I could not think about a proper answer and could only say something along the following lines:
- being a vegetarian is also a lifestyle option and MDs typically refrain from suggesting such changes unless it is to prevent a disease from occurring or getting worse
- science based nutrition is not a focus in medical training, AFAIK (might apply only when I live and several other countries).
I searched more and there seems to be no consensus about this, so I am asking here because the network format requires more fact based answers than other places.