I have been vegetarian for several years and now I rarely eat meat (never red meat). I have often heard people (including medical doctors) that in order to have a healthy life you should eat a bit of everything (that includes red meat).
People that have little or no medical knowledge have a hard time providing an explanation to support their claims and those having medical knowledge typically talk about risk of developing some deficiencies, although they do not dive into the details.
As far as I know, a vegetarian's greatest risk is related to B12 deficiency since it is virtually found in meat only and they have to compensate through supplements.
Question: How to counter the theory that you need to eat a bit of everything to have a balanced life?
I am interested mostly in answers that cover the social point of view as scientific references do not seem to be very convincing.