But you can just measure whether animal eaters (tend to) have a B12 deficiency or not - you don't need to know any of this. Is your doctor recommending this bas

But you can just measure whether animal eaters (tend to) have a B12 deficiency or not - you don't need to know any of this. Is your doctor recommending this based off a long chain of reasoning or based off of a study just directly measuring the rates of B12 deficiency? Since the latter is much easier and is what you actually care about.