Re: What would cause this
The most common cause of low B12 (from what I've read) is a vegan diet (complete elimination of all animal products).
Other causes could be Crohn's disease, digestive inflammation, alcoholism, liver disease, surgery of the stomach or small intestine, parasites, and a lack of intrinsic factor...which is protein in the stomach lining that helps the intestines utilise/absorb B12.....
Without B12, one can end up with pernicious anemia...which is different than iron deficiency.
Is your B12 very low?
It's all a matter of perspective!