4 Main Etiologies of Vitamin B12 Deficiency:

1. Autoimmune disorders- Autoimmune disorders like pernicious anemia produces antibodies to intrinsic factors (proteins that help the intestine absorb vitamin B12) that results in an inability to absorb vitamin B12
2. Malabsorption-Damage to the terminal ileum of the small intestine may cause malabsorption of vitamin B12 like gastric bypass surgery, surgical resection due to Crohn's disease, or inflammation from celiac disease.
3. Dietary insufficiency-Low intake of vitamin B12 from diet is seen in those that follow vegan diets, chronic alcoholics, and elderly people.
4. Toxin exposure-Exposure to nitrous oxide, metformin, and proton pump inhibitors can cause vitamin B12 deficiency