Dietary risk factors:
(1) vegetarian diet, especially a vegan diet
(2) elderly
(3) severe and chronic malnutrition (developing country, prisoner of war, etc.)
Gastrointestinal risk factors:
(1) deficiency of functional intrinsic factor
(1a) pernicious anemia (autoimmune atrophic gastritis)
(1b) gastric resection
(1c) hereditary defect in intrinsic factor
(1d) extensive diseases of the stomach
(1e) gastric bypass surgery
(2) malabsorption (tropical sprue, celiac sprue, pancreatic insufficiency, other)
(3) extensive ileal resection
(4) Crohn's disease
(5) HIV-1 infection
(6) fish tapeworm (Diphyllobothrium latum)
(7) drug interfering with vitamin B12 absorption (PAS, colchicine, meformin, anticonvulsants, other)
Other:
(1) chronic exposure to nitrous oxide (see Chapter 31)
(2) neonate born to a vegetarian mother
