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Cause	Source
Inadequate diet	Vegan diet Breastfeeding infants of vegan mothers Fad diets
Impaired absorption	Lack of intrinsic factor (due to autoimmune metaplastic atrophic gastritis, destruction of gastric mucosa, gastric surgery, or gastric bypass surgery) Intrinsic factor inhibition Decreased acid secretion Small-bowel disorders (eg, inflammatory bowel disease, celiac disease, cancer, biliary or pancreatic disorders) Competition for vitamin B12 (in fish tapeworm infestation or blind loop syndrome) Advanced HIV infection Antacids
Inadequate utilization	Enzyme deficiencies Liver disorders Transport protein abnormality
Altered metabolism	Metformin Nitrous oxide (repeated exposure)