Table 1.

Causes of vitamin B12 deficiency and underlying mechanisms

Conditions potentially associated with vitamin B12 deficiency	Underlying mechanisms
General malnutrition, chronic alcohol abuse, and vegan or strict vegetarian diets	Low or inadequate dietary intake of foods containing vitamin B12
Older age	Vitamin B12 malabsorption and deficiency due to inadequate dietary intake are common in the elderly
Gastric bypass, partial or complete gastrectomy, gastric reduction, bariatric surgery and chronic gastritis due to Helicobacter pylori infection	Impaired IF secretion
Atrophic gastritis (an autoimmune disease characterized by the presence of antibodies directed against gastric parietal cells and IF)	Immune-mediated destruction of gastric parietal cells, gastric mucosal atrophy, hypochlorhydria, decreased IF production, subsequent vitamin B12 malabsorption, vitamin B12 deficiency and pernicious anemia (a type of megaloblastic anemia)
Long-term use (≥ 12 mo) of drugs altering gastric acid secretion or gastric pH (e.g., PPIs, H2RAs and antacids)	These drugs reduce the production of hydrochloric acid by gastric parietal cells; as a consequence, vitamin B12 is not adequately released from the food matrix due to insufficient hydrochloric acid and low pepsin activity
Long-term use of metformin	The underlying mechanism accounting for metformin-induced vitamin B12 deficiency is not fully understood, although it may involve one or more of the following: (1) Interference with the calcium-dependent binding of the IF-vitamin B12 complex to the cubilin receptor on enterocytes at the ileum level; (2) Interaction with the cubilin endocytic receptor; (3) Alteration in small intestine motility leading to small intestinal bacterial overgrowth and subsequent inhibition of IF-vitamin B12 complex absorption in the distal ileum; (4) Alteration in bile acid metabolism and reabsorption; (5) Increased liver accumulation of vitamin B12; and (6) Reduced IF secretion by gastric parietal cells
Use of medications that affect vitamin B12 absorption or metabolism including the bile acid resin cholestyramine (used to treat hypercholesterolemia), colchicine (used for acute gout) and many antibiotics such as neomycin and the anti-tuberculosis drug para-aminosalicylic acid1	Cholestyramine can chelate IF; colchicine and antibiotics can inhibit endocytosis of the IF-vitamin B12 complex
Bacterial overgrowth syndromes, ileal resection or gastrointestinal diseases such as terminal ileitis, celiac disease, inflammatory bowel disease, Crohn’s disease and tropical sprue	Altered absorption of the IF-vitamin B12 complex in the terminal ileum; intestinal villous atrophy and mucosal injury (celiac disease, Crohn’s disease and tropical sprue)
Intestinal parasitic infestations (often accompanied by eosinophilia) caused by the protozoan Giardia lamblia or the fish tapeworm Diphyllobothrium latum	Vitamin B12 malabsorption through vitamin B12 trapping by the parasites
Disorders of the exocrine pancreas or pancreatectomy	Insufficient pancreatic enzyme activity leads to a reduction in the proteolytic degradation of haptocorrin (mediated by pancreatic proteases in the small intestine); as a consequence, vitamin B12 remains bound to haptocorrin, cannot form the IF-vitamin B12 complex and is not available for absorption by the enterocytes in the distal ileum
Nitrous oxide anesthesia or recreational use of nitrous oxide	Irreversible oxidation and inactivation of the coenzyme form of vitamin B12 (methylcobalamin) at the active site of the vitamin B12-dependent methionine synthase reaction, resulting in increased levels of MMA and homocysteine
Inherited disorders affecting the sequential steps in the assimilation, transport and intracellular processing and metabolism of vitamin B12	Reduced expression, binding activity or affinity of receptors and proteins involved in transport, intracellular processing and metabolism of vitamin B12

¹Unlike long-term use of proton-pump inhibitors, histamine H2-receptor antagonists or metformin, the frequency or duration of use of these drugs is usually insufficient to result in clinical vitamin B12 deficiency. H2RAs: Histamine H2-receptor antagonists; IF: Intrinsic factor; PPIs: Proton-pump inhibitors; MMA: Methylmalonic acid.