Table 2.

Stages of cobalamin metabolism and corresponding causes of cobalamin deficiency

Stage of cobalamin metabolism	Causes of cobalamin deficiency
Intake solely through food	Strict vegetarianism without vitamin supplementation
Digestion brings into play haptocorrin, gastric secretions (hydrochloric acid and pepsin), pancreatic and biliary secretions, enterohepatic cycle	Gastrectomy; pernicious anemia (Biermer’s disease); food-cobalamin malabsorption
Absorption brings into play intrinsic factor and cubilin	Ileal resection; malabsorption; pernicious anemia; Imerslund syndrome
Transport by transcobalamins	Congenital deficiency of transcobalamin II
Intracellular metabolism based on various intracellular enzymes	Congenital deficiency of various intracellular enzymes