Vitamin B₁₂

Vitamin B₁₂, or cyanocobalamin, is a complex corrinoid compound containing four pyrrole rings that surround a single cobalt atom.² Humans obtain vitamin B₁₂ exclusively from animal dietary sources, such as meat, eggs, and milk. Vitamin B₁₂ requires intrinsic factor, a protein secreted by the parietal cells in the gastric mucosa, for absorption. Vitamin B₁₂ and intrinsic factor form a complex that attaches to receptors in the ileal mucosa, where proteins known as transcobalamins transport the vitamin B₁₂ from the mucosal cells to the blood and tissue.^3,4 Most vitamin B₁₂ is stored in the liver as well as in the bone marrow and other tissues.

Vitamin B₁₂ and folate are critical to normal DNA synthesis, which in turn affects erythrocyte maturation.^3,5,6 Vitamin B₁₂ is also necessary for myelin sheath formation and maintenance.⁷ The body uses its B₁₂ stores very economically, reabsorbing vitamin B₁₂ from the ileum and returning it to the liver so that very little is excreted.^4,8

Clinical and laboratory findings for B₁₂ deficiency include neurological abnormalities, decreased serum B₁₂ levels, and increased excretion of methylmalonic acid.^4,8,9 The impaired synthesis associated with vitamin B₁₂ deficiency causes macrocytic anemias. These anemias are characterized by abnormal maturation of erythrocyte precursors in the bone marrow, which results in the presence of megaloblasts and in decreased erythrocyte survival.^3,10

Pernicious anemia is a macrocytic anemia caused by vitamin B₁₂ deficiency that is due to lack of intrinsic factor.^5,6 Low vitamin B₁₂ intake, gastrectomy, diseases of the small intestine, malabsorption, and transcobalamin deficiency can also cause vitamin B₁₂ deficiency.³

Pregnant women need increased amounts of folate for proper fetal development.¹¹ If a woman has a folate deficiency prior to pregnancy, it will be intensified during gestation and may lead to premature birth and neural tube birth defects, such as spina bifida, in the child.¹¹