Vitamin B12

Last revised by Daniel J Bell on 2 Aug 2021

Vitamin B12 (hydroxocobalamin, hydroxycobalamin or B12a) is a water-soluble vitamin, part of the vitamin B complex, and synthesized by intestinal flora that forms a cobalt-based coenzyme that is required for two vital cellular reactions, namely the production of methionine (an amino acid) and the metabolism of odd-number carbon atom fatty acids. Vitamin B12 is one of the hematinics, due to its importance for erythropoiesis.

Related pathology

Vitamin B12 deficiency (hypocobalaminemia) leads to cell membrane dysfunction as it is incorporated with abnormal fatty acids, to which the central nervous system is particularly susceptible.

The commonest cause of deficiency of vitamin B12 is pernicious anemia, an autoimmune condition in which autoantibodies form against intrinsic factor (IF). Intrinsic factor is secreted in the stomach and binds to cobalamin. The  intrinsic factor-cobalamin complex is subsequently absorbed in the distal ileum. Lack of intrinsic factor leads to malabsorption of vitamin B12. Chronic pathology of the terminal ileum (e.g. Crohn disease) and distal ileum surgical resection can also lead to reduced absorption of the complex and, therefore, hypocobalaminemia.

Due to its involvement with folate in the synthesis of amino acids and nucleic bases, B12 deficiency can precipitate folate deficiency.

See also

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