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Normal absorption of vitamin B12

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

  • acidic environment of the stomach facilitates the breakdown of vitamin B12 that is bound to food

    • intrinsic factor, which is released by parietal cells in the stomach, binds to vitamin B12 in the duodenum
    • vitamin B12-intrinsic factor complex subsequently aids in the absorption of vitamin B12 in the terminal ileum

  • in addition to this method of absorption, evidence supports the existence of an alternate system that is independent of intrinsic factor or even an intact terminal ileum

    • approximately 1 percent of a large oral dose of vitamin B12 is absorbed by this second mechanism (1)
      • this pathway is important in relation to oral replacement

Once absorbed, vitamin B12 binds to transcobalamin II and is transported throughout the body

Notes:

  • in humans, only two enzymatic reactions are known to be dependent on vitamin B12

    • methylmalonic acid is converted to succinyl-CoA using vitamin B12 as a cofactor
      • vitamin B12 deficiency can lead to increased levels of serum methylmalonic acid

    • homocysteine is converted to methionine by using vitamin B12 and folic acid as cofactors
      • therefore a deficiency of vitamin B12 or folic acid may lead to increased homocysteine levels

Reference:

  • Elia M. Oral or parenteral therapy for B12 deficiency. Lancet. 1998;352:1721-2.

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