What is the role of vitamin B12 deficiency in the etiology of macrocytosis?

Updated: Dec 16, 2020
  • Author: Vincent E Herrin, MD, FACP; Chief Editor: Emmanuel C Besa, MD  more...
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Vitamin B12 deficiency is a cause of macrocytosis. Because DNA synthesis requires cyanocobalamin (vitamin B12) as a cofactor, a deficiency of the vitamin leads to decreased DNA synthesis in the erythrocyte, thus resulting in macrocytosis. A dietary deficiency of vitamin B12 is rare and usually only occurs in elderly persons on a "tea-and-toast diet" or in strict vegan vegetarians. However, deficiency can result from the following:

  • Lack of intrinsic factor in patients who have undergone gastrectomy or who have pernicious anemia

  • Malabsorption of vitamin B12 secondary to small bowel bacterial overgrowth, tapeworm, familial factors, drugs, ileal bypass, ileal enteritis, or sprue

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