Vitamin B-12 deficiency

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In medicine, vitamin B-12 deficiency, is a "nutritional condition produced by a deficiency of Vitamin B-12 in the diet, characterized by megaloblastic anemia. Since vitamin B 12 is not present in plants, humans have obtained their supply from animal products, from multivitamin supplements in the form of pills, and as additives to food preparations. A wide variety of neuropsychiatric abnormalities is also seen in vitamin B 12 deficiency and appears to be due to an undefined defect involving myelin synthesis."[1]

Manifestations include pernicious anemia and subacute combined degeneration.

Vitamin B-12 deficiency may be combined with iron deficiency anemia.[2]


Vitamin B-12 deficiency may be an explanation for macrocytosis[3]

However, the MCV is not completely accurate.[4][5][6]:

B12 levels are helpful, but imperfect:

Folic acid levels may not help.[11]

Diagnosis may be especially difficult in alcoholics.[12]


Oral therapy may be effective.[13]


  1. Anonymous (2021), Vitamin B-12 deficiency (English). Medical Subject Headings. U.S. National Library of Medicine.
  2. Beyan C, Kaptan K, Beyan E, Turan M (2005). "The platelet count/mean corpuscular hemoglobin ratio distinguishes combined iron and vitamin B12 deficiency from uncomplicated iron deficiency.". Int J Hematol 81 (4): 301-3. DOI:10.1532/IJH97.E0311. PMID 15914358. Research Blogging.
  3. Wymer A, Becker DM (1990). "Recognition and evaluation of red blood cell macrocytosis in the primary care setting.". J Gen Intern Med 5 (3): 192-7. PMID 2341917[e]
  4. Oosterhuis WP, Niessen RW, Bossuyt PM, Sanders GT, Sturk A (2000). "Diagnostic value of the mean corpuscular volume in the detection of vitamin B12 deficiency.". Scand J Clin Lab Invest 60 (1): 9-18. PMID 10757449[e]
  5. 5.0 5.1 5.2 Seward SJ, Safran C, Marton KI, Robinson SH (1990). "Does the mean corpuscular volume help physicians evaluate hospitalized patients with anemia?". J Gen Intern Med 5 (3): 187-91. DOI:10.1007/BF02600530. PMID 2187961. Research Blogging.
  6. 6.0 6.1 Savage DG, Lindenbaum J, Stabler SP, Allen RH (1994). "Sensitivity of serum methylmalonic acid and total homocysteine determinations for diagnosing cobalamin and folate deficiencies.". Am J Med 96 (3): 239-46. PMID 8154512[e]
  7. Pruthi RK, Tefferi A (1994). "Pernicious anemia revisited.". Mayo Clin Proc 69 (2): 144-50. PMID 8309266[e]
  8. Lindenbaum J, Savage DG, Stabler SP, Allen RH (1990). "Diagnosis of cobalamin deficiency: II. Relative sensitivities of serum cobalamin, methylmalonic acid, and total homocysteine concentrations.". Am J Hematol 34 (2): 99-107. PMID 2339684[e]
  9. Naurath HJ, Joosten E, Riezler R, Stabler SP, Allen RH, Lindenbaum J (1995). "Effects of vitamin B12, folate, and vitamin B6 supplements in elderly people with normal serum vitamin concentrations.". Lancet 346 (8967): 85-9. PMID 7603218[e]
  10. Matchar DB, McCrory DC, Millington DS, Feussner JR (1994). "Performance of the serum cobalamin assay for diagnosis of cobalamin deficiency.". Am J Med Sci 308 (5): 276-83. PMID 7977446[e]
  11. Ashraf MJ, Cook JR, Rothberg MB (2008). "Clinical utility of folic acid testing for patients with anemia or dementia.". J Gen Intern Med 23 (6): 824-6. DOI:10.1007/s11606-008-0615-z. PMID 18414954. PMC PMC2517884. Research Blogging.
  12. Savage D, Lindenbaum J (1986). "Anemia in alcoholics.". Medicine (Baltimore) 65 (5): 322-38. PMID 3747828[e]
  13. Vidal-Alaball J, Butler CC, Cannings-John R, Goringe A, Hood K, McCaddon A et al. (2005). "Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency.". Cochrane Database Syst Rev (3): CD004655. DOI:10.1002/14651858.CD004655.pub2. PMID 16034940. Research Blogging. Review in: Evid Based Med. 2006 Feb;11(1):9