Vitamin B12


Vitamin B12, Cyanocobalamin, Cobalamin

  • Physiology
  1. Background
    1. Vitamin B12 is a water soluble Vitamin
    2. Porphyrin ring similar to heme, but contains a central combalt instead of iron
  2. B12 functional roles
    1. Neurologic function
      1. Vitamin B12 Deficiency results in Peripheral Neuropathy, Cognitive Impairment, gait Impairment
    2. Red Blood Cell production
      1. Vitamin B12 Deficiency results in Macrocytic Anemia (as well as Pancytopenia)
    3. DNA Synthesis
  3. B12 dependent reactions in humans
    1. Methylmalonic acid converted to succinyl-CoA
      1. Succinyl-CoA is part of Kreb Cycle in Glycolysis
    2. Homocysteine converted to Methionine (methylation reaction)
      1. Methionine is an Essential Amino Acid, and important for Angiogenesis
    3. 5-Methyltetrahydrofolate converted to Tetrahydrofolate
      1. Required for DNA synthesis and RBC production
  4. Normal B12 absorption pathway
    1. Requires ingestion (animal source or fortified cereal) as cannot be synthesized in humans
      1. Vitamin B12 is sythesized by microorganisms in the Intestine
    2. Acidic Stomach environment breaks down Protein-bound B12
      1. Insufficient acid decreases B12 absorption
      2. Proton Pump Inhibitors suppress B12 absorption
    3. Intrinsic Factor (IF) binds B12 in duodenum
      1. Intrinsic Factor produced in Stomach parietal cells
      2. Pernicious Anemia is autoimmune parietal cell death
      3. Insufficient IF results in decreased B12 absorption
    4. Vitamin B12 absorption via 2 pathways
      1. Primary B12 absorption (from ingested fish, meat and dairy products)
        1. Vitamin B12 with Intrinsic Factor absorbed in terminal ileum
        2. Vitamin B12 dissociates from Intrinsic Factor in erythrocytes
        3. Vitamin B12 enters portal circulation and binds Transcobalamin II
      2. Alternate B12 absorption
        1. Pathway independent of Intrinsic Factor and ileum
        2. Absorption of up to 1% of large oral B12 dose
        3. Allows for oral B12 supplementation
    5. Vitamin B12 storage
      1. Unlike other water soluble Vitamins, the body has capacity to store large quantities of Vitamin B12
      2. Vitamin B12 is stored primarily in the liver
      3. Large hepatic stores may delay B12 Vitamin Deficiency presentation more than 5 years
  • Sources
  • Animal sources only
  1. Liver
  2. Muscle Meats (e.g. beef)
  3. Fish
  4. Eggs
  5. Milk and other dairy products (e.g. Yogurt)
  6. Vitamin B12 fortified foods (Cereals)
  7. Soy Sauce
  8. Sauerkraut
  • Dosing
  1. See Vitamin B12 Supplementation
  2. Recommended daily allowance: 2.4 mcg/day
  3. Elderly, Vegans:
    1. B12 Fortified foods (Cereals)
    2. Daily dietary supplement up to Vitamin B12 1000 mcg orally daily
  • References
  1. Babior in Wilson (1991) Harrisons, McGraw, p. 1523-9
  2. Rendon et al. (2017) Crit Dec Emerg Med 31(6): 15-21
  3. Oh (2003) Am Fam Physician 67(5):979-86 [PubMed]