II. Physiology
- Background
- B12 functional roles
- Neurologic function
- Vitamin B12 Deficiency results in Peripheral Neuropathy, Cognitive Impairment, gait Impairment
- Red Blood Cell production
- Vitamin B12 Deficiency results in Macrocytic Anemia (as well as Pancytopenia)
- DNA Synthesis
- Neurologic function
- B12 dependent reactions in humans
- Methylmalonic acid converted to succinyl-CoA
- Succinyl-CoA is part of Kreb Cycle in Glycolysis
- Homocysteine converted to Methionine (methylation reaction)
- Methionine is an Essential Amino Acid, and important for Angiogenesis
- 5-Methyltetrahydrofolate converted to Tetrahydrofolate
- Required for DNA synthesis and RBC production
- Methylmalonic acid converted to succinyl-CoA
- Normal B12 absorption pathway
- Requires ingestion (animal source or fortified cereal) as cannot be synthesized in humans
- Vitamin B12 is sythesized by Microorganisms in the Intestine
- Acidic Stomach environment breaks down Protein-bound B12
- Insufficient acid decreases B12 absorption
- Proton Pump Inhibitors suppress B12 absorption
- Intrinsic Factor (IF) binds B12 in duodenum
- Intrinsic Factor produced in Stomach parietal cells
- Pernicious Anemia is autoimmune parietal cell death
- Insufficient IF results in decreased B12 absorption
- Vitamin B12 absorption via 2 pathways
- Primary B12 absorption (from ingested fish, meat and dairy products)
- Vitamin B12 with Intrinsic Factor absorbed in terminal ileum
- Vitamin B12 dissociates from Intrinsic Factor in erythrocytes
- Vitamin B12 enters portal circulation and binds Transcobalamin II
- Alternate B12 absorption
- Pathway independent of Intrinsic Factor and ileum
- Absorption of up to 1% of large oral B12 dose
- Allows for oral B12 supplementation
- Primary B12 absorption (from ingested fish, meat and dairy products)
- Vitamin B12 storage
- Unlike other water soluble Vitamins, the body has capacity to store large quantities of Vitamin B12
- Vitamin B12 is stored primarily in the liver
- Large hepatic stores may delay B12 Vitamin Deficiency presentation more than 5 years
- Requires ingestion (animal source or fortified cereal) as cannot be synthesized in humans
III. Signs: Deficiency
IV. Sources: Animal sources only
V. Dosing
- See Vitamin B12 Supplementation
- Recommended daily allowance: 2.4 mcg/day
- Elderly, Vegans:
- B12 Fortified foods (Cereals)
- Daily dietary supplement up to Vitamin B12 1000 mcg orally daily
VI. References
- Babior in Wilson (1991) Harrisons, McGraw, p. 1523-9
- Rendon et al. (2017) Crit Dec Emerg Med 31(6): 15-21
- Oh (2003) Am Fam Physician 67(5):979-86 [PubMed]
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Related Studies
| cyanocobalamin (on 12/21/2022 at Medicaid.Gov Survey of pharmacy drug pricing) | ||
| CYANOCOBALAMIN 1,000 MCG/ML VL | Generic | $3.08 per ml |
| CYANOCOBALAMIN 10,000 MCG/10 ML | Generic | $3.68 per ml |
| CYANOCOBALAMIN 30,000 MCG/30 ML | Generic | $0.97 per ml |