Summary
Vitamin B12 (cobalamin) is a water-soluble vitamin built around a cobalt atom. It is required to form healthy red blood cells, to synthesise DNA, and to maintain the myelin sheath that insulates nerves — so a deficiency shows up as both a megaloblastic anaemia and a progressive neuropathy. Because B12 occurs almost exclusively in animal foods and needs stomach-derived intrinsic factor to be absorbed, vegans, older adults and people with gastrointestinal conditions are most at risk of running low.
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Where Does It Come From?
Vitamin B12 is made by microorganisms and reaches us through animal foods — meat, fish, shellfish, eggs and dairy — where it has accumulated. Plant foods contain essentially none, so fortified foods (some cereals and nutritional yeast) and supplements (cyanocobalamin or methylcobalamin) are the reliable sources for those avoiding animal products.
General Actions
- Red-blood-cell formation
- DNA synthesis
- Maintenance of myelin and nerve function
- Homocysteine metabolism (with folate)
- Energy metabolism from fats and proteins
Specific Actions
B12 is the cofactor for two enzymes: methionine synthase, which (together with folate) remethylates homocysteine to methionine, and methylmalonyl-CoA mutase, which processes certain fatty acids and amino acids. When B12 is low, folate becomes “trapped” and homocysteine and methylmalonic acid rise — the markers used to confirm deficiency.
Indications
- Diagnosed B12 deficiency and pernicious anaemia
- Vegan and vegetarian diets
- Malabsorption (gastric surgery, atrophic gastritis, metformin use)
- Elevated homocysteine or methylmalonic acid
- Peripheral neuropathy attributable to deficiency
Chemical Description
Cobalamin is a corrinoid — a large ring system surrounding a central cobalt ion. Supplemental and natural forms differ by the group attached to the cobalt: cyanocobalamin (stable, synthetic), methylcobalamin and adenosylcobalamin (the two coenzyme forms) and hydroxocobalamin (used by injection).
Extractions/Synthesis
Commercial B12 is produced by bacterial fermentation, then purified — most often as cyanocobalamin because of its stability.
Pharmacology/Medical Research
B12’s coenzyme roles are well characterised. Clinical interest centres on detecting and correcting deficiency (especially the neurological form, which can become irreversible if untreated), the homocysteine pathway it shares with folate, and the high-dose oral regimens that can treat deficiency without injections.
Toxicity
Vitamin B12 has a very low potential for harm; the body excretes what it cannot store, and no Tolerable Upper Intake Level has been established. This is why corrective doses of hundreds to a thousand micrograms are used freely.
Synergy
Functions hand-in-hand with folate (B9) — a deficiency of one can mask the other — and with vitamin B6 in homocysteine metabolism.
References
- National Institutes of Health, Office of Dietary Supplements. Vitamin B12 — Health Professional Fact Sheet. ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional