Vitamin B12 (Cobalamin)
vitaminA water-soluble vitamin essential for nerve function, red blood cell formation, and DNA synthesis.
Definition
A water-soluble vitamin essential for nerve function, red blood cell formation, and DNA synthesis. Found almost exclusively in animal products. DV is 2.4mcg. Vegans are at risk of deficiency.
What Is Vitamin B12 (Cobalamin)?
Vitamin B12, or cobalamin, is a water-soluble vitamin with the most complex chemical structure of all vitamins, featuring a central cobalt atom in a corrin ring. It is the only vitamin produced exclusively by microorganisms (bacteria and archaea); animals and humans obtain it by consuming animal-source foods or fortified products. The two metabolically active coenzyme forms are methylcobalamin and adenosylcobalamin.
The RDA for adults is 2.4 mcg/day. Pregnancy requires 2.6 mcg/day and lactation 2.8 mcg/day. Adults over 50 are advised to obtain most B12 from fortified foods or supplements because crystalline B12 is better absorbed than protein-bound B12 at reduced gastric acid levels.
Key Functions
- Methionine synthesis: Methylcobalamin is a cofactor for methionine synthase, which converts homocysteine to methionine, regenerating the universal methyl donor S-adenosylmethionine (SAM) and recycling folate.
- Myelin maintenance: Adenosylcobalamin participates in the metabolism of methylmalonyl-CoA to succinyl-CoA, essential for normal myelin sheath synthesis. Deficiency causes subacute combined degeneration of the spinal cord.
- Red blood cell formation: Without adequate B12, functional folate deficiency develops, impairing DNA synthesis and causing megaloblastic anemia — indistinguishable from folate-deficiency anemia without laboratory testing.
Deficiency Risk Groups and Food Sources
Groups at elevated risk include strict vegans (plant foods contain no B12 unless fortified), older adults with atrophic gastritis, individuals with pernicious anemia (autoimmune loss of intrinsic factor), and those taking proton pump inhibitors long-term. Neurological damage from B12 deficiency can be irreversible if prolonged, underscoring the importance of monitoring.
Animal-source foods are the primary natural sources: clams, beef liver, salmon, tuna, dairy products, and eggs. Many plant milks, breakfast cereals, and nutritional yeasts are fortified with B12.
Top Food Sources
| # | Food | Amount per 100g | % DV |
|---|---|---|---|
| 1 | ＜魚類＞ （さけ・ます類） しろさけ めふん | 330mcg | 13750% |
| 2 | Mollusks, clam, mixed species, cooked, moist heat | 98.9mcg | 4121% |
| 3 | Beef, New Zealand, imported, variety meats and by-products liver, cooked, boiled | 96.0mcg | 4000% |
| 4 | Lamb, variety meats and by-products, liver, raw | 90.0mcg | 3750% |
| 5 | Lamb, variety meats and by-products, liver, cooked, pan-fried | 85.7mcg | 3571% |
| 6 | Veal, variety meats and by-products, liver, cooked, braised | 84.6mcg | 3525% |
| 7 | Beef, New Zealand, imported, variety meats and by-products, liver, raw | 84.5mcg | 3521% |
| 8 | Beef, variety meats and by-products, liver, cooked, pan-fried | 83.1mcg | 3462% |
| 9 | ＜貝類＞ しじみ 水煮 | 82.0mcg | 3417% |
| 10 | Lamb, variety meats and by-products, kidneys, cooked, braised | 78.9mcg | 3288% |
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