Vitamin B12 (Cobalamin)
Essential Vitamins & Minerals · 8 specs
Direct Answer
Vitamin B12 is a cobalt-containing water-soluble corrinoid synthesized exclusively by microorganisms. Adult RDA is 2.4µg/day (no Upper Limit established), required for DNA synthesis, methionine synthase homocysteine conversion, and myelin sheath maintenance.
Source: NIH Office of Dietary Supplements / EFSA · Last reviewed Aug 28, 2026
Specs
| Nutrient Type | Water-Soluble Corrinoid Vitamin |
| Adult RDA / RDI | 2.4 µg / day (2.6µg pregnancy, 2.8µg lactation) |
| Tolerable Upper Limit (UL) | None Established (Extremely low toxicity) |
| Chemical Forms | Methylcobalamin (Active), Adenosylcobalamin (Active), Cyanocobalamin (Stable synthetic) |
| Primary Biological Function | Methionine synthase cofactor, Methylmalonyl-CoA mutase cofactor, Myelin integrity, Erythropoiesis |
| Deficiency Condition | Pernicious / Megaloblastic Anemia, Irreversible Neurological Peripheral Neuropathy |
| Top Whole Food Sources | Oysters, Beef Liver, Sardines, Salmon, Beef, Dairy, Eggs (Absent in unfortified plant foods) |
| Absorption Mechanism | Requires Gastric Acid, R-Protein, and Intrinsic Factor (IF) via Ileal Cubam receptors |
Intrinsic Factor Absorption Bottleneck
Active receptor-mediated ileal absorption of B12 via gastric intrinsic factor saturates at ~1.5–2.0µg per meal; doses above this absorb via passive diffusion at roughly 1% efficiency.
Technical Comparisons
Compare all in Essential Vitamins & Minerals →
Vitamin B12 (Cobalamin) vs Calcium (Elemental Ca)
→
Vitamin B12 (Cobalamin) vs Iodine (Elemental I)
→
Vitamin B12 (Cobalamin) vs Iron (Heme & Non-Heme)
→
Vitamin B12 (Cobalamin) vs Magnesium (Elemental Mg)
→
Vitamin B12 (Cobalamin) vs Potassium (Elemental K)
→
Vitamin B12 (Cobalamin) vs Selenium (Elemental Se)
→
Vitamin B12 (Cobalamin) vs Sodium (Elemental Na)
→
Vitamin B12 (Cobalamin) vs Vitamin A (Retinol & Beta-Carotene)
→