Cobalamin

Cobalamin, commonly known as vitamin B12, is a water-soluble vitamin that functions as a cofactor in DNA synthesis, and in the metabolism of fats and amino acids. Its molecule contains cobalt within a corrin ring and is among the most chemically complex vitamins. The two physiologically active coenzyme forms are methylcobalamin, which participates in one-carbon metabolism, and adenosylcobalamin, a cofactor in mitochondrial energy production. Cyanocobalamin and hydroxocobalamin are commonly used in supplements and injections; cyanocobalamin is converted to active forms in the body.

Absorption and storage depend on intrinsic factor, a glycoprotein produced by gastric parietal cells. The B12-intrinsic factor complex is absorbed in the distal ileum via receptor-mediated endocytosis. Adequate intrinsic factor is required; its deficiency leads to impaired absorption and pernicious anemia. The body stores cobalamin primarily in the liver, with stores lasting several years. Daily requirements are typically around 2–3 micrograms. Dietary sources are predominantly animal-derived: meat, fish, dairy, and eggs; fortified plant-based foods provide alternatives. Vegans are at risk of deficiency without supplementation.

Clinical significance and deficiency: Inadequate B12 leads to megaloblastic anemia and, with progressive depletion, peripheral neuropathy, cognitive changes, and, if untreated, potentially irreversible neurological damage. Newborns of deficient mothers are at risk. Diagnosis relies on serum cobalamin measurement, with functional indicators such as methylmalonic acid and homocysteine elevation supporting deficiency. Treatment depends on cause and may include oral or parenteral supplementation; pernicious anemia often requires injections. In cases of impaired absorption, high-dose oral therapy can be effective or parenteral administration may be used to restore stores.