B12

Vitamin B12, or cobalamin, is a water-soluble vitamin that functions as a cofactor in two key enzymatic reactions. The body stores substantial amounts in the liver, and several chemical forms exist, including cyanocobalamin, hydroxocobalamin, methylcobalamin and adenosylcobalamin. The coenzyme forms methylcobalamin and adenosylcobalamin participate in methionine synthesis and the conversion of methylmalonyl-CoA to succinyl-CoA, supporting DNA synthesis and myelin maintenance in nerve tissue.

Dietary B12 occurs almost exclusively in animal-derived foods—meat, fish, eggs and dairy—and is added to some fortified foods. The vitamin is produced by gut bacteria and absorbed via a complex process that requires intrinsic factor, a protein produced in the stomach. The B12-intrinsic factor complex is absorbed in the terminal ileum; active absorption is limited to about 1–2 micrograms per meal, though large oral doses can be taken up passively by diffusion (roughly 1% of the dose). The recommended daily intake for adults is about 2.4 micrograms (higher during pregnancy and lactation).

Deficiency risk increases with low intake (notably in vegans), reduced absorption (pernicious anemia, aging with atrophic gastritis, surgical removal of stomach or ileum, Crohn’s disease, pancreatic insufficiency), or certain medications such as metformin or proton pump inhibitors. Symptoms may include fatigue, pallor, glossitis, and if untreated, peripheral neuropathy or cognitive changes.

Diagnosis typically involves measuring serum B12, with methylmalonic acid and homocysteine used to confirm deficiency. Treatment depends on cause and severity and may involve oral high-dose B12 or intramuscular injections; most people respond well. B12 has a wide safety margin in typical therapeutic use.