Vitamin B₁₂, vitamin B12 or vitamin B-12, also called cobalamin, is a water-soluble vitamin that has a key role in the normal functioning of the brain and nervous system, and the formation of red blood cells. It is one of eight B vitamins. It is involved in the metabolism of every cell of the human body, especially affecting DNA synthesis, fatty acid and amino acid metabolism. No fungi, plants, nor animals (including humans) are capable of producing vitamin B₁₂. Only bacteria and archaea have the enzymes needed for its synthesis. Some plant foods are a natural source of B₁₂ because of bacterial symbiosis. B₁₂ is the largest and most structurally complicated vitamin and can be produced industrially only through a bacterial fermentation-synthesis. This synthetic B₁₂ is used to fortify foods and sold as a dietary supplement.