Vitamin B12, 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, or animals (including humans) are capable of producing vitamin B12. Only bacteria and archaea have the enzymes needed for its synthesis. Some substantial sources of B12 include animal products (shellfish, meat), fortified food products, and dietary supplements. B12 is the largest and most structurally complicated vitamin and can be produced industrially only through bacterial fermentation synthesis, typically used to manufacture B12 for fortified foods and supplements.