Claim: Vitamin B12 is not produced by humans.

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 -LRB- including humans -RRB- 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 -LRB- shellfish , meat -RRB- , 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 .   Vitamin B12 consists of a class of chemically related compounds -LRB- vitamers -RRB- , all of which show pharmacological activity . It contains the biochemically rare element cobalt -LRB- chemical symbol Co -RRB- positioned in the center of a planar tetra-pyrrole ring called a corrin ring . The vitamer is produced by bacteria as hydroxocobalamin , but conversion between different forms of the vitamin occurs in the body after consumption .   Vitamin B12 was discovered as a result of its relationship to the disease pernicious anemia , an autoimmune disease in which parietal cells of the stomach responsible for secreting intrinsic factor are destroyed ; these cells are also responsible for secreting acid in the stomach . Because intrinsic factor is crucial for the normal absorption of B12 , its lack in the presence of pernicious anemia causes a vitamin B12 deficiency . Many other subtler kinds of vitamin B12 deficiency and their biochemical effects have since been elucidated . Due to impairment of vitamin B12 absorption during aging , people over age 60 are at risk of deficiency .