Vitamin B12, also called cobalamin (Cbl), is a water-soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins and is normally involved in the metabolism of every cell of the human body, especially affecting DNA synthesis and regulation, but also fatty acid synthesis and energy production. Neither fungi, plants, nor animals are capable of producing vitamin B12. Only bacteria and archaea have the enzymes required for its synthesis.
Cobalamin’s structure contains four reduced pyrrol rings linked together and designated “corrin” because they are the core of the molecule. The prefix “cob” designates the presence of a cobalt atom.
Corrinoids are cobalt-containing cyclic structures in the human body, in foods of animal origin, and in bacteria, which make not only vitamin B12 but also various analogues thereof. Vitamin B12 and all its analogues are corrinoids. The human uses as vitamin B12 only those corrin nuclei to which are added the three other basic parts of the Cbl molecule: aminopropanol, sugar, and a nucleotide. To use it as a vitamin, the human cell must see it as a Cbl with no alterations except in the R adduct. Cobalamins remain vitamin active for humans with a variety of R adducts, which are named cobalamin with whatever is attached to the cobalt as a prefix. Thus, we have hydroxocobalamin, cyanocobalamin, adenosylcobalamin, and methylcobalamin among the naturally occurring human-active and potentially human-active forms of vitamin B12 in various foods.
For healthy adults, dietary vitamin B12 intake of 4-7ug/day is associated with normal vitamin B12 status as measured by all relevant biomarkers.