Vitamin B12 (Cyanocobalamin)
Vitamin B12 serves as a cofactor for enzymes involved in the normal function of the nervous system, the formation of red blood cells and for the metabolism of folate. It is also involved in energy production. Together with folate and vitamin B6, vitamin B12 is required for maintenance of normal blood homocysteine levels. Raised homocysteine is a risk factor for cardiovascular disease.
Dietary deficiency is rare in younger people and only occurs among strict vegans. It is more common in older people as a result of impaired absorption, usually due to chronic inflammation of the stomach lining (atrophic gastritis) or lack of intrinsic factor (the substance required for vitamin B12 absorption). Deficiency results in the development of pernicious anaemia, in which red blood cells are enlarged (megaloblastic), and peripheral neurological damage develops.
There are few reports of any adverse effects of high intakes of vitamin B12.
Vitamin B12 is found in almost all foods of animal origin. Green plants provide none but it can be synthesized by some algae and bacteria, although the bioavailability of such forms has been disputed. Meat, fish, milk, cheese, eggs, yeast extract and fortified breakfast cereals are all dietary sources.