Vitamin B12, also called cobalamin, is a water-soluble vitamin that is involved in the metabolism of every cell of the human body: it is a cofactor in DNA synthesis, and in both fatty acid and amino acid metabolism. It is particularly important in the normal functioning of the nervous system via its role in the synthesis of myelin, and in the maturation of developing red blood cells in the bone marrow.
Vitamin B12 is one of eight B vitamins; it is the largest and most structurally complex vitamin. It consists of a class of chemically related compounds (vitamers), all of which show physiological activity. It contains the biochemically rare element cobalt (chemical symbol Co) positioned in the center of a corrin ring. The only organisms to produce vitamin B12 are certain bacteria, and archaea. Some of these bacteria are found in the soil around the grasses that ruminants eat; they are taken into the animal, proliferate, form part of their gut flora, and continue to produce vitamin B12.
Because there are no reliable vegetable sources of the vitamin, vegans must use a supplement or fortified foods for B12 intake or risk serious health consequences. Otherwise, most omnivorous people in developed countries obtain enough vitamin B12 from consuming animal products including meat, milk, eggs, and fish. Staple foods, especially those that form part of a vegan diet, are often fortified by having the vitamin added to them. Vitamin B12 supplements are available in single agent or multivitamin tablets; and pharmaceutical preparations may be given by intramuscular injection.
The most common cause of vitamin B12 deficiency in developed countries is impaired absorption due to a loss of gastric intrinsic factor, which must be bound to food-source B12 in order for absorption to occur. Another group affected are those on long term antacid therapy, using proton pump inhibitors, H2 blockers or other antacids. This condition may be characterised by limb neuropathy or a blood disorder called pernicious anemia, a type of megaloblastic anemia. Folate levels in the individual may affect the course of pathological changes and symptomatology. Deficiency is more likely after age 60, and increases in incidence with advancing age. Dietary deficiency is very rare in developed countries due to access to dietary meat and fortified foods, but children in some regions of developing countries are at particular risk due to increased requirements during growth coupled with lack of access to dietary B12; adults in these regions are also at risk. Other causes of vitamin B12 deficiency are much less frequent.