Vitamin B12, also known as cobalamin, is a water-soluble vitamin involved in metabolism.[2] It is one of eight B vitamins. It is required by animals, which use it as a cofactor in DNA synthesis, in both fatty acid and amino acid metabolism.[3] It is important in the normal functioning of the nervous system via its role in the synthesis of myelin, and in the circulatory system in the maturation of red blood cells in the bone marrow.[2][4] Plants do not need cobalamin and carry out the reactions with enzymes that are not dependent on it.[5]
Vitamin B12 is the most chemically complex of all vitamins,[6] and for humans, the only vitamin that must be sourced from animal-derived foods or supplements.[2][7] Only some archaea and bacteria can synthesize vitamin B12.[8] Most people in developed countries get enough B12 from the consumption of meat or foods with animal sources.[2] Foods containing vitamin B12 include meat, clams, liver, fish, poultry, eggs, and dairy products.[2] Vitamin B12 is available as a supplement, and for the treatment of deficiency as an intramuscular injection.[2][6]
The most common cause of vitamin B12 deficiency in developed countries is impaired absorption due to a loss of gastric intrinsic factor (IF) which must be bound to a food-source of B12 in order for absorption to occur.[9] A second major cause is age-related decline in stomach acid production (achlorhydria), because acid exposure frees protein-bound vitamin.[10] For the same reason, people on long-term antacid therapy, using proton-pump inhibitors, H2 blockers or other antacids are at increased risk.[11] The diets of vegetarians and vegans may not provide sufficient B12 unless a dietary supplement is consumed. A deficiency in vitamin B12 may be characterized by limb neuropathy or a blood disorder called pernicious anemia, a type of megaloblastic anemia, causing a feeling of tiredness and weakness, lightheadedness, headache, breathlessness, loss of appetite, pins and needles sensations, changes in mobility, severe joint pain, muscle weakness, memory problems, decreased level of consciousness, brain fog, and many others.[12] If left untreated in infants, deficiency may lead to neurological damage and anemia.[2] Folate levels in the individual may affect the course of pathological changes and symptomatology of vitamin B12 deficiency.
Vitamin B12 was discovered as a result of pernicious anemia, an autoimmune disorder in which the blood has a lower than normal number of red blood cells, due to a deficiency in vitamin B12.[5][13] The ability to absorb the vitamin declines with age, especially in people over 60 years old.[14]
Vitamin B12 is a coordination complex of cobalt, which occupies the center of a corrin ligand and is further bound to a benzimidazole ligand and adenosyl group.[15] It is a deep red solid that dissolves in water to give red solutions.
A number of related species are known and these behave similarly, in particular all function as vitamins. This collection of compounds, of which vitamin B12 is one member, are often referred to as "cobalamins". These chemical compounds have a similar molecular structure, each of which shows vitamin activity in a vitamin-deficient biological system, they are referred to as vitamers. The vitamin activity is as a coenzyme, meaning that its presence is required for some enzyme-catalyzed reactions.[10][16]