B12 is required for folate metabolism, cell division and replication. It is also involved in the metabolism of certain amino and fatty acids.
A glycoprotein called intrinsic factor must be present in the gastrointestinal tract in order for B12 absorption to take place. Instrinsic factor is secreted from cells lining the stomach. It then forms a complex with B12, carrying it to the small intestine where B12 is absorbed. In the small intestine, B12 can also be synthesized by bacteria. In the blood, B12 is present as methylcobalamin II, where it circulates bound to a protein called transcobalamin II. In the body, B12 is stored in the liver as adenosylcobalamin and circulates through the biliary system. Very small amounts are lost in the feces or urine and therefore deficiency takes a long time to develop.
Cobalamin is found almost exclusively in foods of animal origin. These include organ meats, milk and eggs.
Where B12 is deficient, folate deficiency may also occur. Anemia may be due to B12 and/or folate deficiency. The anemia due to B12 deficiency is macrocytic anemia. Glossitis (inflammation of the tongue) and gastrointestinal disturbances may occur. Neurological problems may also result such as parasthesia, unsteadiness, poor muscle coordination, moodiness, forgetfulness, depression, hallucinations and psychosis. Deficiency may occur due to inadequate intake (such as in a vegan diet) or where individuals cant afford foods of animal origin as in many developing countries.