A common synthetic form of the vitamin, cyanocobalamin, does not occur in nature, but is used in many pharmaceuticals and supplements, and as a food additive, because of its stability and lower cost. In the body it is converted to the physiological form, methylcobalamin.
Vitamin B12 was discovered from its relationship to the disease pernicious anemia, which is an autoimmune disease that destroys parietal cells in the stomach that secrete intrinsic factor. Intrinsic factor is crucial for the normal absorption of B12, so a lack of intrinsic factor, as seen in pernicious anemia, causes a vitamin B12 deficiency. Many other subtler kinds of vitamin B12 deficiency and their biochemical effects have since been elucidated.
Vitamin B12 is found in foods that come from animals, including fish and shellfish, meat (especially liver), poultry, eggs, milk, and milk products. One half chicken breast provides some 0.3 µg (micrograms) per serving or 6.0% of one's daily value (DV); 85 grams (3 oz) of beef, 2.4 µg, or 40% of one's DV; one slice of liver 47.9 µg or 780% of DV; and 85 grams (3 oz) of molluscs 84.1 µg, or 1,400% of DV.
Eggs are often mentioned as a good B12 source, but they also contain a factor (avidin) that blocks absorption. Certain insects such as termites contain B12 produced by their gut bacteria, in a way analogous to ruminant animals.