Vitamin B12 is a generic term used to describe a variety of cobalamins that are necessary for metabolic processes throughout the body effecting lipid, carbohydrate and protein metabolism. Cyanocobalamin is the commercially available form of B12 used in vitamin preparations and is converted in the body to a usable form. Since methylcobalamin is essential in folate metabolism, folate and B12 interact and are necessary for the normal conversion of homocysteine to methionine, for protein biosynthesis, synthesis of purines and pyrimidines, for methylation reactions and for the maintenance of intracellular levels of folate.
B12 is absorbed in the ileum of the intestine but requires a substance known as gastric intrinsic factor. Intrinsic factor is a highly specific binding protein secreted by the parietal cells of the stomach. For this reason, anything that affects the ability of the body to produce intrinsic actor or affects the ileum may reduce B12 absorption.
Such problems may occur in persons with gastric atrophy as seen in aged individuals and the disease pernicious anemia that results from antibodies directed against the stomach’s intrinsic factor producing parietal cells. This latter disease is also associated with achlorhydria or a reduction in acid production by the stomach. Individuals who have had a gastrectomy or who have had their pancreatic function impaired and thus a reduced production of enzymes necessary for the release of Vitamin B12 from binding proteins may also have a deficiency in Vitamin B12. In cases of Vitamin B12 deficiency, folate is unable to be converted to the form necessary for proper red cell production and thus a megaloblastic anemia may result.
Vitamin B12 is stored in the liver in quantities up to 10 mg. About 3 micrograms of B12 are secreted into the bile each day and largely re-absorbed in the ileum. Between 0.1 and 0.2% of Vitamin B12 is excreted each day and thus the RDA has been set at 2 micrograms per day for adults. Vitamin B12 is derived entirely from animal products including meat, fish, poultry, shellfish, eggs, milk, and dairy products. Since vegans do not consume adequate quantities of Vitamin B12 in their diet, supplements arc essential to ensuring good health. Vitamin B12 is quite stable during food preparation.
Deficiency states of Vitamin B12 are quite common, especially in the elderly who have the highest rate of pernicious anemia. B12 deficiency, however, may also result from lack of intake of B12 containing foods for prolonged periods of time, intestinal tape worms that compete for the B12 itself, and individuals with “blind loop” syndromes from prior intestinal surgery that leads to bacterial overgrowth and increased breakdown of B12 prior to it being able to be absorbed. Common findings associated with deficiency include a sore tongue, paresthesias, or numbness and tingling of the legs, weakness, and dementia. A megaloblastic or macrocytic anemia is characteristic though not essential to the diagnosis, and is associated with classic hyper-segmented poly-morphonuclear neutrophils. Since this may also be seen in persons deicient in folate, it is necessary that both vitamins be assessed early as folate replacement may correct the blood abnormality but not the neurological findings.
Large doses of B12 have not been associated with harmful effects and in fact, it has long been used as a “tonic” in medical practice.