Vitamin-B 12 (Cyanocobalamin)-An Update
Cyanocobalamin (B12) is a cobalt-containing coordination compound produced by intestinal micro-organisms and found also in soil and water. Higher plants do not concentrate vitamin B 12 from the soil and so are a poor source of the substance as compared with animal tissues. INTRINSIC FACTOR is important for the assimilation of vitamin B 12.
Vitamin B12 may be found in liver, kidney, fish, and fortified milk and helps convert folic acid into its active form. Vitamin B12 is essential to synthesize DNA and promotes cellular division and is required for hematopoiesis (development of red blood cells in bone marrow) and to maintain the integrity of the nervous system.
Vitamin B12 is absorbed in the intestine with the aid of an intrinsic factor produced by gastric parietal cells. Once absorbed, vitamin B12 binds to the transcobalamin II protein and is then transferred to tissues. Vitamin B12 is stored in the liver for up to three years during which time it is slowly excreted in urine.
Vitamin B12 deficiency is common in patients who are strict vegetarians and in patients who have malaborption syndromes (cancer, celiac disease), gastrectomy, Crohn’s disease, and liver and kidney diseases. Vitamin B12 (cyanocobalmin) administered both orally and parenterally is equally effective in treating anemia from vitamin B12 deficiency.
However, use of parenteral cyanacobalamin is the most common method of vitamin B12 replacement because it may be more reliable and practical. Subcutaneous or intramuscular administration is appropriate. Vitamin B12 is absorbed completely following parenteral administration, whereas oral vitamin B12 is absorbed poorly via the GI tract.
Furthermore, use of parenteral vitamin B12 to treat megaloblastic anemia may circumvent the need to perform a Schilling test to diagnose lack of intrinsic factor. A typical cyanocobalmin dosing regimen is 800 to 1000 mcg/day for 1 to 2 weeks, followed by 100 to 1000 mcg/day every week until the Hgb/Hct normalizes and maintenance of 100 to 1000 mcg monthly for life. However, a number of dosing regimens exist. In addition, a number of oral vitamin B12 preparations are available, including many over-the-counter formulations.
A common oral dosing regimen is from 1000 to 2000 mcg/day. If parenteral cyanocobalmin is used initially, oral vitamin B12 can be useful as maintenance therapy. Typically, the response to therapy is quick. Neurologic symptoms and megaloblastic cells disappear within a few days, and hemoglobin levels increase after a week of therapy.Vitamin B12 generally is well tolerated and exhibits minimal adverse effects. Injection-site pain, pruritus, rash, and diarrhea have been reported. Drug interactions have been observed with omeprazole and ascorbic acid that decrease oral absorption.
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