Cobalamin, or vitamin B-12, has the most complex molecular structure of all the vitamins. It is the only vitamin that contains an essential mineral -- cobalt -- and it is the only B vitamin that your body stores for many months. However, dietary excesses of B-12 are readily eliminated in your urine. The Linus Pauling Institute at Oregon State University states that methylcobalamin, or methyl B-12, is one of two active forms of vitamin B-12 in your tissues. It is also found in some supplements. The dosage for all cobalamin supplements is the same, and your physician can help you decide what dosage is best for you.
Vitamin B-12 occurs in several different forms in foods, supplements and living tissues. Hydroxocobalamin, the form produced by bacteria, is converted in the laboratory to cyanocobalamin or methylcobalamin for use in supplements. Because it is inexpensive, cyanocobalamin is the form most commonly used for supplements and to fortify foods. Your body readily converts all forms of cobalamin to the active forms -- methylcobalamin and adenosylcobalamin -- used by your cells to perform their functions.
Vitamin B-12 performs only two principal functions in your body, but both are critical for your health. As a cofactor for the enzyme methionine synthase, methylcobalamin cooperates with folic acid -- another B vitamin -- to produce DNA and RNA and to synthesize the amino acid methionine from homocysteine. As part of the methylmalonyl-CoA mutase enzyme, adenosylcobalamin participates in the manufacture of hemoglobin, the oxygen-carrying protein in your red blood cells, and myelin, the protective lipid that coats and insulates your nerve fibers. Thus, vitamin B-12 is necessary for producing healthy red blood cells and maintaining the integrity of your nervous system.
Only bacteria can produce cobalamin. The methylcobalamin and cyanocobalamin found in supplements are ultimately produced from bacterial hydroxocobalamin. Plant foods do not contain B-12, as plants do not produce the vitamin, nor do they incorporate it into their tissues. Only animal-based foods are reliable natural sources of cobalamin. Liver and other organ meats, red meat, fish, shellfish, eggs and dairy products contain vitamin B-12. Tempeh and miso -- products of bacterial fermentation of soy, rice or barley -- also contain some vitamin B-12. Supplements containing methylcobalamin or cyanocobalamin are commercially available sources.
Recommended dietary allowances for cobalamin, regardless of its form, vary from 0.4 mcg daily for infants to 2.8 mcg for nursing mothers; nutritionist Elson Haas, M.D., states that 10 mcg to 20 mcg daily is "a good insurance level." However, these recommendations pertain to people who are efficiently absorbing B-12. Among individuals who don't absorb B-12 well, such as the elderly, daily requirements can be several hundred times greater than the recommended dietary allowance. A study published in the May 2005 issue of "Archives of Internal Medicine" demonstrated that you need to take around 1,000 mcg daily to overcome mild deficiency. Such dosages are well-tolerated and safe. Ask your doctor if you need additional vitamin B-12.