Vitamin B-12
Description
Vitamin B-12 (cyanocobalamin) is a component of the water-soluble vitamin B-Complex. It is essential for normal growth, nucleic acid synthesis, protein and lipid synthesis, and the maintenance of epithelial cells and the nervous system.
Vitamin B-12 was given the name cobalamin because of the presence of cobalt. The active form of the vitamin is hydroxyocobalamin, and the vitamin is produced commercially as cyanocobalamin, the most stable form of the vitamin.
Method of Action
Calcium is necessary for the proper absorption of vitamin B-12. However, the vitamin is poorly absorbed from the intestinal tract unless a mucoprotein intrinsic factor is present. This intrinsic factor is part of the gastric juices and assists in absorption by binding to vitamin B-12, which helps attach the vitamin to a receptor in the membranes of the lower small intestine. The vitamin is then transported into the cells.
Vitamin B-12 is transported in the bloodstream bound to transcobalamin I and II. In this way it circulates to the tissues, and is concentrated in the liver and, to some extent, the kidneys. Vitamin B-12 is then released as needed to other tissues of the body and to bone marrow.
Vitamin B-12 is involved in nucleic acid synthesis as well as protein, fat, and carbohydrate metabolism. Its primary function is the removal and transfer of methyl groups. In folacin metabolism a methyl group is removed by vitamin B-12 to form the active species tetrahydrofolate. Vitamin B-12 is also essential for the normal metabolism and growth of all cells, especially those of the gastrointestinal tract, bone marrow, and nervous system tissues. It has been reported that vitamin B-12 assists in maintaining hormone balance.
Absorption appears to decrease with aging, iron deficiency, and vitamin B-6 deficiency. Tobacco, alcohol, coffee, and laxatives prevent absorption of the vitamin. Vitamin B-12 is destroyed by heating, acid, or alkali. Vitamin B-6, vitamin B-Complex, choline, inositol, vitamin C, potassium, and sodium assist absorption. Absorption increases during pregnancy.
Properties & Uses
Vitamin B-12 has been promoted as a treatment for night blindness, psoriasis, warts, problems related to menopause, and general malaise. However, no scientific evidence suggests these problems result from a lack of vitamin B-12, or they can be cured by its use. Currently the only clinical use for vitamin B-12 is to treat a metabolic defect in its absorption, such as pernicious anemia.
Vitamin B-12 supplements contain the active vitamin, as well as several analogs; the analogs have little or no vitamin activity. The presence of vitamin C, thiamine, and vitamin E in vitamin B-12 supplements may cause the creation of undesirable analogs, which can interfere with normal vitamin function.
Commercially produced vitamin B-12 is usually in the form of cyanocobalamin, which is produced by bacterial fermentation.
Consequence of Deficiency
If normal vitamin B-12 levels of 100 to 1,000 micrograms per milliliter fall to below 100 micrograms, pernicious anemia occurs. Pernicious anemia can result from an inadequate dietary intake of the vitamin; a diminished secretion of intrinsic factor by the glands of the stomach; the partial or complete removal of the stomach; or from a lack of the transport proteins transcobalamin I and II.
An additional cause of vitamin B-12 deficiency is the atrophy of the mucosal cells lining the stomach. This atrophy can result from genetic facts, alcoholism, iron deficiency, or thyroid gland disorders. An infection with the intestinal parasite Diphyllobothrium latum can produce pernicious anemia; the worm absorbs the available vitamin B-12, leaving little or none for the host's use. In the latter case, the anemia is easily cured by expulsion of the worm from the intestines.
Injections of vitamin B-12 can control pernicious anemia; however, the anemia must be detected at an early stage so that treatment starts prior to irreversible nerve degeneration. It is possible to diagnose vitamin B-12 deficiency on the basis of blood levels of the vitamin using microbiological techniques.
Since the body is efficient in storing vitamin B-12, it could take up to six years before vitamin B-12 deficiency symptoms develop in a diet devoid of the vitamin. Children of vegetarian parents can become vitamin B-12 deficient in two to three years, as they have no vitamin B-12 stores in their bodies.
Toxicity Factors
Vitamin B-12 is nontoxic to humans in doses exceeding the RDA by 10,000 times. Water-soluble vitamins in excess tend to be excreted in the urine, rather than stored in the tissues as are fat-soluble vitamins.
Recommended Dietary Allowance
| age | RDA (mcg) | RNI (mcg) | |
| infants/children | |||
| 0-6 months | 0.3 | 0.3 | |
| 6-12 months | 0.5 | 0.4 | |
| 1-3 years | 0.7 | 0.5 | |
| 4-6 years | 1.0 | 0.8 | |
| 7-10 years | 1.4 | 1.0 | |
| males | |||
| 11-14 years | 2.0 | 1.2 | |
| 15+ | 2.0 | 1.5 | |
| females | |||
| 11-14 years | 2.0 | 1.2 | |
| 15+ | 2.0 | 1.5 | |
| pregnancy | 2.2 | - | |
| lactation | 2.6 | 2.0 |
The amount of dietary vitamin B-12 needed for normal human metabolism appears to be very small. Reported minimum requirements have been from 0.6 to 1.2 mmicrograms per day, with a range to approcimately 2.8 micrograms to allow for individual variance. The ordinary diet easily provides this amount. For exammple, one cup of milk, one egg, and four ounces of meat provide 2.4 micrograms. The Recommended Daily Allownce standards recommends a daily intake of 3.0 micrograms for adults.
For over thirty years, Recommended Daily Amounts has existed in the United Kingdom. It has been used to measure the adequacy of an individual's diet. However, in 1991 the Committee on Medical Aspects of Food Policy (COMA) gave forth a whole new set of figures upon the request of the Department of Health's Chief Medical Officer. Reference Nutrient Intake (RNI) is one of these sets collectively known as "Dietary Reference Values." RNI is an amount of a nutrient that is enough for almost every individuals, even someone who has high needs for the nutrient. This level of intake is, therefore, considerably higher than what most people would need. If individuals are consuming the RNI of a nutrient they are most unlikely to be deficient in that nutrient.
Food Sources
Low: (0.5 - 5 mg/100 g)
| Beef | Cheese |
| Chicken | Cod |
| Egg | Flounder |
| Haddock | Halibut |
| Lamb | Lobster |
| Milk | Pork |
| Scallop | Shrimp |
| Swordfish | Tuna |
| Whale |
Medium: (5 - 50 mg/100 g)
| Beef heart | Chicken heart |
| Chicken liver | Clams |
| Crab | Egg yolk |
| Frankfurters | Herring |
| Oysters | Rabbit kidney |
| Rabbit liver | Rabbit heart |
| Salmon | Sardines |
High: (50 - 500 mg/100 g)
| Beef brain | Beef kidney |
| Beef liver | Lamb kidney |
| Lamb liver | Pork liver |
| Veal liver |
References
al-Momen-AK: Diminished vitamin B12 levels in patients with severe sickle cell disease. J-Intern-Med. 1995 Jun; 237(6): 551-5.
Bendich, A., Cohen, M. In: Nutrition and Immunology. Alan R. Liss: New York, 1988, pp. 101-123.
Borrman, W.R. 1979. Comprehensive Guide to Nutrition. New Horizons Pub Corp. Chicago, Illinois.
Brinton, L.A., et al. Br J Cancer, 1989; 59: 810-813.
Carethers, M. Geriatrics, 1988; 43: 89-112.
Chu, R.C., Hall, C.A. Am J CLin Pathol., 1988; 90: 446-449.
Cunha-UG et al: Vitamin B12 deficiency and dementia.. Int-Psychogeriatr. 1995 Spring; 7(1): 85-8.
Dickinson-CJ. Does folic acid harm people with vitamin B12 deficiency? QJM. 1995 May; 88(5): 357-64.
Doscherholmen, A., Swaim, W.R. Gastroenterol., 1973; 64: 913-919.
Gaby, S.K., Bendich, A. In: Vitamin Intake and Health: A Scientific Review. Marcel Dekker: New York, 1991, pp. 193-197.
Glusker-JP: Vitamin B12 and the B12 coenzymes. Vitam-Horm. 1995; 50: 1-76.
Guthrie, Helen A. Introductory Nutrition. 5th edition. St. Louis: C.V. Mosby Co., 1971.
Hathcock, J.N., Troendle, G.J. JAMA, 1991, 265: 96-97.
Heimburger, D.C., et al. J Am Med Assoc., 1988; 259: 1525-1530.
Herb Teas: How Safe? Consumer's Research Magazine, March 1977.
Hoerauf-K et al: [Nitrous oxide exposure to personnel in a recovery room with modern climate control]. Anaesthesist. 1995 Aug; 44(8): 590-4.
Inada, M., Toyoshima, M., Kameyama, M. J Nutr Sci Vitaminol., 1982; 28: 351-357.
Kirschmann, J.D. Nutrition Almanac: Nutrition Search. McGrew-Hill: New York. 1990.
Kirschmann, John D.. Nutrition Almanac: Nutrition Search Inc. New York: McGraw-Hill, 1975.
Krause, M.V. & L.K. Mahan. 1979. Food, Nutrition and Diet Therapy. 6th ed. W.B. Saunders Company, Philadelphia. 963.
Kutsky, R.J. 1973. Handbook of Vitamins and Hormones. Van Nostrand Reinhold, Co. New York, New York. 278.
Lederle, F.A. JAMA, 1991; 265: 94-95.
Murray, M.T. & Pizzorno, J.E. Encyclopedia of Natural Medicine. Rocklin, CA: Prima Publishing,1991.
Naurath-HJ et al: Effects of vitamin B12, folate, and vitamin B6 supplements in elderly people with normal serum vitamin concentrations. Lancet. 1995 Jul 8; 346(8967): 85-9.
Nilsson-Ehle, H., et al. Dig Dis Sci., 1989; 34: 716-723.
Olszewski, A.J., et al. Atherosclerosis, 1989; 75: 1-6.
Omaye, S.T. In: Nutritional and Toxicological Aspects of Food Safety. Plenum Press: New York, 1984, pp. 169-203.
Paltiel-O et al: Clinical correlates of subnormal vitamin B12 levels in patients infected with the human immunodeficiency virus. Am-J-Hematol. 1995 Aug; 49(4): 318-22.
Recommended Dietary Allowances. 1989. National Academy of Science, National Academy Press, Washington, D.C.
Richardson, L.R., Brock, R. J Nutr., 1956; 58: 135-145.
Schilling-RF & Williams-WJ: Vitamin B12 deficiency: underdiagnosed, overtreated? Hosp-Pract-Off-Ed. 1995 Jul 15; 30(7): 47-52; discussion 52, 54.
Shils, M.E., Young, V.R. Modern Nutrition in Health and Disease, 7th Edition. Lea and Febiger, 1988, pp. 401-404.
Swift, M.E., Schultz, T.D. Nutr Rep Int., 1986; 34: 1-14.
Veilleux-M et al: Sensorimotor neuropathy and abnormal vitamin B12 metabolism in early HIV infection.. Can-J-Neurol-Sci. 1995 Feb; 22(1): 43-6.
Walji, H., Vitamin Guide: Essential nutrients for healthy living., Element: Dorset, U.K. 1992.
Walji, H., Vitamin Minerals & Dietary Supplements., Hodder Headline Plc.: London, U.K. 1994.
Wardinsky-TD et al: Vitamin B12 deficiency associated with low breast-milk vitamin B12 concentration in an infant following maternal gastric bypass surgery [letter] Arch-Pediatr-Adolesc-Med. 1995 Nov; 149(11): 1281-4.
Williams, Sue Rodwell. Nutrition And Diet Therapy. 5th edition. St Louis: Times Mirror Mosby 1985.
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