| VITAMIN B12 cyanocobalimin |
VITAMIN B12 cyanocobalimin
Vitamin B-12 (cobalamin), stored in the liver, is a cofactor for two coenzymes. Methyl-cobalamin catalyzes methyl group transfer from a folic acid cofactor to form methionine; the unmethylated folate cofactor then participates in single carbon reactions for nucleic acid synthesis. Thus some B-12 and folic deficiency symptoms are similar. The B-12 coenzyme deoxyadenosylcobalamin catalyzes amino acid and fatty acid breakdown.
Deficiencies: Stages of deficiency symptoms include Stage I (early deficiency), lower serum holoTC II (<60 pg/mL); Stage II, lower serum vitamin B-12 (<300 pg/mL) and holoTC II (<40 pg/mL); Stage III, serum B-12 <200 and holoTC II <40 pg/mL, neutrophil hypersegmentation, elevated serum and urinary methylmalonic acid and homocysteine; and Stage IV (severest deficiency), also megaloblastic, macrocytic anemia. Around Stage III (before anemia), potentially irreversible demyelination of spinal cord, brain, and optic and peripheral nerves produces peripheral neuropathy progressing to subacute combined degeneration. Dementia, poor attention span, and depression may be early symptoms.
The stomach secretes intrinsic factor that binds B-12 and mediates its absorption at receptor sites in the ileum. Inadequate intrinsic factor secretion occurs in pernicious anemia, an autoimmune disease. In the elderly, atrophic gastritis is commonly associated with B-12 malabsorption and deficiency. Because the absorbed vitamin is secreted in bile and subsequently reabsorbed, deficiency symptoms can take 20 years to develop from low intakes, e.g., in strict vegetarians. However, in malabsorption, deficiency occurs in months or a few years because absorption from both the diet and enterohepatic circulation is impaired.
Diet recommendations: The Recommended Daily Allowances (RDAs) are (µg/day): 0.3 at age 0-6 months, 0.5 for 6-12 months, 0.7 for 1-3 years, 1.0 for 4-6 years, 1.4 for 7-10 years, 2.0 for adolescents and adults, 2.2 in pregnancy and 2.6 in lactation. Usual intakes are about 4-8 µg/d. Pregnant, lactating, and long-term strict vegetarians should take supplements providing the RDA.
Food sources: Vitamin B-12 is found only in animal products. Excellent sources (>10 µg/100g) include organ meats and bivalve mollusks such as clams and oysters. Moderate amounts (1-10 µg/100g) are contained in egg yolks, muscle meats and poultry, fish, fermented cheeses and dry milk. Milk and milk products contain <1 µg/100g. There is no human-active form of B-12 in algae such as nori and spirulina; the forms are all analogues.
Toxicity: No toxic effects have been reported when up to 100 µg/day are consumed. Intramuscular injections of 100 µg are usually given once/month to individuals who cannot absorb the vitamin through their intestine, because of pernicious anemia or other problems.
Recent research: Vitamin B-12 deficiency may increase the risk of neural tube defects in pregnant women with a high risk of this condition. Vitamin B-12 deficiency may be common in developing countries, perhaps due to malabsorption and low intakes.
For further information:
Herbert, V. (1996) Vitamin B-12. In: Present Knowledge in Nutrition (Ziegler, E. E. & Filer, L. J., Jr., eds.), 7th ed., pp. 191-205. International Life Sciences Institute Press, Washington, DC.
Allen, L. H. & Casterline, J. (1994) Vitamin B-12 deficiency in the elderly: diagnosis and requirements. Am. J. Clin. Nutr. 60: 12-14.
Prepared By:
Lindsay Allen, Ph.D.
Professor, Department of Nutrition
University of California at Davis
Meyer Hall
Davis, CA 95616-8669
Phone: 530-752-5920
FAX:530-752-3406
Email: lhallen@ucdavis.edu
Victor D. Herbert, M.D., J.D.
Professor
Hematology and Nutrition Lab
Bronx VA Medical Center (151E)
130 W Kingsbridge Road
Bronx, NY 10468-3904
Phone: 718-364-5799
FAX: 718-562-9120
Email: nutristar@aol.com
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