VITAMIN B-12

CV#748. Allen L, Herbert V. Vitamin B₁₂. In: Freeland-Graves J, Ed. Nutrient Summaries, 1996.

VITAMIN B-12

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.

Deficiency symptoms: Stage I (early deficiency), lower serum holoTC II (<60 pg/mL); II, lower serum vitamin B-12 (<300 pg/mL) and holoTC II (<40 pg/mL); III, serum B-12 <200 and holoTC II <40 pg/mL, neutrophil hypersegmentation, elevated serum and urinary methylmalonic acid and homocysteine; IV (severest deficiency), 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, 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.

Recommended and usual intakes: The 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 claims 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 B-12 in algae such as nori and spirulina. It is 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 vitamins 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 high risk of this condition. 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. Seventh edition. ILSI-Nutrition Foundation, 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.