Function

Vitamin B12, or cobalamin, is a coenzyme for methylmalonyl-CoA mutase and methionine synthetase. Methionine synthetase is involved in the synthesis of DNA and RNA via purine and pyrimidines. The enzyme influences the entry of folate into cells. Folate demethylates by the conversion of homocysteine to methionine via methionine synthetase. The enzyme methylmalonyl-CoA mutase is involved in the conversion of propionic to succinic acid, a factor of fatty acid metabolism.

Absorption and excretion

Vitamin B12 has a high affinity for glycoproteins. Intrinsic factor is a glycoprotein that is secreted by the gastric cells and is required for absorption of vitamin B12. It moves into the ileum where it is absorbed. The vitamin is transported to cells by a protein carrier called transcobalamin I.

Clinical conditions

Megaloblastic anemia is a condition that results from a deficiency in B12. It is actually a deficiency in intracellular folate accompanied by B12 deficiency. Vitamin B12 deficiency can also result in neuropathy and with high homocysteine levels. Causes of B12 deficiency are: malabsorption, pancreatic insufficiency, pernicious anemia, AIDS, and an abnormal connection between the colon and small intestine.

Recommended intake

The 1999 recommendations for vitamin B12 intake are: 0.4-0.5 mcg for infants, 0.9-1.8 mcg for children, 2.4 mcg for adults, 2.6 mcg for pregnant women, and 2.8 mcg for lactating women.

Food Sources

Vitamin B12 is only found in animal tissue. Liver is the primary source of vitamin B12. Other sources are meat and fish.

Figure %: Vitamin B12 Content of Selected Foods

Supplementation

Vitamin B12 injections are commonly given intramuscularly by physicians to prevent pernicious anemia. Oral vitamin B12 can be used effectively in raising serum vitamin B12 levels. Other indications for which supplemental B12 has been used are AIDS, Alzheimer's disease, asthma, and depression, although scientific information is limited.

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