Vitamin B12

It represents a group of cobalt-containing substances, which play an important role in protein and fatty acid metabolism and red blood cell production. It catalyses the methylation of homocysteine to methionine and the conversion of methylmalonyl CoA to succinyl CoA.

After ingestion, vit B12 binds to intrinsic factor, a protein secreted by gastric parietal cells. It can also bind to other proteins, called R-factors, in which case the protein-vitamin complex is not absorbable. The vit.-intrinsic complex is absorbed in the terminal ileum by specific receptors and binds to three plasma transport proteins, called transcobalamins 1,2,3. 1 and 3 are secreted by white blood cells and bind about 90% of vitamin B12, but only 2 is able to transport cyanocobalamin into cells.

Liver stores contain 2000 to 5000 microg. of vit., while the average daily consumption is 3-5 microg., so the body has a large reserve.

Its RDA ranges from 2 to 3 microg in both children and adults. Its serum levels range from 150 to 300 pg/ml. High-fat diets reduce the production of the intrinsic factor necessary for the absorption of vitamin B12; this disrupts the immune system by providing it with insufficient amounts of vitamin B12 for the transport of folic acid, which is largely dependent on it. Vitamin B12 deficiency leads to pernicious anaemia.

It is not synthesised by either plants or humans. It has no known side effects.

Drug interactions: phenobarbital, phenytoin and primidone reduce serum levels, neomycin, aminosalicylic acid, chloramphenicol, cycloserine, methotrexate, colchicine and cholestyramine reduce absorption.

The main natural sources are: liver, seafood, red meat in general, milk and dairy products.