Vitamin B12 status is typically assessed by measurements of serum or plasma vitamin B12 levels.
#VITAMIN B12 500 MCG FREE#
If vitamin B12 is added to fortified foods and dietary supplements, it is already in free form and therefore does not require the separation step. The resulting complex is absorbed in the distal ileum by receptor-mediated endocytosis. In the duodenum, digestive enzymes free the vitamin B12 from haptocorrin, and this freed vitamin B12 combines with intrinsic factor, a transport and delivery binding protein secreted by the stomach’s parietal cells. More vitamin B12 is released from its food matrix by the activity of hydrochloric acid and gastric protease in the stomach, where it then binds to haptocorrin. The freed vitamin B12 then binds with haptocorrin, a cobalamin-binding protein in the saliva. The process starts in the mouth when food is mixed with saliva. Vitamin B12 is bound to protein in food and must be released before it is absorbed. L-methylmalonyl-CoA mutase converts L-methylmalonyl-CoA to succinyl-CoA in the metabolism of propionate, a short-chain fatty acid. Methionine is required for the formation of S-adenosylmethionine, a universal methyl donor for almost 100 different substrates, including DNA, RNA, proteins, and lipids. Methionine synthase catalyzes the conversion of homocysteine to the essential amino acid methionine. Vitamin B12 functions as a cofactor for two enzymes, methionine synthase and L-methylmalonyl-CoA mutase.
Vitamin B12 is required for the development, myelination, and function of the central nervous system healthy red blood cell formation and DNA synthesis. However, two others forms, hydroxycobalamin and cyanocobalamin, become biologically active after they are converted to methylcobalamin or 5-deoxyadenosylcobalamin. Methylcobalamin and 5-deoxyadenosylcobalamin are the metabolically active forms of vitamin B12. Because vitamin B12 contains the mineral cobalt, compounds with vitamin B12 activity are collectively called “cobalamins”. Vitamin B12 is a water-soluble vitamin that is naturally present in some foods, added to others, and available as a dietary supplement and a prescription medication.
For a reader-friendly overview of Vitamin B12, see our consumer fact sheet on Vitamin B12. This is a fact sheet intended for health professionals. Health Risks from Excessive Vitamin B12.Groups at Risk of Vitamin B12 Inadequacy.
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