As a cofactor for methylmalonyl-CoA mutase enzymes, vitamin B12 helps convert odd chain fatty acids and branched chain amino acids into succinyl-CoA, a common citric acid cycle intermediate. Vitamin B12 is also required for nucleic acid (DNA) synthesis, methionine synthesis from cysteine, and normal myelin synthesis in the nervous system. Along with vitamin B6 and folic acid, adequate levels of vitamin B12 are required to maintain normal plasma homocysteine levels. Elevated plasma homocysteine may be an independent risk factor for developing heart conditions.ƒ There are two distinct mechanisms for intestinal vitamin B12 absorption; receptor-mediated absorption and passive diffusion. In the first, vitamin B12 attaches to a salivary ﬁR-binderﬂ protein which transports it into the small intestine, where vitamin B12 is released. The vitamin then binds to ﬁIntrinsic Factorﬂ (IF), a glycoprotein normally produced by the gastric parietal cells. This vitamin B12-IF complex is carried down to the ileum, where it binds to mucosal receptors. Finally, the complex is absorbed and bound to serum vitamin B12-binding proteins. The second absorption mechanism, passive diffusion, does not require any carriers, such as B-binder or IF. The elderly and strict vegetarians are often at risk for vitamin B12 deficiency, either due to low dietary intake or impaired absorption.