The incorporation of vitamin B12 into bone marrow and other tissues is a multistep process. Initially, the vitamin is taken in from the diet and separated from food by hydrochloric acid, synthesized by gastric cells. Next B12 is transported to the stomach and combines with intrinsic factor, a substance secreted by the parietal cells of the stomach. Intrinsic factor and B12 form a complex that proceeds to the ileum. Vitamin B12 is absorbed through the brush borders of the ileum, and intrinsic factor is neutralized. Once the vitamin leaves the ileum, it is carried across the stomach wall and into the plasma to form a complex with transcobalamin II (TCII), which transports it to the circulation.2 From the circulation, vitamin B12 is transferred to the liver, the bone marrow, and other tissues (Fig. 6.3).
Moving folic acid into the circulation and tissues occurs with a little more ease. Once folic acid is ingested and absorbed through the small intestine, it is reduced to methyl tetrahydrofolate through dihydrofolate reductase, an enzyme available in mucosal cells. It is the reduced form that is delivered to the tissues. Once
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