Vitamin B12 is involved in purine and DNA synthesis and in methyl group transfer reactions, notably to homocysteine. Deficiency results in megaloblastic anemia and also in degeneration of the myelin sheath in nerve cells. In individuals with achlorhydria, or with advancing age where achlorhydria progression develops, intrinsic factor (which is produced in the stomach and is essential for absorption of B12) is substantially inactivated, resulting in poor B12 absorption. As such, B12 supplementation is often necessary for adequate B12 intake with advancing age.
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