nutritional disorders
Vitamin B12 Deficiency
Introduction
Dietary vitamin B12 deficiency usually results from inadequate absorption, but deficiency can develop in vegans who do not take vitamin supplements. Deficiency causes megaloblastic anemia, damage to the white matter of the spinal cord and brain, and peripheral neuropathy. Diagnosis is usually made by measuring serum vitamin B12 levels. The Schilling test helps determine etiology. Treatment consists of oral or parenteral vitamin B12. Folate (folic acid) should not be used instead of vitamin B12 because folate may alleviate the anemia but allow neurologic deficits to progress. Cobalamin is a general term for compounds with biologic vitamin B12 activity. These compounds are involved in nucleic acid metabolism, methyl transfer, and myelin synthesis and repair. They are necessary for the formation of normal red blood cells and normal neural function (see table). Dietary sources of vitamin B12 include meats (especially beef, pork, and organ meats [eg, liver]), poultry, eggs, fortified cereals, milk and milk products, and seafood such as clams, oysters, mackerel, and salmon. Food-bound vitamin B12 is released in the stomach’s acid environment and is bound to R protein (haptocorrin). Pancreatic enzymes cleave this B12 complex (B12-R protein) in the small intestine. After cleavage, intrinsic factor, secreted by parietal cells in the gastric mucosa, binds with vitamin B12. Intrinsic factor is required for absorption of vitamin B12, which takes place in the terminal ileum. Vitamin B12 in plasma is bound to transcobalamins I and II. Transcobalamin II is responsible for delivering vitamin B12 to tissues. The liver stores large amounts of vitamin B12. Enterohepatic reabsorption helps retain vitamin B12. Liver vitamin B12 stores can normally sustain physiologic needs for 3 to 5 years if B12 intake stops (eg, in people who become vegans) and for months to 1 year if enterohepatic reabsorption capacity is absent. Large amounts of vitamin B12 seem to be nontoxic but are not recommended for regular use (ie, as a general tonic). (See also Overview of Vitamins .) 