Practice Question # 767.
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VITAMIN B12 DEFICIENCY
A deficiency of vitamin B12 can occur in several ways. Inadequate dietary intake is rare but can develop in strict vegetarians who consume no meat or dairy products. Faulty absorption from the gastrointestinal tract is more common. This occurs in conditions such as Crohn’s disease, or after ileal resection or gastrectomy.
Another cause is the absence of intrinsic factor, as in pernicious anemia. Intrinsic factor is normally secreted by cells within the gastric mucosa; normally it binds with the dietary vitamin B12 and travels with it to the ileum, where the vitamin is absorbed. Without intrinsic factor, orally consumed vitamin B12 cannot be absorbed, and RBC production is eventually diminished. Even if adequate vitamin B12 and intrinsic factor are present, a deficiency may occur if disease involving the ileum or pancreas impairs absorption.
Pernicious anemia, which tends to run in families, is primarily a disorder of adults, particularly the elderly. The abnormality is in the gastric mucosa: the stomach wall atrophies and fails to secrete intrinsic factor. Therefore, the absorption of vitamin B12 is significantly impaired.