The classic deficiency of vitamin B12 (cobalamin) absorption called pernicious anemia is often seen in elderly persons resulting from loss of acid-secreting cells of the stomach which also secrete a protein called "intrinsic factor". Intrinsic factor is required for the efficient intestinal absorption of cobalamin. Injection of B12 effectively circumvents the deficiency. The same problem is encountered by persons who have partial or total removal of their stomach, for instance to treat stomach cancer or to bypass the stomach for weight loss.

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