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vitamin B12 deficiency

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Vitamin B12 deficiency causes distinctive dyserythropoietic abnormalities in the bone marrow - megaloblastic erythropoiesis - associated with abnormally large red cells in the peripheral blood i.e. macrocytosis.

Vitamin B12 deficiency in developed countries is commonly due to pernicious anaemia; in developing countries, often more vegan-based, undernourishment is a more common cause.

Vitamin B12 is found in foods of animal origin including milk, cheese, yoghurt and eggs

  • recommended daily requirement of vitamin B12 is small (1-2 µg/day) compared with total body stores (2000-5000 µg) much of which is stored in the liver. This explains why it takes a long time, usually years, for vitamin B12 deficiency to develop
  • dietary vitamin B12 is freed from the food protein by pepsin in the acid gastric environment and binds to haptocorrin, a protein secreted in saliva. In the small intestine, haptocorrin is degraded by pancreatic enzymes, the vitamin B12 is released and binds with intrinsic factor (IF), itself secreted by gastric parietal cells
    • the IF-B12 complex is carried through the small intestine and binds to receptors in the terminal ileum where it is actively absorbed. A small fraction (1-2%) of the daily intake is passively absorbed across the entire absorptive surface of the intestinal tract

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