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Notes: Abstract The intestinal absorption of 59Fe from a diagnostic 10 μmole (0.56 mg) 59Fe2+ test dose and 5 mg Fe equivalents of 59Fe-labeled pork, hog liver and hemoglobin was measured with a 4 π geometry whole-body radioactivity detector with liquid organic scintillator and the results compared with the stainable amounts of diffuse cytoplasmatic (non-heme) storage iron in the bone marrow macrophages, serum iron concentrations and total iron-binding capacity, erythropoietic activity and other hematological parameters. Children with mild hypochromic, microcytic anemia due to heterozygous β-thalassemia absorbed normal amounts of 59Fe from the diagnostic dose of 59Fe2+ if their bone marrow macrophages contained normal or mildly increased amounts of diffuse cytoplasmatic storage iron. Depleted iron stores also caused increased absorption of diagnostic 59Fe in children with heterozygous β-thalassemia, so that prelatent iron deficiency can be diagnosed by measurement of increased iron absorption. The diagnostic 59Fe2+ absorption was increased to 70–100% in children with homozygous β-thalassemia when it was measured 64–300 days after the last blood transfusion. Since all these children contained normal to increased amounts of diffuse cytoplasmatic storage iron in the bone marrow macrophages, the high intestinal iron absorption was not caused, as is usual, by depleted iron stores but coincided with considerable hyperplasia of ineffective erythropoiesis demonstrated by the increased numbers of normoblasts in the bone marrow and peripheral blood during a period of severe anemia. Blood transfusions which elevate the hemoglobin levels from 4–5 g/100 ml to 8.5–9.9 g/100 ml and suppress the erythroblastic hyperplasia in the bone marrow also reduce the increased absorption of inorganic and food iron to normal values. Interruption of blood transfusions always results in the next erythroblastic hyperplasia and a simultaneous increase of intestinal iron absorption. Mild hyperplasia of the erythropoietic system such as is observed in megaloblastic anemia due to vitamin-B12 deficiency or in hereditary spherocytosis is not sufficient to increase iron absorption. Anemia per se does not increase iron absorption, as is borne out by the observation that patients with severe aregenerative anemia or megaloblastic anemia absorb 59Fe2+ according to their normal iron stores within the normal range. The messenger which signalizes depleted iron stores or erythropoietic hyperplasia to the duodenal and jejunal mucosa so that intestinal iron absorption is increased is not yet known. The food iron absorption from 59Fe-labeled pork and hog liver was increased to 2–4 times the normal average when measured in homozygous β-thalassemia with severe anemia and erythroblastic hyperplasia 28–42 days after the last blood transfusion. This increased food iron absorption was normalized by blood transfusion which suppressed the erythropoietic hyperplasia and raised the hemoglobin levels to 9.7–10.4 g/100 ml. The development of hemosiderosis either from blood transfusions or from increased food-iron absorption seems to be unavoidable in patients with homozygous β-thalassemia. Regular transfusion of 2–8 l blood per year is necessary to keep the hemoglobin at a level of at least 9 g/100 ml, which does not induce erythrocytopoietic hyperplasia and increased food-iron absorption. After 18 years on such a transfusion schedule, between 18 and 73 g iron have been incorporated into the body and produced transfusion hemosiderosis. Patients with homozygous β-thalassemia who survive without blood transfusions and have low hemoglobin levels (〈8 g/100 ml) absorb 3 times more iron from a mixed diet and after 18 years have accumulated about 19 g iron from increased food-iron absorption, which is also enough to produce hemosiderosis.