1307 1307 1308 1308 Deficiency of iron is the most common nutritional disorder in the world and results in clinical signs and symptoms that are mostly re-lated to inadequate hemoglobin synthesis. Although the prevalence of iron deficiency anemia is higher in low-resource countries, this form of anemia is common in the United States, particularly in toddlers, adoles-cent girls, and females of childbearing age. The factors underlying iron deficiency differ somewhat in various population groups and can be best considered in the context of normal iron metabolism. 1309 1309 1310 1310 1311 1311 1312 1312 *TABLE 14.6 Iron Distribution in Healthy Young Adults (mg)* 1313 1313 1314 1314 1315 1315

Pool	Males	Females
Total	3450	2450
Functional
Hemoglobin	2100	1750
Myoglobin	300	250
Enzymes	50	50
Storage
Ferritin, hemosiderin	1000	400

1316 1316 1317 1317 Iron Metabolism. Iron balance is maintained by regulating the absorption of dietary iron in the proximal duodenum (described later). By contrast, there is no regulated pathway for iron excretion and daily losses are limited to the 1 to 2 mg found in shed mucosal and skin epithelial cells. The normal daily diet in the United States contains 10 to 20 mg of iron, mostly in the form of heme in animal products, with the remainder being inorganic iron in vegetables. About 20% of heme iron (in contrast with 1% to 2% of nonheme iron) is absorbable, so the average diet in the United States contains sufficient iron to balance fixed daily losses. By contrast, in low-resource parts of the world, iron intake is often inadequate to make up for normal daily losses. This is compounded in some lower-resource areas by the high burden of parasites, such as hookworm, that may be associated with significant blood loss. 1318 1318 1319 1319 In the United States, average total body iron content is as high as 6 g in healthy males but is only 2.5 g in healthy young females because of blood loss during menstruation and increased demands during pregnancy. Most of the total body iron exists in the functional compartment (Table 14.6), consisting of iron in hemoglobin, myoglobin, and iron-containing enzymes such as catalase and cytochromes. Most remaining iron is found in the storage compartment, in which it exists in two major protein-bound forms: 1320 1320 1321 1321 Ferritin is a protein-iron complex found at the highest levels in hepatocytes and splenic and bone marrow macrophages. Iron bound to ferritin is tightly sequestered, which is important because free iron is highly toxic (Chapter 18). In individuals with normal iron stores, most storage iron is bound to ferritin. Cell turnover releases some ferritin into the plasma, and in general plasma ferritin levels correlate with body iron stores. In iron deficiency, serum ferritin is below 12 μg/L, whereas in iron overload values approaching 5000 μg/L may be seen. 1322 1322 1323 1323 Hemosiderin granules are created by the aggregation of ferritin following partial degradation within lysosomes. Iron in hemosiderin is chemically reactive and turns blue-black when exposed to potassium ferrocyanide, which is the basis for the Prussian blue stain. In iron-overloaded cells, most iron is stored as hemosiderin. 1324 1324 1325 1325 Of physiologic importance, storage iron can be readily mobilized if iron requirements increase, as may occur after loss of blood. 1326 1326 1327 1327 Transfer of iron between [... Output truncated due to length. 1842 tokens/6926 characters removed ...]