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Abstract
Iron is an essential element for microbes, plants, and higher animals. It is a component of heme and iron-sulfur centers in many key redox enzymes and is an essential component of oxygen storage and transporting proteins such as hemoglobin and myoglobin.
In humans, iron is strictly conserved, in large part, by recycling the iron (about 20 mg/day) from hemoglobin of senescent red blood cells to provide iron for new red blood cells. Smaller amounts of iron from myoglobin and various redox enzymes are also recycled. In most circumstances, human diets contain more iron than is necessary to replace the small daily losses (1-2 mg/day). Dietary iron is absorbed predominantly in the duodenum. Humans and other mammals lack mechanisms to excrete excess iron, and, therefore, intestinal iron absorption must be regulated by a feedback mechanism.
One of the most distinguishing features of iron metabolism is the degree to which body iron is conserved. Of the typical 3 to 4 g of iron contained in the normal adult human, only about 0.03% (or ~1 mg) is lost per day, mainly the result of obligatory losses of exfoliated mucosal cells, bile, and extravasated red cells. To replace these basal losses and remain in iron balance, the body must absorb a roughly equivalent amount of iron from the diet. This relatively small daily exchange of iron between body and environment contrasts sharply with the comparatively large exchange of this metal between internal organs. For example, each day the bone marrow utilizes approximately 24 mg of iron to produce over 200 billion new erythrocytes.