Iron has profound effects on the human body. Too little and you'll be anemic, too much and you'll rust from the inside (hemochromatosis). By far the most common route to iron absorption is through ingestion.
Insoluble Fe3+ enters the GI tract, where it is almost entirely absorbed in the duodenum by enterocytes of the duodenal lining. Before it can be absorbed it must be reduced to Fe2+ by a ferric reductase enzyme on the brush border of the enterocytes. From here a divalent metal transporter protein then transports the iron across the enterocytes cell membrane and into the cell. This is the big step, the body now chooses to either store the iron as ferritin or ferroportin.
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If the iron is stored as ferritin--a globular protein that is the main intracellular iron storage protein that keeps the iron soluble and non-toxic--the iron will leave the body when the cell dies and is sloughed off into the feces. Ferroportin is a transmembrane protein that transports iron from the inside of the cell to the outside of it .
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If the iron is stored as ferroportin, the iron will be moved into the body. Once in the body, the only way to lose that iron is through the loss of hair, skin, nails or blood; which combined amounts to a nearly negligible 1 mg/day.
Absorption is therefore the most important regulatory step in maintaining iron homeostasis. It does this by increasing the production of ferroportin, the divalent metal transporter protein, and the ferric reductase enzyme if the body is deficient in Iron. If there is plenty of systemic iron, the body allows it to pass through the GI tract and out with the feces.