Physiology

Iron is essential to life for humans. Iron is incorporated into red cells, hemoglobin, myoglobin, cytochromes, and other enzymes and cofactors. Since excess iron is toxic, the body utilizes several mechanisms to maintain appropriate iron availability while preventing toxicity. These mechanisms include serum protein binding, intracellular storage and, most importantly, regulation of gastrointestinal (GI) absorption.

The oral bioavailability of inorganic iron is less than 10 percent, and ferrous iron (Fe +2) is better absorbed than ferric (Fe+3) iron. Chelated iron, most readily found in heme, is best absorbed, accounting for one beneficial effect of eating meat. In the duodenum and jejunum, a mucosal carrier protein takes up iron and immediately stores it as ferritin. In times of iron need, the body produces transferrin, which removes iron from the cells. Alternatively, the iron is stored in the intestinal cell and is eventually eliminated from the body when the mucosa is sloughed. This is the principle mechanism for limiting absorption, and failure of this mechanism is critical following iron overdose.

Transferrin, the carrier protein that binds ferric iron, is the major mechanism for safe iron transport through the body. The total iron-binding capacity (TIBC) primarily measures the amount of serum transferrin. The TIBC is generally two to three times the normal iron concentration.

Ferritin is a large intracellular storage protein that can reversibly bind as many as 4500 molecules of iron. When an iron deficit exists, iron is transported from ferritin and the GI tract to the liver, spleen, and bone marrow, where it is incorporated into appropriate molecules. 1

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