Metabolism and Excretion of Iron
Despite the importance of dietary iron in maintaining the long term adequacy of body iron, the amount of iron needed were it not for the avid conservation and constant recycling of body iron.
The daily enteric absorption of iron amounts to only about 0.06% of the total body iron content. The iron from degraded hemoglobin is continually reused for erythropoiesis.
The red blood cells (RBCs) live for about 120 days and are then trapped and phagocytosed by the reticuloendothelial cells (RE), primarily in the spleen.
Although transferrin accepts iron from the gastrointestinal tract, most of the iron entering the plasma for distribution by transferrin is contributed by sites of hemoglobin destruction (phagocytes of RE cells) and sites of stored iron (ferritin and hemosiderin).
The RE serves as storage site for ferritin and hemosiderin in the bone marrow, spleen and to a large extent in the liver.
There is also some evidence that iron stored in skeletal muscle is located in the RE cells lying between the muscles fibers. In the liver, parenchyma is also a storage site for iron.
The requirement for transferrin iron is determined by the needs of the bone marrow for red cell synthesis. Therefore in chronic hemolysis, the quantity of iron passing through the plasma can expand six to eight times normal.
On the other hand, when erythropoiesis declines dramatically, the quantity of iron in the plasma pool may decrease to as little as one third of normal.
In normal adult male, daily iron losses are approximately between 0.9 an 1.0 mg/d (12 to 14 ug/kg/d). Most of these losses are via the gastrointestinal tract, with about 0.45 due to blood loss, which occurs even in healthy individual.
About 0.2 to 0.3 mg iron is lost by desquamated of surface cells from the skin, and a very small amount, about 0.1 mg, is lost in the urine.
Total losses of premenopausal women, however, are estimated at about 1.3 to 1.4 mg/d because of iron loss in menses. Average daily iron loss in menses is about 0.6 mg, but this amount can vary considerably. In some women iron loss due to menses alone may exceed 1.4 mg/d.
The increased excretion of iron in normal persons with the excessive intakes is due to the above average concentration of iron in the ferritin of desquamated mucosal cells.
Metabolism and Excretion of Iron
