ÝApproximately 80% of total body iron is ultimately incorporated into red cell hemoglobin. An average adult produces 2 x 1011 red cells daily, for a red cell renewal rate of 0.8 percent per day. Each red cell contains more than a billion atoms of iron, and each ml of red cells contains 1 mg of iron. To meet this daily need for 2 x 1020 atoms (or 20 mg) of elemental iron, the body has developed regulatory mechanisms whereby erythropoiesis profoundly influences iron absorption. Plasma iron turnover (PIT) represents the mass turnover of transferrin-bound iron in the circulation, expressed as mg/kg/day (Huff et al., 1950). Accelerated erythropoiesis increases plasma iron turnover, which is associated with enhanced iron uptake from the gastrointestinal tract (Weintraub et al., 1965). The mechanism by which PIT alters iron absorption is unknown.