Storage Iron Kinetics

S ummary . Iron metabolism in the rat has been examined by chemical and isotopic measurements. Iron stores were larger in the female than the male, but in neither sex did stores play a dominant role in meeting increased iron needs. A nearly two‐fold higher plasma iron turnover in growing rats as compared with adult animals was accounted for largely by growth requirements. In both sexes and in both young and adult rats, the erythroid marrow assimilated more than two‐thirds of the plasma iron turnover. The source and distribution of transferrin iron was assessed in relation to the erythroid marrow, gastrointestinal tract, hepatic parenchyma and remaining body tissues. Absorption from the gastrointestinal tract provided about one‐third of the iron entering the plasma in adult rats and more than half in growing or phlebotomized rats. Iron absorption was closely related to the erythroid marrow requirements, but did not correlate with either the plasma iron level or tissue iron stores. The rat depends on dietary iron rather than iron stores to meet increased requirements and shows a much greater capacity to absorb iron than does man.