Studies on the bioavailability of zinc in humans: effects of heme and nonheme iron on the absorption of zinc.

The change in the concentration of plasma zinc after oral administration of zinc was used as the index of zinc absorption in the study of the interaction of zinc and iron in the human intestine. With zinc sulfate as the source of inorganic zinc, and ferrous sulfate as the source of nonheme iron, a ratio of Fe/Zn of 1:1 slightly inhibited zinc absorption while Fe/Zn ratio of 2:1 and 3:1 substantially inhibited zinc uptake. No effect on zinc absorption was observed, however, when heme iron, as heme chloride, was ingested in a 3:1 Fe/Zn ratio with inorganic zinc. Atlantic oysters providing about 54 mg of zinc were consumed with or without 100 mg of ferrous iron; Fe/Zn ratio was 2:1. With this "organic" form of zinc, iron did not significantly affect zinc absorption. The evidence for competitive interaction of zinc and iron was strongest with nonheme iron and inorganic zinc. Thus, the possibility that intrinsic iron in formulas for feeding infants and in vitamin-mineral supplement might inhibit the absorption of zinc justifies concern about the Fe/Zn ratio in the formulation of these products.