Competitive interaction of iron and zinc in the diet: consequences for human nutrition.

The degree to which inhibitors of zinc bioavailability actually influence the zinc status of humans who consume usual meals and diets is not known. The interaction of iron and zinc and competitive inhibition of zinc uptake by excess iron in ratios of 2:1 or greater, when the total amount of ionic species is greater than 25 mg, appear to have a measurable effect on human zinc nutriture. The physiological basis is the competition of these chemically similar ions for some portions of a common absorptive pathway shared between inorganic (nonheme) iron and zinc; this has been demonstrated in animal experiments and in zinc absorption studies in human volunteers. Thus, studies involving formula-fed infants, experimental zinc-depletion diets and pregnant women who took prenatal vitamin-mineral supplements containing high levels of iron have shown growth delay (infants) and a decreased circulating zinc pool (all age groups), suggesting a determinant impact of excessively high Fe/Zn ratios in the diet. Consideration of solutions to these problems, including conscious adjustment of the Fe/Zn ratios in human diets, foods and therapeutic nutrient supplements in order to reduce the zinc-inhibiting effects of iron, should become a priority in policy and marketing discussions within government regulatory agencies, industry and the scientific community of human and clinical nutritionists.