Zinc inhibits nonheme iron bioavailability in humans

There is increasing concern about potential negative interactions in combined iron and zinc supplementation. The aim of the present study was to determine the dose-response effect of zinc, given as a solution, on iron bioavailability. Twenty-two healthy adult women were selected to participate in the study. Iron, with or without zinc was given as an aqueous solution on d 1, 2, 14, and 15 of the study. Iron bioavailability was measured on the basis of erythrocyte incorporation of Fe-55 or Fe-59 14 d after administration. Subjects received 0.5 mg of iron together with graded zinc concentrations (0-11.71 mg). No significant effect of zinc on iron absorption was found at Zn: Fe molar ratios up to 2: 1. At 5: 1, 10: 1, and 20: 1 molar ratios, a dose-dependent inhibitory effect on iron absorption was observed (28-40% of iron absorption inhibition; one-way repeated-measures ANOVA, F = 4.48, p = 0.02). In conclusion, zinc administration combined with iron in an aqueous solution leads to the inhibition of iron bioavailability, which occurs in a dose-dependent way. This negative interaction should be considered for supplementation programs with both microminerals.