Elevated intakes of zinc in infant formulas do not interfere with iron absorption in premature infants

Background: Zinc and iron may share common pathways for absorption and compete for uptake into mucosal cells. We determined whether elevated ratios of zinc to iron would interfere with erythrocyte incorporation of iron in premature infants bath during and between feeds. Methods: In the first experiment, five premature infants (<2500 g birth weight) were enrolled, once receiving full oral feeds by nasogastric tube. They received either high (1200 ug/kg, ratio 4:1) or low (300 ug/kg, ratio 1:1) doses of oral zinc sulfate, together with 300 ug/kg oral Fe-58 as chloride in saline with 10 mg/kg vitamin C, between designated feeding periods. Each infant served as its own control and randomly received either high or low doses of zinc or iron and then the alternate dose after 2 weeks. In the second experiment, nine additional premature infants were assigned to the same zinc:iron intake protocol except zinc and iron were given with usual oral feeds (premature formula or human milk) equilibrated before feeding. Iron absorption was measured by the erythrocyte incorporation of Fe-58. Results: High doses of zinc given between feeds significantly inhibited erythrocyte incorporation of iron. Fe-58 incorporation (%) with the 1:1 ratio of zinc:iron intake was 7.5 (5.7, 10; geometric mean, -I SD, +1 SD). The percentage of Fe-58 incorporation on the 4:1 ratio of zinc:iron intake was 3.6 (2.6, 5.1). Given with feeds, the percentage of Fe-58 incorporation on low zinc:iron intake was 7.0 (2.6, 19). Finally, the percentage of Fe-58 incorporation on high zinc:iron intake was 6.7 (2.5, 19). Conclusion: Elevated intakes of zinc do not interfere with erythrocyte incorporation of iron in premature formulas.