https://examine.com/supplements/zinc/research/#nutrient-nutrient-interactions
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Both zinc and iron are transported by the bivalent cation transporter DMT1 and Nramp2. As transporters can get saturated to capacity with excessive levels of substrate, it is thought high levels of both minerals may impair absorption of one of them.
Supplementation of zinc in solution appears to inhibit the absorption of iron. with low doses of iron (500mcg) requiring 5.7-fold as much zinc to inhibit absorption while higher doses (10mg) only required a 1:1 ratio which has been replicated. Elsewhere, nonheme iron specifically, where a solution of zinc and iron caused a dose-dependent inhibition of 28-40% with increasing zinc ratios (5:1 to 20:1).
Iron and zinc in solution appear to compete for absorption, and while this is not a concern with low levels of iron intake it does appear to be concernable when both minerals are above 10mg. This may be relevant to taking supplements containing them both in a fasted state
Studies using fortified milk (4:1 ratio with 300mcg/kg iron in infants or simply 10mg/L iron with a 2:1 ratio in women have failed to replicate the inhibition of iron absorption.
Studies using iron and zinc both fortified into food products have failed to find a zinc-mediated inhibition of iron absorption. It is possible that this is related to a reduced rate of absorption (due to solid food particles in the intestines) and the reduced rate avoiding transporter saturation
Link:
https://pubmed.ncbi.nlm.nih.gov/17350802/
New insights about iron bioavailability inhibition by zinc
Objective: We measured the effects of lower and higher doses of zinc (Zn) given as an aqueous solution on the bioavailability of iron (Fe).
Methods: Fourteen healthy subjects received a solution with 0.5 mg of elemental Fe as ferrous sulfate given alone or with 0.59 mg of Zn as zinc sulfate (molar ratio Zn:Fe 1:1). Fourteen days after they received a second solution with 10 mg of Fe given alone or with 11.71 mg of Zn (molar ratio Zn:Fe 1:1). Iron bioavailability was assessed by erythrocyte incorporation of iron radioisotopes (55)Fe and (59)Fe.
Results: No significant effect of Zn on Fe bioavailability was observed at lower doses; however, at higher doses Fe bioavailability was inhibited by 56% (P < 0.001, repeated measures analysis of variance).
Conclusion: The inhibitory effect of Zn on Fe bioavailability depends on the total amount of both minerals present in the intestinal lumen. This fact should be considered when designing a supplementation program if Fe and Zn are to be provided together.
Link:
https://pubmed.ncbi.nlm.nih.gov/17873388/
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 55Fe or 59Fe 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.
