Iron Bioavailability from Ferric Pyrophosphate in Extruded Rice Cofortified with Zinc Sulfate Is Greater than When Cofortified with Zinc Oxide in a Human Stable Isotope Study

Abstract

Background: Extruded rice grains are often cofortified with iron and zinc. However, it is uncertain if the addition of zinc to iron-fortified rice affects iron absorption and whether this is zinc-compound specific.Objective: We investigated whether zinc, added as zinc oxide (ZnO) or zinc sulfate (ZnSO₄), affects human iron absorption from extruded rice fortified with ferric pyrophosphate (FePP).Methods: In 19 iron-depleted Swiss women (plasma ferritin ≤16.5 μ/L) aged between 20 and 39 y with a normal body mass index (in kg/m²; 18.7-24.8), we compared iron absorption from 4 meals containing fortified extruded rice with 4 mg Fe and 3 mg Zn. Three of the meals contained extruded rice labeled with FePP (⁵⁷FePP): 1) 1 meal without added zinc (⁵⁷FePP-Zn), 2) 1 cofortified with ZnO (⁵⁷FePP+ZnO), and 3) 1 cofortified with ZnSO₄ (⁵⁷FePP+ZnSO₄). The fourth meal contained extruded rice without iron or zinc, extrinsically labeled with ferrous sulfate (⁵⁸FeSO₄) added as a solution after cooking. All 4 meals contained citric acid. Iron bioavailability was measured by isotopic iron ratios in red blood cells. We also measured relative in vitro iron solubility from ⁵⁷FePP-Zn, ⁵⁷FePP+ZnO, and ⁵⁷FePP+ZnSO₄ expressed as a fraction of FeSO₄ solubility.Results: Geometric mean fractional iron absorption (95% CI) from ⁵⁷FePP+ZnSO₄ was 4.5% (3.4%, 5.8%) and differed from ⁵⁷FePP+ZnO (2.7%; 1.8%, 4.1%) (P < 0.03); both did not differ from ⁵⁷FePP-Zn: 4.0% (2.8%, 5.6%). Relative iron bioavailabilities compared with ⁵⁸FeSO₄ were 62%, 57%, and 38% from ⁵⁷FePP+ZnSO₄, ⁵⁷FePP-Zn, and ⁵⁷FePP+ZnO, respectively. In vitro solubility from ⁵⁷FePP+ZnSO₄ differed from that of ⁵⁷FePP-Zn (14.3%; P < 0.02) but not from that of ⁵⁷FePP+ZnO (10.2% compared with 13.1%; P = 0.08).Conclusions: In iron-depleted women, iron absorption from FePP-fortified extruded rice cofortified with ZnSO₄ was 1.6-fold (95% CI: 1.4-, 1.9-fold) that of rice cofortified with ZnO. These findings suggest that ZnSO₄ may be the preferable zinc cofortificant for optimal iron bioavailability of iron-fortified extruded rice. This trial was registered at clinicaltrials.gov as NCT02255942.

Keywords: fortification; human iron absorption; iron deficiency; rice; zinc-iron interaction.