Iron, zinc, and copper concentrations in breast milk are independent of maternal mineral status

Background: Little is known about the regulation of iron, zinc, and copper in breast milk and the transport of these minerals across the mammary gland epithelium. Objective: The objective was to study associations between breast-milk concentrations of iron, zinc, and copper and maternal mineral status. Design: Milk samples from 191 Swedish and Honduran mothers were collected at 9 mo postpartum. Iron, zinc, and copper concentrations were measured by atomic absorption spectrometry. Blood samples from mothers were analyzed for plasma zinc and copper and 4 indexes of iron status: hemoglobin, plasma ferritin, soluble transferrin receptors, and zinc protoporphyrin. Complementary food energy (CFE) intake was used as an inverse proxy for breast-milk intake. Results: Mean (±SD) breast-milk concentrations of iron were lower in the Honduran than in the Swedish mothers (0.21 ± 0.25 compared with 0.29 ± 0.21 mg/L; P < 0.001), and mean breastmilk concentrations of zinc and copper were higher in the Honduran than in the Swedish mothers [0.70 ± 0.18 compared with 0.46 ± 0.26 mg/L (P < 0.001) and 0.16 ± 0.21 compared with 0.12 ± 0.22 mg/L (P = 0.001), respectively]. Milk iron was positively correlated with CFE intake (r = 0.24, P = 0.001) but was not significantly correlated with any iron-status variable. Milk zinc was negatively correlated with CFE intake (r = -0.24, P = 0.001) but was not significantly correlated with maternal plasma zinc. Milk copper was not significantly correlated with CFE intake or maternal plasma copper. Conclusions: Milk iron, zinc, and copper concentrations at 9 mo postpartum are not associated with maternal mineral status, which suggests active transport mechanisms in the mammary gland for all 3 minerals. Milk iron concentrations decrease and milk zinc concentrations increase during weaning.