Is the Antioxidant Capacity of Stored Human Milk Preserved?

In the neonatal intensive care unit (NICU) expressed mothers' milk usually is stored frozen until used. We found that when human milk was stored at -20°C for up to 9 months there were reduced bacterial counts and pH, increased free fatty acids, but unchanged immune proteins. Antioxidant protection is an important benefit of human milk. Few studies have evaluated long-term effects of cold storage on the antioxidant capacity of human milk. We hypothesized that the antioxidant capacity of human milk is affected adversely by long-term storage at -20°C. To study the impact of long-term cold storage on the oxidative capacity of human milk and the biological impact of these changes on macromolecular constituents of human milk. Freshly expressed milk was obtained from mothers in the NICU, stored at -20°C for 6 months, and compared with the baseline. Paired samples were analyzed for glutathione, hydrogen peroxide (H₂O₂), 8-isoprostane, catalase, and superoxide dismutase. There was no change in H₂O₂ concentration between baseline and 6 months. Significant reductions from baseline in both catalase and superoxide dismutase concentrations and activities, total glutathione, oxidized glutathione, reduced glutathione, and the ratio of reduced to oxidized glutathione were observed (p < 0.05). There was a significant increase in 8-isoprostane concentrations (p < 0.001). These data indicate significant changes in antioxidant capacity of human milk, including oxidation of macromolecules, after storage at -20°C for 6 months. The clinical implication of these findings may explain the nonuniform protection against oxidant disease in preterm infants fed human milk.