Is the Antioxidant Capacity of Stored Human Milk Preserved?

Abstract

Background:

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.

Objective:

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.

Methods:

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.

Results:

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).

Conclusion:

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.