Aim: To investigate to what extent formula milk and stored breast milk, commonly used in hospitals, could be pro‐oxidant sources for newborn babies. Methods: We determined total anti‐oxidant capacity and lipid peroxidation products, such as lipid peroxides, TBARS and conjugated dienes, in fresh and stored (at ‐20°C) samples of breast milk and in different brands of formula milk. Results: There were notable differences in the oxidation parameters in several brands of formula milk, particularly concerning the levels of lipid peroxides and total antioxidant capacity. No difference was found in the mean total antioxidant capacity between formula and breast milk, even if the vitamin content is much higher in formula milk than in breast milk. On the contrary, all the considered lipid peroxidation products were higher in human milk (HM) than formula milk (FM), and lipid peroxides were much higher in HM stored at ‐20°C. Many differences were found between different formula milks. Conclusion: There was a conspicuous formation of lipid peroxides in HM stored at ‐20°C, which was probably caused by an increased presence of free fatty acids due to lipoprotein lipase activity during storage. Unexpectedly, even fresh HM had a higher concentration of lipid peroxidation products when compared to FM. This could be ascribed to the higher susceptibility of HM to degradation during analysis because of manipulation and light exposure. However, it is also interesting that the high content of lipid peroxides did not correspond to a low total antioxidant capacity in either breast or formula milk. This could signify that such levels of lipid peroxidation products might be present naturally in milk and HM after expression is subject to a strong peroxidation either at room temperature or at ‐20°C.

There was a conspicuous formation of lipid peroxides in HM stored at -20 degrees C, which was probably caused by an increased presence of free fatty acids due to lipoprotein lipase activity during storage. Unexpectedly, even fresh HM had a higher concentration of lipid peroxidation products when compared to FM. This could be ascribed to the higher susceptibility of HM to degradation during analysis because of manipulation and light exposure. However, it is also interesting that the high content of lipid peroxides did not correspond to a low total antioxidant capacity in either breast or formula milk. This could signify that such levels of lipid peroxidation products might be present naturally in milk and HM after expression is subject to a strong peroxidation either at room temperature or at -20 degrees C.