Another analysis found that infants who were not directly breastfed have a significant increase in coughing and wheezing episodes compared to those who were. Some research suggests storing expressed breast milk may interfere with its beneficial properties. Freezing can break down its immunological cells and lipids (but doesn’t affect its antimicrobial proteins), refrigeration reduces ascorbic acid concentrations, and both storage methods reduce antioxidant activity. Microwave thawing, which is not recommended, drastically decreases breast milk’s anti-infective elements. The effects of these changes, if any, are unknown.
The content of the breast milk itself may differ when it is exclusively pumped. A mother’s breast milk changes according to a baby’s needs, as it ages, as well as throughout the course of a day and the course of each feeding session. Research has confirmed that the fat concentration of expressed milk increases with the baby’s age in the same way that breast-fed milk does. But if mothers don’t pump for long enough at each session, their infants may receive predominantly fore milk (which is high in carbohydrates) and not get enough hind milk (which is high in fat).
As for contamination, everything breast milk touches outside the body during the process of expression and bottle-feeding is a chance for it to obtain harmful bacteria. Bacterial counts are higher in milk expressed with a pump than in milk expressed by hand.
Thorley adds that breastfeeding protects against ear infections not just because of breast milk’s anti-infective elements, but because of the posture of the baby’s head and the dynamics of the suck/swallow/breathe cycle while nursing. She says babies fed by bottle miss out on these aspects.
