Protein levels and protease activity in milk from mothers of pre-term infants: A prospective longitudinal study of human milk macronutrient composition

The composition and enzymology of human milk changes throughout the lactation period, and differ for mothers who give birth prematurely compared to those who deliver at full-term. Understanding the composition of milk from mothers of very low birth weight premature infants is of great significance, and the objective of this study was to evaluate the composition, protein profile and plasmin activity of milk from mothers who delivered infants at different gestational ages. Samples of human milk were donated by women (n = 74) in the Cork, Ireland, area who gave birth to full-term (>37 weeks gestation, FT), pre-term (32–37 weeks, PT) and very pre-term (≤32 weeks, VPT) infants. FT milk was collected at 1, 3, 6 and 10 weeks post-partum (PP), while PT and VPT milk was collected weekly until the FT due date of the infant and subsequently followed the FT protocol. Gestational age did not significantly affect lactose or fat content or total energy content of milk. However, protein content, and levels of some individual proteins, were significantly affected by both gestational age at birth and duration of lactation, with significantly higher protein levels in PT or VPT milk samples at 0–7 days and 1–2 months, respectively. Plasmin activity was significantly higher in VPT milk, indicating differences in proteolytic processing in milk. Compositional differences between the milk of mothers of term and pre-term infants were greatest in terms of the protein profile, which showed both qualitative and quantitative differences, as well as difference in proteolytic activity.