The impact of breast milk relaxin 2, oxytocin, and insulin-like peptide 3 on neonatal growth parameters in early life

Objectives: This study aims to quantify the levels of relaxin 2 (RLX2), oxytocin (OXT), and insulin-like peptide 3 (INSL3) in colostrum (postpartum days 1–5, Visit 1) and mature breast milk (postpartum days 21–35, Visit 2), and to evaluate their associations with neonatal growth outcomes. Methods: During two postpartum visits, anthropometric measurements were obtained from 60 healthy infants, and the concentrations of RLX2, OXT, and INSL3 in both foremilk and hind milk were analyzed. Results: At both Visit 1 and Visit 2, post-breastfeeding body weight and body mass index (BMI) were significantly higher than pre-breastfeeding values. Furthermore, all anthropometric measurements, including head circumference, length, and pre- and post-breastfeeding body weight and BMI, were significantly greater at Visit 2 compared to Visit 1. In foremilk, concentrations of RLX2, OXT, and INSL3 were significantly lower at Visit 2 than at Visit 1 (p = 0.001, p = 0.001, and p = 0.05, respectively). Similarly, in hindmilk, RLX2 and OXT concentrations were significantly lower at Visit 2 than at Visit 1 (p = 0.001 for both). In contrast, INSL3 concentrations in hindmilk did not differ between visits (p > 0.05). Breastmilk OXT and INSL3 were positively correlated with BMI, length, and head circumference, whereas RLX2 concentrations were not significantly associated with these outcomes. Conclusions: The findings of this study suggest that RLX2, OXT, and INSL3 concentrations in breast milk show significant changes during lactation. Furthermore, it suggests that OXT, and more prominently INSL3, are associated with neonatal growth.