Background: The source and regulatory mechanisms that elevate beta-endorphin (β-EP) approximately twofold higher than circulating plasma levels in the colostrum of lactating mothers are still unknown, and no studies have examined β-EP availability previously during maturation phases of human milk. Therefore, the aim of this study was to determine whether concentrations of β-EP vary over time between colostrum, transitional, and mature breast-milk and to evaluate whether this depends on the method of delivery.
Methods: Mothers of healthy full-term and pre-term newborn infants who planned to breast-feed their newborn infants were considered for this study. They were consecutively recruited in one of 3 groups of 14, according to delivery method: group 1, vaginal delivery at term (gestational age 40.2 ± 0.3 weeks; birth weight, 3.48 ± 0.09 kg); group 2, preterm vaginal delivery (gestational age, 35.6 ± 0.3 weeks; birth weight, 2.49 ± 0.08 kg); and group 3, at-term elective cesarean section (gestational age, 39.0 ± 0.3 weeks; birth weight, 3.32 ± 0.14 kg). Three consecutive breast milk samples were obtained on the fourth day after birth, before each mother's discharge, and thereafter on the 10th and 30th postpartum days, close to expression of the colostrum, transitional, and mature milk production phases, respectively, to test β-EP concentrations (β-Endorphin 125I RIA; INCSTAR Corporation, Stillwater, MN). Data are presented as mean ± standard deviation. Statistical comparison of β-EP concentration among the three lactating mother groups was performed using the Kruskal-Wallis nonparametric test. In addition, to test the hypothesis of a trend toward smaller values with time of β-EP, the authors computed within each mother group a P value per trend (Kruskal-Wallis test) of β-EP concentration averages on the 4th, 10th, and 30th days, respectively. Student's t test for independent samples was used for the analysis of the other data. The 0.05 significance level was used in the statistical analysis. All computations were made by computer.
Results: Colostrum β-EP concentrations on the fourth postpartum day of group 1 and group 2 mothers who were delivered of a neonate vaginally, at term, or prematurely were significantly higher (P < 0.01) than colostrum levels of group 3 mothers who underwent cesarean section. Group 2 mothers who were delivered of a neonate vaginally and prematurely presented the highest β-EP concentrations (P < 0.05), lasting until the transitional milk phase (10th day). No significant differences were found across all 3 groups of lactating mothers in mature milk (30th day) β-EP concentrations. In addition, the β-EP trend toward smaller values with time within each of the three groups on days 4, 10, and 30 was statistically significant (P < 0.01 per trend).
Conclusions: It is hypothesized that elevated β-EP concentrations in colostrum and transitional milk of mothers who were vaginally delivered of infants may contribute to postnatal fetal adaptation, to overcoming birth stress of natural labor and delivery, and at the same time to the postnatal development of several related biologic functions of breast-fed infants.
*Departments of Pediatrics and Physics and †Institute of Laboratory Medicine, Padua University, Padua, Italy
Received May 19, 2000; accepted May 15, 2001.
Address correspondence and reprint requests to Dr. Vincenzo Zanardo, Department of Pediatrics, Padua University School of Medicine, Via Giustiniani 3, 35128 Padova, Italy (e-mail vincenzo. firstname.lastname@example.org).