Beta endorphin concentrations in human milk

Background: The source and regulatory mechanisms that elevate beta-endorphi n (beta -EP) approximately twofold higher than circulating plasma levels in the colostrum of lactating mothers are still unknown, and no studies have examined beta -EP availability previously during maturation phases of human milk. Therefore, the aim of this study was to determine whether concentrat ions of beta -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 plan ned to breast-feed their newborn infants were considered for this study. Th ey were consecutively recruited in one of 3 groups of 14, according to deli very method: group 1, vaginal delivery at term (gestational age 40.2 +/- 0. 3 weeks; birth vaginal delivery weight, 3.48 +/- 0.09 kg); group 2, preterm . vaginal (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 m ilk 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 phas es, respectively, to test beta -EP concentrations (beta -Endorphin I-125 RI A; INCSTAR Corporation, Stillwater, MN). Data are presented as mean +/- sta ndard deviation. Statistical comparison of beta -EP concentration among the three lactating mother groups was performed using the Kruskal-Wallis nonpa rametric test. In addition, to test the hypothesis of a trend toward smalle r values with time of beta -EP, the authors computed within each mother gro up a P value per trend (Kruskal-Wallis test) of beta -EP concentration aver ages on the 4th, 10th, and 30th days, respectively. Student's t test for in dependent samples was used for the analysis of the other data. The 0.05 sig nificance level was used in the statistical analysis. All computations were made by computer. Results: Colostrum beta -EP concentrations on the fourth postpartum day of group 1 and group 2 mothers who were delivered of a neonate vaginally, at t erm, or prematurely were significantly higher (P < 0.01) than colostrum lev els of group 3 mothers who underwent cesarean section. Group 2 mothers who were delivered of a neonate vaginally and prematurely presented the highest <beta>-EP concentrations (P < 0.05), lasting until the transitional milk p hase (10th day). No significant differences were found across all 3 groups of lactating mothers in mature milk (30th day) <beta>-EP concentrations. In addition, the beta -EP trend toward smaller values with time within each o f the three groups on days 4, 10, and 30 was statistically significant (P < 0.01 per trend). Conclusions: It is hypothesized that elevated <beta>-EP concentrations in c olostrum and transitional milk of mothers who were vaginally delivered of i nfants may contribute to postnatal fetal adaptation, to overcoming birth st ress of natural labor and delivery, and at the same time to the postnatal d evelopment of several related biologic functions of breast-fed infants.