Biological and Social Factors Influencing Glucocorticoid Concentrations in Human Breast Milk

Abstract

Background: In addition to providing nutrients, human breast milk (HBM) contains a wide array of non-nutritive bioactive elements, including glucocorticoid hormones (GCs; cortisol and cortisone). Milk-borne GCs regulate infant stress-mediated responses and may influence physiological and psychological development. Many factors are known to influences maternal stress, and hence potentially impact on the GC composition of breast milk. However, few studies have addressed the multiple biological and social factors which may influence GC concentrations of human breast milk. Objective: To measure the GC (cortisol and cortisone) levels in the breast milk samples from lactating mothers participating in the Finnish Cohort STEP study (Steps to the Healthy Development and Well-being of Children) and to determine the influence of biological (maternal age, pre-pregnancy BMI, gestational weight gain and infant gender) and social (education, occupation and sibling number) factors on these hormones. Methods: Samples were obtained from lactating mothers (n=658) when the infants were 12-16 weeks old. GC concentrations were measured using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Maternal demographics, biological and social factors were obtained using self-reported diaries and hospital records. Results: Cortisone (mean 9.5 ng/ml; SD= 3.41) and cortisol (mean 7.5 ng/ml; SD=5.62) were present in all analysed HBM samples. Our results reveal that maternal biological factors (including; age, BMI, weight gain during pregnancy, and gestational diabetes) and social factors (including; sibling number, educational standards) were not statistically related to the difference in GC concentrations in HBM. However, a number of factors were significantly correlated to the breast milk GC levels. These included family structure (p= 0.001), marital status (p=0.01) gestational age (p=0.03) and mode of infant delivery (p=0.08) which were significant predictors of cortisol and cortisone variation in breastmilk. Compared to normal infant delivery, mothers’ with caesarean infant delivery had a statistically significant higher mean level of cortisol (7.33 ng/ml vs. 9.05 ng/ml) in HBM samples at 12- 16 weeks after delivery. Cortisone levels tended to also be higher (9.45 ng/ml vs. 10.18 ng/ml), but this difference was not statistically significant. Conclusion: Milk corticosteroids are affected by multiple factors. Mode of infant delivery and family arrangement appear to play an overlooked role in milk composition. The mechanisms are yet to be defined but these findings provide insight into the unappreciated roles of social and biological factors in orchestrating the hormonal composition of mothers’ milk. Further analysis is ongoing to determine the impact of these variations on child health.