Bibian van der Voorn

86 CHAPTER 6 in our milk samples. Furthermore, the maternal HPA axis could be suppressed by synthetic glucocorticoids. These were administered to the majority of mothers who delivered very preterm for acceleration of fetal lung maturation, whereas only one mother who delivered at term received synthetic glucocorticoids. However, this short course is unlikely to result in long-lasting HPA axis suppression 30 . During early development, cortisone concentrations are much higher than cortisol concentrations in the majority of tissues 31 . After birth, tissue concentrations of cortisol gradually increase, which could either be attributed to an increase in 11βHSD type 1 activity or a decrease in 11βHSD type 2 activity 31 . In our study samples, similar to saliva but contrasting with serum, breast-milk cortisone concentrations exceeded those of milk cortisol. It is unknown whether this reflects increased 11βHSD type 2 activity in the mammary gland or better penetrance of serum cortisone into breast milk relative to cortisol. Animal studies indicate that hormones present in breast milk can be absorbed by the neonatal gut, which is accompanied by an attenuated gastric acid secretion 32 . Locally 32 , as well as after absorption in the systemic circulation 33 , milk-borne hormones such as glucocorticoids are postulated to play a role in the proliferation and differentiation of the intestinal epithelium. This might be an explanation for the reduction in necrotizing enterocolitis and late-onset sepsis in breastfed, compared with formula fed, very preterm infants 4-6 . In our laboratory, milk cortisol and cortisone concentrations were below the lower limit of quantitation in standard infant formulas (unpublished observations of our research group, 2015). Breast-milk glucocorticoids have been suggested to affect neurodevelopment in primate offspring 29,34 . Studies in humans are limited but suggest similar effects 35,36 . If these findings in animals apply to humans, there might be implications for donor milk banks. Milk banks often provide very preterm newborns with milk from mothers who delivered at term, which we have shown to be richer in glucocorticoids. Heating during pasteurization has the potential to affect the concentrations and activities of hormones such as insulin and leptin 8 . Whether this also applies to steroid hormones is not exactly known, although they tend to be more stable. The major strength of this study is the detailed assessment of the diurnal variation and the use of our recently developed, reliable isotope dilution LC–MS/MS assay 24 . However, a limitation of our study is the small sample size. Nevertheless, our study was adequately powered and significant results were found. In addition, our findings can be used to design a larger study, aimed at exploring whether the diurnal rhythm in breastmilk glucocorticoids exerts biologically relevant effects on the developing