Bibian van der Voorn

77 MILK GLUCOCORTICOID DIURNAL RHYTHMICITY INTRODUCTION In term infants, human milk is recommended for its beneficial effect on growth, neurodevelopment, the immune system, and cardiovascular health, all of which are attributable to nutritional and nonnutritive, bioactive components 1-3 . Similar benefits of human milk are observed in very preterm infants 4-7 . Therefore, when breast milk is not yet available after very preterm delivery, donated human milk is recommended 8,9 . Humanmilk is known for its dynamic composition, often filling neonatal requirements 1 . Its nutritional composition is determined by stage of lactation, maternal diet, body composition, and genetics 10 . Human milk has been associated with postnatal growth restriction in very preterm infants, because it does not meet the very preterm (GA <32 wks) infants nutritional requirements 4 . Still, compared with formula feeding, donor milk reduces morbidity, likely by delivering protective factors to the immature gut mucosa 1,9,11 . However, little is known about factors that could influence the composition of nonnutritive, bioactive components (e.g., hormones). Many ill, very preterm newborns have inappropriately low cortisol concentrations. This might result in increased vulnerability to potentially life-threatening illnesses 12,13 . Glucocorticoids in breast milk 1,2 could provide resilience against these illnesses. Therefore, we conducted 2 studies with the aim of exploring variations in glucocorticoid concentrations in human milk. First, we studied whether breast- milk cortisol and cortisone concentrations in the first month postpartum differed between mothers who delivered very preterm and those who delivered at term. We hypothesized that very preterm delivery would be associated with higher breast- milk glucocorticoid concentrations. Results from this study showed that breast-milk cortisol and cortisone concentrations vary widely, not only between mothers but also within mothers. In a post hoc analysis, milk cortisol and cortisone concentrations were found to be dependent on time of collection. Subsequently, in a second study, we explored whether breast-milk glucocorticoid concentrations follow a diurnal rhythm, and whether they correspond to the circadian rhythm of maternal HPA axis activity. METHODS PARTICIPANTS For both studies, mothers were recruited at the maternity department of VUMC in Amsterdam, Netherlands. Both studies were approved by the Medical Ethics