Acute decrease in mothers' cortisol following nursing and milk expression.

The glucocorticoid hormone cortisol is an integral component of signaling pathways related to stress reactivity, energy balance, immune function, and other processes. In animal models, lactation is robustly associated with alterations in glucocorticoid signaling, and limited data suggest that similar changes may occur across human lactation. We asked whether milk letdown/secretion in breastfeeding mothers was associated with changes in cortisol, and whether such effects required presence of an infant. We measured changes in maternal salivary cortisol concentrations before and after nursing, the expression of breastmilk with an electric pump, or control activities. Participants conducted pre-session and post-session sampling (at 30 min) for all conditions, and provided a sample of pumped milk from one session. Both nursing and mechanical expression of breastmilk but not control were associated with equivalent declines in maternal cortisol concentration from pre-session values, indicating an effect of milk letdown on circulating cortisol independent of infant contact. Pre-session maternal salivary cortisol concentration was strongly and positively correlated with cortisol concentration in pumped milk samples, indicating that cortisol ingested by offspring provides a signal of maternal cortisol levels. Self-reported maternal stress was associated with higher pre-session cortisol concentrations, as well as with a larger drop in cortisol following nursing or pumping. These findings demonstrate that milk release-in the presence or absence of a suckling infant-regulates cortisol in mothers, and supports the potential for maternal signaling through breastmilk.