Maternal cardiorespiratory fitness is associated with metabolic hormone concentrations in human milk

The composition of human milk reflects maternal cardiometabolic health, of which cardiorespiratory fitness is a key determinant. Yet, the influence of maternal fitness on milk-borne hormones remains largely unexplored. In this cross-sectional study, we measured peak oxygen uptake during a maximal exercise test in 149 lactating participants and quantified human milk adiponectin and insulin concentrations using enzyme-linked immunosorbent assays. Higher maternal cardiorespiratory fitness was associated with lower concentrations of milk adiponectin after adjustment for body composition and weeks since delivery (β -0.02, 95% CI -0.04 to -0.001). Milk insulin concentrations were inversely associated with fitness after adjustment for body mass index (β -0.02, 95% CI -0.04 to -0.001). This work identifies maternal cardiorespiratory fitness as a previously unrecognised factor associated with human milk hormone concentrations, providing a potential pathway linking maternal physiology with early-life metabolic development.