HORMONAL AND METABOLIC RESPONSES TO EXERCISE ACROSS TIME OF DAY AND MENSTRUAL-CYCLE PHASE

Two studies, each utilizing short-term treadmill exercise of a differe nt intensity, assessed the metabolic and hormonal responses of women t o exercise in the morning (AM) and late afternoon (PM). In study I, pl asma concentrations of growth hormone, arginine vasopressin, catechola mines, adrenocorticotropic hormone, cortisol, lactate, and glucose wer e measured before, during, and after high-intensity exercise (90% maxi mal O-2 uptake) in the AM and PM. In study 2, plasma concentrations of adrenocorticotropic hormone, cortisol, lactate, and glucose were meas ured before, during, and after moderate-intensity exercise (70% maxima l O-2 uptake) in the AM and PM in the follicular (days 3-9), midcycle (clays 10-16), and luteal (days 18-26) phases of the menstrual cycle. The results of studies 1 and 2 revealed no significant diurnal differe nces in the magnitude of responses for any measured variable. In addit ion, study 2 revealed a significant time-by-phase interaction for gluc ose (P = 0.014). However, net integrated responses were similar across cycle phases. These data suggest that metabolic and hormonal response s to short-term, high-intensity exercise can be assessed with equal re liability in the AM and PM and that there are subtle differences in bl ood glucose responses to moderate-intensity exercise across menstrual cycle phase.