Substrate responses to submaximal exercise in the midfollicular and midluteal phases of the menstrual cycle.

This study examined substrate metabolism responses of eumenorrheic women to different intensities of submaximal exercise at the midfollicular (MF) and the midluteal (ML) phases of the menstrual cycle. Nine women performed a 30-min treadmill run in which the exercise intensity was made more difficult every 10 min (35%, 60%, and 75%). Carbohydrate (CHO) utilization and oxidation rates for the 35% and 60% intensities during the ML session were significantly lower than during the comparable intensities in the MF. Conversely, lipid utilization and oxidation were significantly greater during the 35% and 60% ML session than in the MF session. At 75%, however, the ML and MF CHO-lipid utilization and oxidation rates were not significantly different from one another. Thus, the phase of the menstrual cycle in eumenorrheic women does influence metabolic substrate usage during low- to moderate-intensity submaximal exercise, probably due to changes in the endogenous levels of the female sex hormones.