Comparison of respiratory responses to exercise under different heat-stressful environments between oral contraceptive and naturally menstruating ovulating female athletes.

The present study conducted a secondary analysis of previous data (Lei et al., 2017, 2019) to compare exercise-induced respiratory responses under heat stress between the early follicular (EF)/quasi-EF and mid-luteal (ML)/quasi-ML phases in naturally menstruating and oral contraceptive pill (OCP)-using female athletes. Additionally, the study aimed to examine the associations between respiratory responses, thermoregulation, and hormonal profiles within each group. Sixteen endurance-trained female athletes participated: eight with a natural menstrual cycle (NM group) and eight using OCP (OCP group). Each participant completed four trials, comprising fixed-intensity cycling (at 125 and 150 W) followed by a 30-min self-paced cycling session (with maximal effort; OCP ≈144-151 W, NM ≈138-148W). Trials were conducted during the EF/quasi-EF and the ML/quasi-ML phases under two WBGT-matched heat-stress environments: hot, dry (≈34 °C, ≈41 % RH) and warm, humid (≈29 °C, ≈81 % RH). Respiratory parameters did not differ significantly between the menstrual/OCP phases. However, during both exercises in the heat, the OCP group exhibited a significantly lower percentage of maximal oxygen uptake (V˙O2max) and oxygen uptake compared to the NM group, despite no significant differences in V˙O2max or workload between the groups. In the OCP group, changes in mean body temperature during fixed-intensity exercise were associated with the ventilatory equivalent for oxygen uptake and tidal volume. Endogenous progesterone concentration was negatively correlated with changes in minute ventilation. These results suggested that external factors, such as exogenous hormones and environmental heat stress, may have a greater influence on respiratory responses during exercise in female athletes than endogenous hormonal fluctuations.