Menstrual cycle effects on thermoregulation while exercising in the heat

Women may be challenged to maintain thermoregulation due to hormonal changes associated with the menstrual cycle. The purpose of this study was to assess the effect of the menstrual cycle phase on core temperature, hydration status, and perceived exertion while exercising under uncompensable heat gain. Eleven eumenorrheic women (24.4 ± 1.1 yrs, 65.7 ± 2.4 kg, 22.7 ± 1.5% body fat) walked for two 180-min trials in a heat chamber (35 °C and 30% relative humidity) during early-follicular (EF) and mid-luteal (ML) phases. Subjects completed three intervals of 50 min of exercise at 50% VO₂max. Physiological strain index (PSI), core temperature (T_C), perceived heat (PH), and rating of perceived exertion (RPE) were measured throughout both trials. Nude body weight (NBW) and blood samples were collected pre- and post-trial. Blood samples were analyzed for hematocrit (Hct), hemoglobin (Hb), serum estrogen, progesterone, and aldosterone. NBW showed a main effect of time (p = 0.002, η_p² = 0.62). Aldosterone showed main effect of time (p = 0.004, η_p² = 0.59) and phase (p = 0.014, η_p² = 0.47), peaking post exercise in both EF and ML (527.6.1 ± 89.0 pg·mL^-1 vs 827.4 ± 129.5 pg mL^-1 respectively, p = 0.014). Estradiol and progesterone showed main effects of phase (p = 0.007, η_p² = 0.53; p = 0.045, η_p² = 0.30) but not time (p = 0.68, p = 0.32). T_C showed main effect of time (p < 0.001, η_p² = 0.89) and phase, peaking at 170 min (EF: 37.8 ± 0.1 °C vs. ML: 38.0 ± 0.1 °C, p = 0.032, η_p² = 0.38). Main effect of time was seen for PSI (p = 0.002, η_p² = 0.88), PH (p = 0.004, η_p² = 0.66), and RPE (p = 0.026, η_p² = 0.80). Sweat rate, Hct, Hb, and percent dehydration were not different between the phases. In conclusion, subjects demonstrated elevated Tc and basal aldosterone in ML corresponding with elevations in estrogen and progesterone. Aldosterone significantly increased following exercise in the heat but remained elevated in ML. These results indicate that elevated Tc during ML is maintained during exercise in the heat despite similar perceived heat and effort between phases.