Fluid Balance Across the Menstrual Cycle During Exercise in the Heat

Background: Attenuating core temperature elevations is crucial for wildland firefighters during field operations. Fluid intake and retention is one component to address rises in core temperature. Female wildland firefighters (WLFF) might be challenged to maintain fluid balance due to hormonal fluctuations throughout their menstrual cycle. Long periods of exercise in the heat could alter fluid balance and core temperature and risk WLFF safety in the field. The purpose of this study is to examine the effect of uncompensable heat stress on eumenorrheic females across the two phases of the menstrual cycle. Methods: In a crossover design, 12 females walked for two 180-minute trials in a heat chamber (35ºC and 30% relative humidity) during early follicular (FP) and mid luteal (LP) phases of their menstrual cycle. Following a 10-minute seated acclimation, subjects completed three 50-minutes intervals of exercise at 50% of their maximum aerobic capacity and 10 minutes of rest. Physiological strain index (PSI), core temperature (TCORE), skin temperature (TSKIN), and perceived heat (PH) were measured throughout both trials. Nude body weight (NBW), blood samples, and urine were collected pre- and post- trial. Blood samples were analyzed for aldosterone. Results: Aldosterone was 34% higher at rest in the LP compared to FP at rest. After the 180-minute trial, aldosterone was elevated from baseline in both phases. Similarly, TCORE was 0.3 ºC higher in the LP at rest and rose similarly across the two phases. PSI and TSKIN increased throughout the exercise, peaking at 170 minutes in both LP and FP. Perceived heat showed a time*phase interaction. Sweat rate and percent dehydration were not different between the trials. Significance: This study is the first to examine the impact of the menstrual cycle on fluid balance hormones during exercise. These data suggest that females exhibit thermoregulatory stress during prolonged exercise, exacerbated in the LP due to elevated basal core temperature. Despite this, increased fluid retention hormones (aldosterone) in the LP indicates a greater potential for combating dehydration. To further this point, subjects’ perceived heat was lower in this phase. This study provides an important step in identifying a previously unrecognized hormonal influence on fluid retention