PURPOSE: The elevated hormones and core body temperature after ovulation in eumenorrheic women may influence sweat losses and fluid requirements during exercise. This study aimed to determine if sweat rate and sodium concentration, hydration and thirst differ during exercise across the high and low hormone phases of the menstrual cycle.  

METHODS: Sixteen trained eumenorrheic female cyclists (n=8, VO_2max 53 ± 7 mL/kg/min) and runners (n=8, VO_2max 53 ± 6 mL/kg/min) completed 24 h dietary and fluid standardisation prior to either 75 min fixed intensity (60% VO_2max) cycling or a self-paced 10 km running time trial (63 ± 11 min) in a heat chamber (34°C, 60% relative humidity, 4.5 m/s airflow) during phase 1 (day 3-6, low hormones) and phase 4 (7-9 days after positive urinary luteinising hormone test, high hormones) of their menstrual cycle. Urine specific gravity (USG) was measured pre-exercise, whilst nude body mass, plasma osmolality, haemoglobin and haematocrit were measured pre- and post-exercise. Sweat rate and plasma volume change were calculated from the change in body mass (corrected for urine loss) and haemoglobin and haematocrit, respectively. Absorbent patches were used to collect sweat at four sites and sodium concentration was analysed using a LAQUAtwin Na+ pocket meter. Thirst rating (11-point scale) was measured pre-exercise and every 10 min during cycling and every 1 km during running. Repeated measures ANOVA was used to analyse differences between menstrual cycle phase and interactions with exercise type (fixed intensity cycling vs self-paced running). Data are reported as mean ± standard deviation.

RESULTS: Baseline measures of hydration were similar between phase 1 and 4 of the menstrual cycle (body mass 60.1 ± 5.7kg vs 60.0 ± 5.3kg; p=0.544, plasma osmolality 287 ± 6 mOsmol/kg vs 285 ± 8 mOsmol/kg; p=0.091,  and USG 1.013 ± 0.008 vs 1.013 ± 0.008; p=0.763, respectively). There were no differences between phase 1 and 4 of the menstrual cycle for % body mass loss (1.8 ± 0.5% vs 1.9 ± 0.6%; p=0.136), sweat rate (1.0 ± 0.4 L/h vs 1.0 ± 0.4 L/h; p=0.745), change in plasma osmolality (10 ± 7 mOsmol/kg vs 9 ± 4 mOsmol/kg; p=0.901), plasma volume change (-10.6 ± 7.1% vs -10.8 ± 5.7%; p=0.857), whole body sweat sodium concentration (1479 ± 400 mg/L vs 1506 ± 409 mg/L; p=0.833), or cumulative thirst rating (32 ± 13 vs 36 ± 12; p=0.100). There were no interaction effects for exercise type with menstrual cycle phase (all p>0.05).

CONCLUSION: Practitioners and eumenorrheic female athletes are not required to account for the high and low hormones phases of the menstrual cycle when assessing sweat rates and sodium losses or planning fluid requirements.

Disclosure(s): The authors have nothing to disclose.