Fluid intake behaviors and hydration biomarkers across the menstrual cycle in naturally cycling females

Evidence supports an impact of exogenous sex hormones on body water regulation, but research on endogenous hormones is limited. This study assessed whether estradiol and progesterone explain associations between fluid intake and hydration biomarkers in females. Seventeen females (mean ± SD; age, 25 ± 5y; height, 163.0 ± 7.3 cm; body mass, 71.3 ± 24.4 kg; body fat, 24.6 ± 7.1%) completed assessments of total water intake (TWI) and 24-h urinary hydration biomarkers during three menstrual-cycle windows (Days 4–6, 12–14, and 19–21 from the onset of menstruation). Estradiol, progesterone, and copeptin (surrogate of arginine vasopressin) were measured on day 2 of each window. Mixed-effects models examined whether within- or between-subject hormone variability improved TWI-hydration models. Progesterone partially explained the association between prior-day TWI and next-morning copeptin (χ 2 (2) = 7.75, p = 0.021, marginal R 2 = 0.198). Every 1 nmol L −1 greater progesterone concentration (within-subject) was associated with a 23 mOsm kg −1 higher next-day 24-h urine osmolality ( p = 0.028), whereas higher progesterone compared to the group mean was associated with a 20 mOsm kg −1 lower urine osmolality ( p = 0.040). Estradiol did not influence any model. Progesterone, but not estradiol, helped explain the relationships between TWI and copeptin, and between TWI and urine osmolality. Effects were not enough to influence fluid intake behaviors, suggesting hormonal variability is unlikely to confound measurements of fluid intake among females.