Mitchell E. Zaplatosch1, Emily E. Bechke1, Samantha J. Goldenstein1, Madelyn G. Biffle1, Laurie Wideman, FACSM1, William M. Adams, FACSM2. 1University of North Carolina at Greensboro, Greensboro, NC. 2United States Olympic & Paralympic Committee, Colorado Springs, CO.

BACKGROUND: Variations in female sex hormones across the menstrual cycle (MC) influence the osmotic threshold for the release of the fluid regulatory hormone arginine vasopressin, which has been shown to alter fluid retention and thirst perception. However, fluid intake behaviors across the MC have yet to be explored. Thus, the purpose of this study was to determine differences in fluid intake behaviors and hydration status across the menstrual cycle in naturally cycling females. METHODS: Seventeen naturally cycling females (mean±SD; age, 24±5 y; height, 163±7 cm; mass, 71.3±24.4 kg; body fat, 22.7±9.3%) provided 24h urine samples over three consecutive days during three timepoints throughout their MC; early follicular (MCEF, days 3-5), late follicular (MCLF, days 11-13), and mid-luteal (MCML days 18-20) phases, where day 0 was defined as the start of menstruation. Participants also completed daily fluid and dietary intake logs to assess fluid intake behavior. Urinary hydration biomarkers assessed included urine volume (UVOL), urine osmolality (UOSM), urine specific gravity (USG), and urine color (UCOL). MC phase was used in separate linear mixed effects models as a fixed effect predictor of total fluid intake (TFI, all fluids from beverages and foods) and urinary hydration biomarkers, with a random effect of participant. RESULTS: There was no significant effect of MC phase on TFI (MCEF, 3472±1505 mL; MCLF, 3416±1410 mL; MCML, 3331±1703 mL; β [95% CI], -171.59 [-348.7, 6.8], p=0.172). UVOL was significantly greater during MCEF (2.36±1.33 L) compared to MCLF (2.09±1.41 L) and MCML (2.20±1.40 L), β=-0.145 [-0.268, -0.022], p=0.022. UOSMwas significantly greater during MCML (465±217 mOsm∙kg-1) than MCEF (393±193 mOsm∙kg-1) or MCLF (426±203 mOsm∙kg-1),(β=44.90 [19.0, 70.8], p=0.001. However, there was no impact of MC phase on UCOL (β=0.064 [-0.12, 024], p=0.489) or USG (β=0.0003 [-0.001, 0.002], p=0.751). CONCLUSIONS: Our results indicate fluid intake behaviors do not change throughout the menstrual cycle in naturally cycling females despite differences in 24h urine volume and urine osmolality. Since mean total fluid intake exceeded daily adequate intake recommendations with corresponding 24h urinary hydration markers indicating euhydration, the potential clinical implications of menstrual phase on hydration status may be minimal, however this remains to be fully elucidated.

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