The Influence Of Menstrual Cycle Phase On Fluid Intake And Urinary Hydration Markers

Abstract

Variations in female sex hormones across the menstrual cycle (MC) influence the osmotic threshold for release of the hormone arginine vasopressin, which alters fluid retention and thirst perception. However, fluid intake behaviors across the MC have yet to be explored. PURPOSE: 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 24 h 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. 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. UOSM was 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). CONCLUSION: Our results indicate fluid intake behaviors do not change across the menstrual cycle in naturally cycling females despite differences in 24 h urine volume and osmolality. Since mean total fluid intake exceeded daily adequate intake recommendations and 24 h urinary hydration markers indicated euhydration, the clinical implications of menstrual cycle phase on hydration status may be minimal, but this remains to be fully elucidated in women not meeting fluid intake guidelines.