INTRODUCTION: Reproductive hormones influence body fluid regulation, although there have been no studies examining the effect of menstrual cycle phase on drinking response during exercise.
PURPOSE: To determine whether differences exist in ad-lib ingestion of water when consumed during exercise during different phases of the menstrual cycle. Secondarily, we examined whether fluid consumption differed between a flavored fitness water (FW) and water (W) during the follicular phase (F).
METHODS: 20 women (mean age: 32.8 ± 9.0 y), who exercise on a regular basis, completed three trials consisting of one hour of interval exercise on the treadmill, stationary bicycle, and elliptical trainer (20 minutes on each) under warm environmental conditions (WBGT: 24.3 ±1.9° C). During each trial, the women were given a coded, unmarked bottle of either FW or W to drink ad-lib during the exercise session. Two trials were scheduled during F (Days 1–4) and a third trial during the luteal phase (L) of the menstrual cycle (Days 18–22) during which subjects received only water (WL). The order of trials was randomized. Nude body weights were recorded pre- and post-exercise and all fluid containers were weighed pre- and post-consumption. To assess sweat electrolytes, sterile absorbent patches were placed on the women's forearms during the W and WL trials after 20 minutes of exercise and were removed at the end of the exercise period. Sweat rates were calculated and corrected for fluid consumption and urine output. Sweat samples were analyzed for sodium and potassium concentrations (flame photometry). ANOVA was used to test for differences among FW, W and WL (P<0.05).
RESULTS: There was no difference in sweat rates among the three trials (FW = 944 ± 227 mL/h; W = 821 ± 192 mL/h; WL = 896 ± 227 mL/h). Mean consumption during exercise was 668 ± 373 mL for FW, 564 ± 359 mL for W, and 589 ± 336 mL for WL (P= 0.055). When consuming water, there was a trend for a difference in sweat sodium concentrations: L, 44.3 ± 19.2 mEq/L and F, 39.2 ±18.7 mEq/L (P=0.08).
CONCLUSION: Drinking response for water is consistent between the menstrual phases; however, in the follicular phase there is a strong trend towards better fluid replacement with FW verses W.