Objective: To determine whether fluid replacement after exercise induced dehydration varies over the normal menstrual cycle.
Methods: Five subjects, with a regular menstrual cycle lasting 28 (SEM 2) d, were dehydrated by 1.8(0.1)% of their pre-exercise mass by cycle exercise in the heat. Trials were undertaken 2 d before (trial -2) and 5 and 19 d after the onset of menses (trials 6 and 20 respectively). After exercise, subjects ingested a fixed volume, equivalent to 150% of mass loss, of a commercially available sports drink over a 60 min period.
Results: Cumulative urine output [median (range)] over the 6 h following ingestion was the same on all trials: 714(469-750) ml on trial -2; 476(433-639) ml on trial 6; 534(195-852) ml on trial 20. There was no menstrual cycle effect on urinary electrolyte (Na+, K+, Cl-) excretion or serum electrolyte (Na+, K+, Cl-) concentrations. Plasma volume increased by 8-12% of the postexercise value following rehydration. The percentage of ingested fluid retained did not differ between trials at any time. Six hours after drink ingestion, net fluid balance was not different from the initial value on any of the trials.
Conclusions: Acute replacement of exercise induced fluid losses is not affected by the normal menstrual cycle.