Effect of Menstrual Cycle Phase on Fluid Intake during Exercise in the Heat

Menstrual status is known to influence thermoregulation, and fluid loss during exercise/heat stress may vary at different stages of the ovarian cycle in healthy women. It has been established that the steroid hormones, estrogen and progesterone, have an effect on fluid and electrolyte balance, and that there is a tendency toward significant free water retention during the luteal phase when estrogen levels are high. These findings raise questions of whether there is an acute effect of endogenous hormone status on fluid balance, and whether hydration during exercise is affected by the phase of the menstrual cycle. PURPOSE: To determine whether ad libitum fluid intake during exercise induced sweat loss varies over the course of the normal menstrual cycle. METHODS: Seven eumenorrheic (31±2 d) females (22±1 yrs) underwent 3 exercise/rest trials (treadmill walking at 3% grade, 30-35% VO2max, 30 min•h-1 followed by 30 min of sitting during a 3 h period in 36°C, rh 50-70%) representing different stages of the menstrual cycle. Trials were undertaken 3 (menses) and 8-10 d (midfollicular) after the onset of menstrual bleeding, and 7-9 d following the LH peak (midluteal) over 3 consecutive menstrual cycles. RESULTS: Total fluid intake was similar in all phases; 978 ± 195.9 ml during menses; 1136.7 ± 338.7 ml during follicular; 1282.9 ± 473.7 ml during luteal. Stomach fullness ratings were higher during the luteal phase (6 ± 0.4) compared to the follicular (4 ± 0.5) and menses phases (3 ± 0.5, P < 0.05). There was no significant menstrual cycle phase effect on perceived thirst ratings, plasma vasopressin concentrations, Serum osmolarity, plasma volume, or fluid loss across experimental days. CONCLUSIONS: Ad libitum replacement of exercise induced fluid losses is not affected by the normal menstrual cycle in healthy young women.