Influence of Follicular vs. Luteal Phases on Sweat Rate and Estimated Sodium Loss in University Female Football Players: A Field-Based Within-Subject Study

This study examined the influence of the follicular (FP) and luteal phases (LP) of the menstrual cycle on sweat rate, estimated sweat sodium concentration, heart rate, hydration status, fluid intake, and perceived exertion in first-division university female football players. A small sample of eight athletes completed two monitored training sessions, one in each estimated-menstrual phase, following a repeated-measures field-based design under habitual training conditions. Sweat rate was determined using pre- to post-exercise body mass changes and microfluidic sweat patches, while estimated sweat sodium concentration was obtained via wearable colorimetric sensors. Heart rate was continuously monitored, hydration status was assessed using urine specific gravity, fluid intake was recorded, and perceived exertion was evaluated using the Borg CR-10 scale. Sweat rate was significantly higher during LP compared with FP (0.83 ± 0.20 vs. 0.55 ± 0.25 L·h−1, p = 0.026), alongside greater estimated sweat sodium concentration (695 ± 305 vs. 404 ± 159 mg·L−1, p = 0.031) and higher perceived exertion (4.63 ± 1.41 vs. 3.13 ± 0.83, p = 0.021). Fluid intake was also significantly greater during LP (0.99 ± 0.19 vs. 0.49 ± 0.25 L, p < 0.001). No significant differences were observed for urine specific gravity, mean heart rate, or total body mass change (p > 0.05). These findings suggest that the luteal phase may be associated with higher thermoregulatory and perceptual responses during football training, highlighting the potential importance of menstrual cycle-informed hydration and training management strategies in female athletes.