Purpose: This study examined hydration responses to sodium hyperhydration in female athletes at rest and during cycling across the early follicular and mid-luteal phases of the menstrual cycle. Methods: Twelve cyclists/triathletes consumed 30 mL·kg-1 fat-free mass fluid with either sodium chloride (7.5 g·L-1) or placebo (sucrose) 2 h before 75 min of steady-state cycling (60% VO2peak) and a 200 kJ time trial (TT) in a hot environment (34 °C, 60% RH). Body mass was measured, and urine was collected every 30 min, whilst blood samples were taken hourly pre-exercise, post-steady-state, and post-TT. Results: During pre-exercise, sodium hyperhydration increased fluid retention (509.0 mL, 95% CI: [349.0, 669.0]; p < 0.001), while reducing urine volume (-107.4 [-146.7, -68.1] mL; p < 0.001). During exercise, body mass loss was lower with sodium during steady-state (-0.20%; p = 0.001) and TT (-0.15%; p = 0.037), but sweat rates were reduced with sodium only during steady-state (-0.08 L·h-1; p = 0.001). Exploratory analyses showed greater effects in the early-follicular phase, with reductions in body mass loss (-0.26%; p = 0.004), sweat rate (-0.1 L·h-1; p = 0.003), and post-TT arginine vasopressin (-10.8 [-19.2, -2.3] pg·mL-1; p = 0.013). Conclusions: Sodium hyperhydration effectively enhanced fluid retention and reduced body mass loss during exercise in the heat. While no consistent main effects of menstrual cycle phase were observed, some phase-specific differences emerged, particularly in the early-follicular phase. These findings highlight the importance of monitoring hydration responses across the menstrual cycle and tailoring strategies to individual needs, rather than applying universal phase-specific recommendations.
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