Sodium Hyperhydration Improves Performance With No Change in Thermal and Cardiovascular Strain in Female Cyclists Exercising in the Heat Across the Menstrual Cycle

This study investigated the effect of sodium hyperhydration on thermal and cardiovascular strain and exercise performance in unacclimatized endurance-trained females exercising in the heat and whether effects differ between menstrual cycle (MC) Phase 1 (low estrogen and progesterone) and MC Phase 4 (moderate estrogen and high progesterone). Twelve female cyclists/triathletes completed four trials in a randomized, double-blinded, crossover design. Participants consumed 30 ml·kg-1 fat-free mass fluid with either sodium chloride (7.5 g·kg-1) or placebo (sucrose) 2 hr prior to 75 min of steady-state cycling (60% V˙O2peak) followed by a 200-kJ time trial (TT) in 34 °C and 60% relative humidity, with both interventions completed during MC Phase 1 and Phase 4. Rectal temperature and heart rate were measured at baseline, every 5 min during steady state, every 50 kJ of TT, and TT completion. Body mass was measured every 30 min preexercise and pre and post steady state and TT to assess hydration status. Linear mixed models were fitted to estimate intervention and MC phase effect. There were no significant sodium hyperhydration or MC phase effects on rectal temperature or heart rate (p > .05). Body mass increased with sodium versus placebo (0.38 [0.02, 0.74] kg; p = .04), with a greater increase in MC Phase 4 (0.69 [0.17, 1.2] kg; p < .001). TT performance improved with sodium versus placebo (-1.55 [-2.46, -0.64] min; p = .001), with a greater improvement in MC Phase 4 (-1.85 [-3.16, -0.55] min; p = .005). Sodium hyperhydration is a promising heat mitigation strategy for females undertaking prolonged exercise in the heat, especially during MC Phase 4 and when fluid access is limited.