Effects of a High-Sodium, Carbohydrate-Free Multi-Electrolyte Supplement on 15 km Treadmill Time-Trial Performance in Warm and Humid Conditions: A Randomized, Single-Blind, Placebo-Controlled Crossover Trial

Sodium ingestion may support fluid balance and endurance performance in the heat, but most previous studies have used sodium-loading, hyperhydration, or carbohydrate–electrolyte protocols. This study examined the effects of a high-sodium, carbohydrate-free multi-electrolyte supplement (HSES) on 15 km treadmill time-trial performance and physiological responses in trained male runners under warm-humid conditions. In a randomized, single-blind, counterbalanced crossover design, ten trained male runners (V̇O2max: 57.3 ± 8.1 mL·kg−1·min−1) completed two 15 km treadmill time trials at 26.7 °C and 60% relative humidity. Participants received HSES or a flavor-matched placebo (PLA) 30 min before exercise and every 15 min of running time. Repeated-measures analysis of cumulative completion time showed a significant main effect of condition (p = 0.034) and a significant condition × distance interaction (p = 0.023). Segment pace also showed a significant condition × segment interaction (p = 0.001); participants ran faster during HSES over the opening 0–5 km (p < 0.001), pace did not differ during 5–10 km (p = 0.091), and participants ran faster during PLA over the final 10–15 km (p = 0.032). Mean final 15 km completion time was 68.76 ± 12.28 min during HSES and 70.18 ± 13.33 min during PLA, a mean difference of 1.42 min (~2.0%). However, the direct paired comparison of final completion time did not reach statistical significance (HSES − PLA: −1.42 min; 95% CI, −3.60 to 0.77 min; p = 0.176; Cohen’s dz = −0.46). No significant between-condition differences were observed in measured hydration-related, cardiovascular, metabolic, or perceptual outcomes. Estimated core temperature was higher during HSES at 15 km and 10 min post-exercise. Pacing differed between conditions during the 15 km time trial, with a faster opening 5 km during HSES and a faster final 5 km during PLA. Final 15 km completion time was approximately 2.0% faster during HSES, although the difference was not statistically significant.