Purpose: We examined the effect of glycerol- and sodium-induced hyperhydration on a field-based half-marathon in warm conditions. Methods: Endurance runners (N = 13) completed a 180-minute hyperhydration (HYP) and control (CON) protocol before a maximal-effort half-marathon time trial (~24 °C wet-bulb globe temperature) in a randomized crossover order. HYP involved ingesting 25 mL·kg body mass (BM)-1 fluid with glycerol (1.2 g·kg BM-1) and sodium chloride (7.5 g·L-1) in 4 equal boluses across 60 minutes, with CON matching fluid ingestion for volume and timing. Every 20 minutes, urine samples were analyzed for volume (UVol; in milliliters) and fluid retention (fluid ingested minus UVol; in milliliters), while gastrointestinal symptoms were recorded using a questionnaire. Changes in plasma volume were calculated from hematocrit and hemoglobin at baseline and every 60 minutes. Core temperature (gastrointestinal temperature) and heart rate were recorded during the half-marathon. Intervention effects were estimated using linear mixed models and are presented as mean differences with 95% CIs. Results: HYP increased fluid retention compared with CON by 1189 mL, 95% CI 987-1391 (P < .001), and plasma volume by 9.0%, 95% CI 3.6-14.4 (P = .001). However, there were no statistically significant differences in exercise performance, gastrointestinal symptoms, gastrointestinal temperature, and heart rate (P > .05 for mean values and at all time points). Conclusions: Glycerol- and sodium-induced hyperhydration improved hydration status without changing gastrointestinal symptoms prior to endurance exercise in warm conditions but did not improve half-marathon performance. This strategy requires further investigation in environments with greater heat stress.
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