Associations Between Hydration, Sodium Intake, and Body Mass in Ultra-Endurance Trail Runners Under Ecological Race Conditions: A Cross-Sectional Field Study

Background: Hydration and electrolyte strategies are critical in mountain ultra-endurance events, yet field-based evidence from trail races remains limited. This study examined the relationship between fluid intake, sodium consumption, and body mass changes in trail runners competing under real environmental conditions. Methods: A cross-sectional field study was conducted during La Misión Brasil 2024. Athletes of both sexes competing in the endurance race (35 km; EG: n = 15; age = 37.0 [29.5–46.0] years; 12 men and 3 women) and the ultra-endurance race (80 km; UEG: n = 13; age = 42.0 [37.0–46.0] years; 11 men and 2 women) were included in the study. Pre- and post-race body mass were assessed, and in-race fluid and food intake were collected using an adapted 24-h dietary recall. Water and sodium intake were expressed as total (L and mg, respectively) and per-hour (mL/h and mg/h, respectively) values. Environmental temperature and humidity were obtained from a local weather station. Group comparisons were performed using the Mann–Whitney U test, and associations were examined with Spearman’s correlation (p < 0.05). Results: EG (n = 15) and UEG (n = 13) showed similar absolute and relative body mass changes (2.6% to −3.0%; p > 0.05). EG runners presented greater weight loss rate (−270 vs. −115 g/h; p = 0.002), while UEG consumed higher total water (7.11 vs. 4.14 L; p = 0.008) and sodium (5789 vs. 2857 mg; p = 0.003). Water intake per hour was higher in EG (626 vs. 427 mL/h; p = 0.017). Body Mass Index was negatively correlated with hourly weight loss (r = −0.605; p < 0.001). Water and sodium intake per hour were positively correlated (r = 0.607; p < 0.001), though neither predicted hourly weight loss. Conclusions: Hydration responses may differ according to environmental stress and pacing demands. Changes in body mass may not necessarily reflect hydration adequacy, suggesting a possible multifactorial nature of hydroelectrolyte balance during mountain endurance events.