Introduction: Oral rehydration reduces thirst sensation (TS), which may negatively affect autonomic function, measured by heart-rate variability (HRV). However, it is unclear if this effect is independent of hydration changes. This study examines whether TS influences autonomic function between intravenous and oral rehydration. Methods: Twelve males (mean ± SD; age, 29 ± 12 years; 74.7 ± 7.9 kg; 179.4 ± 7.0 cm; VO2_max, 49.8± 6.6 mL·kg⁻¹·min⁻¹) cycled at 55% VO_2max for 90 min followed by a 12 km time trial. Two experimental conditions were performed in a counterbalanced, randomized order; (a) the high thirst (HT) group were infused 25 mL of isotonic saline every 5 min via an intravenous tube, and (b) the low thirst (LT) group ingested 25 mL of water every 5 min. TS and heart rate were collected every 5 min. HRV was assessed pre exercise, post steady-state exercise, and post time trial. HRV parameters included time domain, frequency domain, and non-linear measures analyzed by two-way repeated measures ANOVA. Results: There was a significant time x condition for the root mean square of successive RR interval differences (RMSSD_log), high-frequency (HF) power, and SD1 (p < 0.05). In LT, RMSSD_log decreased from Pre to Mid (3.71 ± 0.61 ms to 2.53 ± 1.15 ms, p < 0.01) and Pre to Post (2.18 ± 0.90 ms, p < 0.01) but stabilized from Mid to Post (p = 0.39). High-frequency (HF) power in HT was maintained from Pre (3.7 ± 0.6 nu) to Mid (3.4 ± 0.8 nu, p = 0.21) but decreased from Pre to Post (2.5 ± 0.7 nu, p < 0.01) and Mid to Post (p < 0.01). LT decreased in HF power from Pre (3.7 ± 0.5 nu) to Mid (3.0 ± 0.8 nu, p < 0.01) and Pre to Post (3.0 ± 0.7 nu, p < 0.01); Mid and Post was maintained (p = 0.99). SD1, decreased in HT (Pre: 3.4 ± 0.4 ms, Mid: 2.0 ± 1.1 ms, Post: 1.1 ± 0.5 ms; all comparisons p < 0.05). In LT, SD1 decreased from Pre (3.4 ± 0.6 ms) to Mid (2.18 ± 1.15 ms, p < 0.01) and Pre to Post (1.83 ± 0.90 ms, p < 0.01), but stabilized Mid to Post (p = 0.39). Conclusion: Satiating thirst through oral rehydration increases parasympathetic activity post exercise, reducing stress and increasing recovery between exercise bouts. These findings have implications for optimizing rehydration strategies in sports and occupational settings.

Background: Oral rehydration reduces thirst sensation, which potentially influences cognitive performance. Aim: This study examines whether thirst influences cognitive performance between intravenous and oral rehydration. Methods: Twelve men cycled for 90 min at 55% VO _2max in 35°C and 30% r.h, followed by a 12 km cycling time trial. Two experimental trials were conducted: participants consumed 25 mL of water every 5 min (low thirst: LT) or were infused with 25 mL of isotonic saline every 5 min via intravenous tube (high thirst: HT). Thirst was measured every 5 min with visual analog scales. The Environmental Symptoms Questionnaire (ESQ), Profile of Mood States (POMS), a modified flanker task, body mass, and urine specific gravity (USG) were recorded before and the end of each trial. Summary: There were no significant differences in body mass loss and USG ( p ≥ 0.14), demonstrating similar hydration states. Thirst was higher in HT compared to LT after 15 min and throughout the 90 min and 12 km time trial ( p = 0.006). Participants reported feeling performance decrements (mean ± SD; HT: 3.7 ± 0.2, LT: 3.1 ± 0.3, p = 0.012), difficulty concentrating (HT: 1.8 ± 0.2, LT: 1.4 ± 0.1, p = 0.032), and an overall heightened subjective thermal strain (total ESQ, HT: 27.3 ± 1.1, LT: 24.8 ± 1.0, p < 0.045) during HT trials compared to LT for the ESQ. No significant differences were observed in the flanker task and POMS between trials ( p > 0.05). Thirst may not affect executive function performance, though it may increase subjective thermal strain during exercise in the heat. This study suggests that strategies for mitigating thirst are as important as maintaining hydration status to maintain concentration and subjective thermal strain.