Oral Rehydration Improved Exercise Performance and Decreased Core Temperature and Heartrate Compared With Intravenous Rehydration in Recreationally Active Men

Background: The purpose of this study was to examine the effect of oral versus intravenous (IV) rehydration during exercise in the heat on exercise performance and physiological responses. Hypothesis: Oral rehydration will lead to better performance and physiological outcomes. Study Design: Cross-over study. Level of Evidence: Level 2. Methods: A total of recreational active men (age, 29 ± 12 years; peak oxygen consumption [VO2peak], 49.8 ± 6.6 ml kg−1 min−1) performed 90 minutes of cycling exercise at 55% watts at VO2peak (WVO2peak) followed by a 12-km time trial in the heat (34.9 ± 0.6°C; 30.3 ± 0.9%; wind speed, 3.4 m sec−1). Two experimental trials were performed in random order: (1) drank 25 ml of water every 5 minutes (ORAL) and (2) 25 ml of isotonic saline infused intravenously every 5 minutes (IV). Rectal temperature (Trec) and thirst sensation were measured every 5 minutes. Urine specific gravity (USG) and body mass loss were measured before and after trials. Changes (Δ) were calculated based on the resting value for each trial. Results: Participants started both trials euhydrated (USG < 1.020), and no differences were found in hydration status between ORAL and IV after trials (USG:ORAL, 1.012 ± 0.006; P = 0.14; IV, 1.013 ± 0.007; body mass loss: ORAL, 2.4 ± 0.8%; IV, 2.3 ± 0.5%, P = 0.68). Thirst levels were higher in IV than ORAL from 20 to 50 minutes (P = 0.03 to 0.05) and after 65 minutes throughout 90 minutes of cycling exercise and the 12-km time trial (P < 0.001 to 0.04). ΔTrec was also significantly higher in IV after 20 minutes throughout 90 minutes of cycling exercise and a 12-km time trial (P = 0.003 to 0.04). The time trial was significantly faster in ORAL (17.7 ± 4.6 minutes) compared with IV (19.6 ± 6.2 minutes, P = 0.05). Conclusion: Oral rehydration enhances exercise performance and decreases Trec and heartrate compared with IV rehydration. Clinical Relevance: Oral rehydration might have more benefits than IV in exercise performance and physiological response.