Thirst Sensation Decreases Exercise Performance And Increases Core Temperature And Heart Rate Independent Of Hydration Status

PURPOSE: The purpose of this study was to examine the effects of thirst sensation and oropharyngeal reflexes on exercise performance and physiological markers during exercise in the heat. METHODS: Twelve males (mean (standard deviation; age, 29 ± 12 years; body mass, 74.7 ± 7.9 kg; height, 179.4 ± 7.0 cm; maximal oxygen consumption [VO2max], 49.8 ± 6.6 ml·kg−1·min−1) performed 90 minutes of cycling exercise at 55% wVO2max followed by 12 km time trial in the heat (ambient temperature, 34.9 ± 0.6 °C; relative humidity, 30.3 ± 0.9%; wind speed, 3.4 miles ·h-1). Two experimental trials were performed in random order; a) participants drank 25 ml of water every 5 minutes (NT), b) participants were infused with 25 ml of isotonic saline every 5 minutes via intravenous tube (T). Rectal temperature (Trec) heart rate (HR), and thirst sensation were measured every 5 minutes throughout the trial. Urine specific gravity (USG), urine osmolality (UOsmo), serum osmolality (SOsmo), and body mass loss were measured before and after trials to assess hydration status. Δ was calculated based on the change from the resting value for each trial. RESULTS: Participants started both trials euhydrated (USG, NT, 1.009 ± 0.005, T, 1.008 ± 0.007; UOsmo, NT, 409 ± 205, T, 467 ± 248 mOsmol; SOsmo, NT, 290 ± 5, T, 286 ± 5 mOsmol). There were no differences in hydration status between NT and T after trials (USG, NT, 1.012 ± 0.006, T, 1.013 ± 0.007; UOsmo, NT, 451 ± 217, T, 467 ± 248 mOsmol; SOsmo, NT, 293 ± 4, T 295 ± 4 mOsmol; P > 0.05). Also, there were no differences in body mass loss (NT, 2.4 ± 0.8%; T, 2.3 ± 0.5%; p = 0.759). Thirst levels and Δ Trec were significantly higher in T than NT after 15 minutes throughout 90 minutes of cycling exercise and a 12 km time trial (p ≤ 0.05). Δ HR was significantly higher in T compared to NT at 55 minutes to 65 minutes and before the 12 km time trial started (p < 0.05). The time trial was significantly faster in NT (17.7 ± 4.6 min) compared to T (19.6 ± 6.2 min, p = 0.048). CONCLUSION: Independent of hydration status, thirst sensation decreases exercise performance and increases Trec and HR. Therefore, frequent fluid intake and drinking before feeling thirsty are as important as the total amount of fluid replacement to optimize exercise performance.