Context: Hydration has been shown to be an important factor in performance; however, the effects of mild dehydration during intense cycling are not clear.

Objective: To determine the influence of mild dehydration on cycling performance during an outdoor climbing trial in the heat (ambient temperature = 29.0°C ± 2.2°C).

Design: Crossover study.

Setting: Outdoor.

Patients or other participants: Ten well-trained, male endurance cyclists (age = 28 ± 5 years, height = 182 ± 0.4 cm, mass = 73 ± 4 kg, maximal oxygen uptake = 56 ± 9 mL·min(-1)·kg(-1), body fat = 23% ± 2%, maximal power = 354 ± 48 W).

Intervention(s): Participants completed 1 hour of steady-state cycling with or without drinking to achieve the desired pre-exercise hydration level before 5-km hill-climbing cycling. Participants started the 5-km ride either euhydrated (EUH) or dehydrated by -1% of body mass (DEH).

Main outcome measure(s): Performance time, core temperature, sweat rate, sweat sensitivity, and rating of perceived exertion (RPE).

Results: Participants completed the 5-km ride 5.8% faster in the EUH (16.6 ± 2.3 minutes) than DEH (17.6 ± 2.9 minutes) trial (t1 = 10.221, P = .001). Postexercise body mass was -1.4% ± 0.3% for the EUH trial and -2.2% ± 0.2% for the DEH trial (t1 = 191.384, P < .001). Core temperature after the climb was greater during the DEH (39.2°C ± 0.3°C) than EUH (38.8°C ± 0.2°C) trial (t1 = 8.04, P = .005). Sweat rate was lower during the DEH (0.44 ± 0.16 mg·m(-2)·s(-1)) than EUH (0.51 ± 0.16 mg·m(-2)·s(-1)) trial (t8 = 2.703, P = .03). Sweat sensitivity was lower during the DEH (72.6 ± 32 g·°C(-1)·min(-1)) than EUH (102.6 ± 54.2 g·°C(-1)·min(-1)) trial (t8 = 3.072, P = .02). Lastly, RPE after the exercise performance test was higher for the DEH (19.0 ± 1.0) than EUH (17.0 ± 1.0) participants (t9 = -3.36, P = .008).

Conclusions: We found mild dehydration decreased cycling performance during a 5-km outdoor hill course, probably due to greater heat strain and greater perceived intensity.