Influence of Hydration Status on Thermoregulation and Cycling Hill Climbing

Purpose: Although dehydration can impair endurance performance, a reduced body mass may benefit uphill cycling by increasing the power-to-mass ratio. This study examined the effects of a reduction in body mass attributable to unreplaced sweat losses on simulated cycling hill-climbing performance in the heat. Methods: Eight well-trained male cyclists (mean ± SD: 28.4 ± 5.7 yr; 71.0 ± 5.9 kg; 176.7 ± 4.7 cm; V?O2peak: 66.2 ± 5.8 mL·kg-1·min-1) completed a maximal graded cycling test on a stationary ergometer to determine maximal aerobic power (MAP). In a randomized crossover design, cyclists performed a 2-h ride at 53% MAP on a stationary ergometer, immediately followed by a cycling hill-climb time-to-exhaustion trial (88% MAP) on their own bicycle on an inclined treadmill (8%) at approximately 30°C. During the 2-h ride, they consumed either 2.4 L of a 7% carbohydrate (CHO) drink (HIGH) or 0.4 L of water (LOW) with sport gels to match for CHO content. Results: After the 2-h ride and before the hill climb, drinking strategies influenced body mass (LOW -2.5 ± 0.5% vs HIGH 0.3 ± 0.4%; P < 0.001), HR (LOW 158 ± 15 vs HIGH 146 ± 15 bpm; P = 0.03), and rectal temperature (Tre: LOW 38.9 ± 0.2 vs HIGH 38.3 ± 0.2°C; P = 0.001). Despite being approximately 1.9 kg lighter, time to exhaustion was significantly reduced by 28.6 ± 13.8% in the LOW treatment (LOW 13.9 ± 5.5 vs HIGH 19.5 ± 6.0 min, P = 0.002), as was the power output for a fixed speed (LOW 308 ± 28 vs HIGH 313 ± 28 W, P = 0.003). At exhaustion, Tre was higher in the LOW treatment (39.5 vs HIGH 39.1°C; P < 0.001), yet peak HR, blood lactate, and glucose were similar. Conclusion: Exercise-induced dehydration in a warm environment is detrimental to laboratory cycling hill-climbing performance despite reducing the power output required for a given speed.