Effects Of Exercise-induced Dehydration On Thermoregulation And Cycling Hill-climbing Performance

Abstract

Dehydration is generally thought to impair endurance performance. However, losses in body mass due to sweat loss may be beneficial for cyclists racing uphill because of an improved power:mass ratio. PURPOSE To explore the concept of “functional dehydration” by examining the effects of exercise-induced dehydration on cycling hillclimbing performance. METHODS This study used a repeated measures cross-over design. Eight trained male endurance cyclists (28.4 ± 5.7 yr; 71.0 ± 5.9 kg; 176.7 ± 4.7 cm; VO2peak: 66.2 ± 5.8 ml·kg−1·min−1) completed a maximal graded exercise test (GXT) and familiarization trials riding a bicycle uphill on a treadmill. Cyclists then performed a 2 h ride (53% GXTWATTSPK) immediately followed by a laboratory-based cycling hillclimb performance trial (exercise time until exhaustion) in a heat chamber (∼30°C). During the 2 h ride, cyclists either consumed 2.4 L of a 7% CHO sport drink (HIGH) or 0.4 L of water (LOW). Sport gels were used in the LOW trial to match CHO content. RESULTS At the end of the 2-h ride, drinking strategies had influenced body mass (LOW −2.5% v HIGH 0.3%; p <0.001), plasma volume (LOW −4.9% v HIGH −1.9%; p = 0.02), HR (LOW 158 v HIGH 146 bpm; p = 0.008) and core temperature (TREC: LOW 38.9°C v HIGH 38.3°C; p = 0.001) prior to the hill-climb performance trial. Despite being lighter, cycling hill-climbing performance was worse for all subjects in the LOW trial (13.9 min v HIGH 19.5 min; p = 0.002). Interestingly, upon reaching exhaustion, TREC was higher in LOW (39.5°C v HIGH 39.1°C; p = 0.0006) with a slightly higher HR (LOW 187 v HIGH 183 bpm; p = 0.02), yet peak Lactate (LOW 11.7 mM v HIGH 11.6 mM; p = 0.78), Glucose (LOW 8.8 mM v HIGH 7.9 mM; p = 0.13) and Prolactin (LOW 35.8 ng.mL−1 v HIGH 36.0 ng.mL−1; p = 0.93) were similar. CONCLUSIONS Although exercise-induced dehydration (∼2 kg) reduced the absolute power output required to ride uphill, this loss in body water in a warm environment impairs thermoregulation which appears to be detrimental to cycling hill-climb performance in the laboratory. Future research is required to establish whether these findings are relevant for cyclists competing in the field. Supported by the Australian Sports Commission, Flinders University and Cycling Australia