Glycerol hyperhydration fails to improve endurance performance and thermoregulation in humans in a warm humid environment

It is equivocal whether glycerol hyperhydration improves exercise performance and thermoregulation in the heat. The purpose of this study was to compare the effectiveness of glycerol with water hyperhydration, using a reliable, self-paced variable-intensity cycling protocol under hot, humid conditions. Seven moderately-to-well trained subjects ingested either a solution consisting of 1.2 g kg -1 body mass (BM) glycerol mixed with 21 ml kg -1 BM flavoured water (GLY) or placebo (PL), which was flavoured water of equal volume to the GLY trial, 2.5 h before exercise. Following hyperhydration, subjects undertook a self-paced, variable-intensity cycling protocol designed to simulate racing, with the aim being to cycle as great a distance as possible over 60 min. There were no differences in total distance cycled between conditions (29.7±5.7 km for PL, 28.9±5.7 km for GLY). Power output was not different at any time between conditions. Terminal rectal temperatures were 39.0±0.5 °C for PL and 38.8±0.7 °C for GLY and were not significantly different. Heart rate was significantly higher for GLY only during the high-intensity efforts. The sweat rate for GLY was 1.72±0.28 l h -1 ( P<0.01) compared with 1.15±0.29 l h -1 for PL. It is concluded that glycerol hyperhydration has no significant advantage over water hyperhydration on performance or thermoregulation during a 1-h, variable-intensity exercise performance.