The primary objective of this study was to investigate the effects of a hyperhydration strategy on running economy in cool and hot conditions. The hyperhydration regimen consisted of a combination of Creatine and glycerol which previously has been shown to each have a hyperhydrating effect and if applied together this effect is additive. Seven well-trained athletes were invited to participate in this study which included a pre- and post-supplementation experimental trial consisting of running at an intensity of 60% VO2max for 30 minutes in moderate (Ta = 10.5 ± 0.2 °C; RH = 72.0 ± 1.1%) and another 30 minutes in hot (Ta = 34.72 ± 0.19 °C and RH = 71.5 ± 0.9%) conditions with a 30 min break in between during which subjects were given water to replace any fluid lost in the first part of the trial. Between the last two experiments subjects followed a seven-day supplementation regime, i.e. consuming a daily dose of 11.4 g of Cr∙H2O (equivalent to 10g of Creatine) and one dose of glycerol (1g•kg-1 BM) on the day of the final experiment. The measurements of total body water (TBW) indicated that not all subjects responded to the hyperhydration regimen. Hence, only the subjects considered responders were taken into analysis (n=7). In this group of subjects we found an increase of 0.8 ± .0.3 kg in BM, 0.64 ± 0.18 L in TBW of which 0.20 ± 0.12 L could be assigned to an increase in extracellular water (ECW), and 0.44 ± 0.09 L were due to an increase in intracellular water (ICW). No significant changes could be observed in thermoregulatory (core body temperature), cardiovascular (heart rate) or pulmonary responses (O2 consumption, CO2 production, minute ventilation, respiratory exchange ratio), and rating of perceived exertion. Nevertheless, in the trial after the supplementation the subjective perception of heat, i.e. thermal comfort, was significantly reduced after 5 and 10 minutes in the bout at 35°C. As no change in VO2 could be observed, we conclude that there is no negative impact of a slightly increased body weight on oxygen consumption. Compared to previous studies, this study could not achieve a hyperhydration sufficient enough to observe positive thermoregulatory and cardiovascular responses as reported by other studies investigating water pre-loading. These differences are possibly due to a change in the administration protocol of glycerol
