The purpose of the current study was to investigate the effectiveness of combining glycerol hyperhydration and a practical precooling strategy on performance during a cycling time trial that simulated a real-life event in hot and humid environmental conditions. The main findings of this study were that: i) a hyperhydration strategy, with or without the addition of glycerol, in addition to an established precooling technique, failed to achieve a clear enhancement of cycling time trial performance in hot humid conditions, ii) the ingestion of a large volume of chilled (4°C) fluid prior to the time trial (CON) induced a clear and sustained large reduction in body temperature, and iii) when precooling, involving the application of iced towels and the ingestion of a slushie, was performed after consumption of a hyperhydration solution without, but not with glycerol, a further “small” reduction in deep body temperature, reduced perceived exertion and improved performance on the second half of the time trial (i.e., climb 2) occurred.
Our original hypotheses were that our precooling strategy would result in lower body temperatures compared with the control condition and the prior ingestion of a hyperhydration strategy would be further enhanced with the addition of glycerol. While glycerol hyperhydration resulted in an increased fluid balance of ~330 ml (10%) and the precooling technique caused a further small to moderate reduction in deep body temperature, together these alterations did not lead to a clear improvement in overall performance. In fact, on further inspection of performance data, a possible (49% chance) performance benefit (2%) was observed on climb 2 following hyperhydration, without glycerol, plus precooling (PC intervention) over the control trial. This improved performance was associated with subjects reporting a lower perception of effort over the first 10 km of the time trial (2.5 km short of the top of the climb), despite similar pacing strategies and physiological perturbations (i.e., rectal temperature, heart rate, thermal comfort and stomach fullness) across all trials. As such, it appears that benefits associated with hyperhydration plus precooling offered some advantage in attenuating the perception of effort during the initial portion of the trial, allowing for improved performance in the later stages of the trial when thermal load was greatest. These results may be partially explained by the pre-trial brief, in which subjects were instructed “if feeling good, to save the big effort for the second lap”.
Despite lower core body temperature and improved thermal comfort as a result of precooling and hyperhydration with the co-ingestion of glycerol, performance was not significantly different to the control trial over any section of the course. Moreover, although subjects received the same precooling intervention, the magnitude of cooling was greater in the PC+G trial compared with the PC trial (a moderate versus small reduction in rectal temperature, respectively). We are unable to provide a clear explanation into the potential mechanism of this enhanced effect. However, the differences in performance among trials in the present study, despite differing core body temperatures are commensurate with those from our previous (unpublished) observations, whereby a greater reduction in rectal temperature did not lead to greater performance effects. These results thus provide further data to refute the existence of a direct relationship between magnitude of cooling and the functional outcome [8, 35]. In fact, we may have induced a magnitude of cooling that surpassed a threshold temperature, in which performance may be impaired during self-paced endurance exercise, however this currently remains speculative.
While results of the present study may indicate that the precooling and hyperhydration interventions used are ineffective in enhancing real life sporting performance, an unexpected finding from this study was that the ingestion of the pre-event fluid in the control trial, also induced a clear and sustained large reduction in body temperature. A chilled beverage was selected as the control condition for hyperhydrating subjects to mask the flavor characteristics of the glycerol in the sports drink in PC+G trial, to standardize total fluid intake, and to simulate the conditions of a real-life event. Indeed, when performing in hot and humid conditions, participants are usually exposed to the environmental conditions for more than 2 hr prior to the event and in most circumstances would preferentially ingest a cool beverage. It is possible that the large reduction in rectal temperature observed in the control trial may have provided a benefit to performance and thus reduced the likelihood of observing clear physiological or performance effects. Indeed, this protocol and magnitude of cooling observed is similar to studies that have shown improvements in endurance capacity following cold fluid ingestion precooling [36–38]. These studies used ~20.5 to 22.5 ml.kg-1 fluid served at 4°C in the 90 min before  and/or during [37, 38] an exercise task performed in hot and humid conditions. Interestingly, we observed a sustained cooling effect with mean baseline rectal temperature (t=−65 pre time trial) remaining below pre-hydration levels, despite subjects being exposed to the hot and humid conditions for ~60 min following consumption. Although we cannot determine whether the reduction in core body temperature improved performance in the present study, we have previously shown that the same precooling strategy resulted in a 3% increase in average cycling power output of similar calibre cyclists over the same course , when compared to a control trial without any fluid intake. Collectively these results indicate that hyperhydration with or without glycerol, plus precooling through the application of iced towels and the ingestion of a slushie, may provide minimal performance benefit, over the ingestion of a large cool beverage.
Although the focus of precooling was the optimization of thermoregulation, we acknowledge the composition of the slushie, in the current study, provided additional fluid and carbohydrate; nutritional components that may also enhance performance. However, as we have previously discussed [11, 12], it is unlikely that performance of our cycling protocol would be influenced by providing euhydrated subjects with further fluid or having greater carbohydrate availability associated with this strategy, at least within the limits of detection of our protocol and under the control conditions of nutritional preparation (i.e., following a carbohydrate rich mean, well hydrated). Furthermore, this study design was representative of real-life circumstances, whereby cyclists simply added the precooling strategy to a hyperhydration strategy.
In summary, the current study does not support the hypothesis that hyperhydration, with or without the addition of glycerol, plus an established precooling strategy is superior to hyperhydration, in reducing thermoregulatory strain and improving exercise performance. Despite increasing fluid intake and reducing core body temperature, hyperhydration plus precooling failed to improve performance when compared with the consumption of a large cool beverage prior to the trial. These results indicate that a combined precooling technique (i.e., ice towel application and slushie ingestion) results in minimal performance benefit over and above the typical real-life pre-race preparations (i.e., consumption of a cold fluid). Further research is warranted in order to examine the influence of fluid temperature and volume on the success of glycerol hyperhydration and precooling strategies, presumably because the control condition, chosen to standardize total fluid intake, also involved a substantial precooling effect. Specifically, further studies could be undertaken to compare glycerol hyperhydration using a tepid beverage to distinguish the effects of this strategy on fluid status from its thermoregulatory impact and allow separation of the different elements that may underpin a performance change.