Expired gas data collected during the study are presented in Table 1. Oxygen uptake increased over time during exercise in each trial (P < 0.001). The ingestion of the experimental solutions seemed to influence this response, with mean V˙O2 during exercise increasing with a higher concentration of CHO ingested, but this marginally failed to reach significance (P = 0.056). RER was not different between trials (P = 0.294), but there was a significant reduction in RER as exercise progressed (P < 0.001). This response was greater during the 0% trial than during the trials where CHO was ingested. Rates of CHO (P = 0.244) and fat (P = 0.136) oxidation during exercise were not influenced by the beverage ingested.
Study 2: warm (30°C)
There was a significant difference between trials in TTE (P = 0.043; Fig. 1B). Further analysis of the performance data was undertaken by calculating the difference between the times recorded during each CHO trial and that attained during the 0% trial. This was 9.6 ± 16.0 min (P = 0.643, ES = 0.48) in the 2% trial, 10.9 ± 12.1 min (P = 0.188, ES = 0.41) in the 4% trial, and 17.6 ± 21.3 min (P = 0.045, ES = 0.62) in the 6% trial. There were no statistical differences in TTE apparent when comparisons were made between pairs of trials where CHO was ingested (all P > 0.05).
There was a tendency for greater total sweat losses during exercise with higher drink CHO content (P = 0.072), but there was no difference in the calculated sweat rate (0% = 1.51 ± 0.32 L·h−1, 2% = 1.46 ± 0.31 L·h−1, 4% = 1.46 ± 0.27 L·h−1, 6% = 1.46 ± 0.32 L·h−1; P = 0.496). There was a clear elevation in HR during exercise (P < 0.001), but the beverage ingested did not alter this response (P = 0.464; Fig. 2B). There was no difference in core temperature between trials before exercise (Fig. 3B). Although the CHO content of the drink ingested did not influence the core temperature response (P = 0.516), there was a progressive increase during exercise in all trials (P < 0.001), reaching 38.4°C ± 0.5°C, 38.8°C ± 0.7°C, 38.4°C ± 0.7°C, and 38.5°C ± 0.7°C in the 0%, 2%, 4%, and 6% trials, respectively. Although there was a significant increase in RPE and perceived thermal stress as exercise progressed observed during all trials (P < 0.001), neither perceived exertion (P = 0.802) nor thermal stress (P = 0.356) was influenced by the drink ingested.
Expired gas data collected during the warm study are presented in Table 2. Oxygen uptake increased during exercise in each trial (P < 0.001) but was not influenced by the CHO content of the drink ingested (P = 0.109). RER was not different between trials (P = 0.226), but there was a reduction in RER as exercise progressed in all trials (P = 0.034). Rates of CHO (P = 0.105) and fat (P = 0.339) oxidation during exercise were not influenced by the drink ingested.
Most commercially available sports drinks have a CHO concentration of around 6%–8%. The formation of these beverages has been based on an abundance of published data supporting the benefits of CHO on exercise performance. However, evidence suggests that more dilute beverages empty rapidly from the stomach (40) and may promote better water absorption (26,38). Consequently, a lower CHO concentration may be just as effective in enhancing exercise performance in some situations. The aim of the present study was to systematically investigate the effects of solutions containing between 0% and 6% CHO on exercise performance in cool and warm environments.
Cool study (10°C)
During prolonged exercise in temperate conditions lasting longer than 90 min, fatigue is characterized by low muscle glycogen availability and a fall in circulating blood glucose concentrations (12). The ingestion of exogenous CHO before and during exercise has been demonstrated to prolong exercise TTE (8,9,13,19,28,31,39) through maintenance of circulating blood glucose levels and to increase the rate of CHO use late in exercise. The results of the present study demonstrate a significant improvement over the 0% trial when the 4% and 6% CHO solutions were ingested. The difference in exercise capacity between the 0% and 6% trials was 21.5 ± 24.2 min, representing a 19.3% improvement in TTE. As highlighted previously, many studies have reported similar findings when comparing a placebo to 6%–8% CHO solutions, but the performance effects of more dilute CHO solutions are not well defined. The 4% CHO solution also resulted in an enhanced exercise capacity over that observed during the 0% trials (+18.4 min, P = 0.032). The ingestion of the 2% CHO solution resulted in a mean increase in exercise capacity of 6.6 min over the 0% trial; the variability in this response meant this was not statistically significant (P = 0.427, ES = 0.20). Although there was a difference in TTE between the 2% and 6% trials (P = 0.025, ES = 0.45), no further statistical differences were apparent between the CHO trials, suggesting that when exercising in cool conditions, a 4% CHO solution may be just as effective at maintaining performance as a more concentrated 6% solution. Significant improvements in the performance of a preloaded time trial after the ingestion of relatively small quantities of glucose (15, 30, and 60 g·h−1) compared with a CHO-free placebo have also been recently reported (39).
No differences were observed in the estimated rates of CHO and fat oxidation. CHO ingestion is widely reported to enhance overall CHO oxidation rate, while also suppressing lipolysis and consequently attenuating the rate of fat oxidation (12,27). Several other investigators have reported the absence of differences in rates of CHO and fat oxidation with ingestion of 6% and 8% CHO solutions (31,34). The difference between studies may be explained by the quantities of CHO administered, with the absence of response potentially related to the relatively small amount of CHO ingested in the present study. Studies reporting a marked effect of CHO ingestion on rates of CHO and fat oxidation have typically administered 60–75 g CHO·h−1 (10), whereas subjects in the present study ingested around 14, 29, or 43 g CHO·h−1 during the 2%, 4%, and 6% trials, respectively (assuming the ingestion of ∼120 mL every 10 min for an 80-kg subject). This view is supported by recent data demonstrating no change in total CHO oxidation after the ingestion of glucose at a rate of 30 g·h−1, but a significant increase compared with a placebo control was apparent when 60 g CHO·h−1 was ingested (39). These data also suggest that higher rates of CHO ingestion result in a reduction in the use of endogenous glucose from the liver, with no change in muscle glycogen use. Maximal rates of exogenous glucose oxidation during exercise are typically reported at 1.0–1.1 g·min−1, whereas sugars requiring hepatic metabolism before muscle use (e.g., fructose and galactose) seem to be limited to around 0.4–0.7 g·min−1 (27). The delivery of ingested CHO to the muscle seems to be a limiting factor in determining exogenous CHO oxidation rates. Recent evidence suggests that the inclusion of multiple transportable substrates, as used in the present study, can increase the rate of CHO delivery to the circulation and may enhance oxidation rates beyond 1.0–1.1 g·min−1 (22).
There was no significant effect of the treatment on the subjects’ thermoregulatory response to the exercise: no differences between trials were apparent in core temperature, although there was a tendency for perceived thermal stress to be higher throughout the 6% trial than throughout the other trials. Similar responses have been reported by Galloway and Maughan (18) after the ingestion of CHO solutions during exercise.
Warm study (30°C)
The capacity to perform prolonged exercise is reduced in a warm environment (17,35), but the underlying mechanisms behind this response are still poorly understood. Potential peripheral mechanisms include impaired substrate availability or use, accumulation of metabolic waste products, or the loss of body fluids, but these do not adequately explain this reduction in performance, leading to the suggestion that the CNS may be important (30). Despite evidence suggesting that substrate availability is not limiting, several studies have reported improvements in performance after the ingestion of a high-CHO diet (37) and the use of CHO supplements before and during exercise (2,5,18,32). Certainly, the ingested fluid attenuates the increase in core temperature and HR typically observed during exercise in the heat (11,20,33). Consequently, a solution that can rapidly deliver fluid to the circulation is likely to be more beneficial than a concentrated CHO-containing fluid during prolonged exercise in the heat (18).
This line of thought is supported by data presented by Febbraio et al. (15) demonstrating no difference in exercise TTE compared with a control condition when 4.2%, 7%, and 14% CHO solutions were ingested during exercise undertaken at 33°C. There was a tendency for exercise capacity to be reduced at the highest drink CHO concentration (14%), and this was attributed by the authors to a greater reduction in plasma volume and a faster rate of core temperature rise resulting from a slower rate of gastric emptying and intestinal absorption. Perhaps contrary to these findings, the present study reported longer exercise times with increasing CHO concentration (Fig. 1B), although this effect reached statistical significance only when comparing the 0% and 6% trials. In fact, ingestion of the 6% CHO solution resulted in an 18.6% improvement in exercise capacity over the 0% trial, which is strikingly similar to the difference between these two trials observed in the cool study. It is worth noting that the highest beverage CHO concentration (6%) used in the present study is lower than the 14%–15% used in previous reports (15,18).
Several studies have reported an increase in total CHO oxidation when exercise is performed in warm conditions when compared with exercise in a temperate environment (15,16,22). This response seems to be caused by increased rates of muscle glycogen oxidation (+25%) and is accompanied by a reduction in exogenous CHO usage (22). Although the rate muscle glycogen use is increased during exercise undertaken in warm conditions (16), relatively high muscle glycogen concentrations have been reported at the point of fatigue (35). Despite the apparent reduction in exogenous CHO oxidation rates, the present study and several previous publications have reported a benefit of ingesting CHO solutions before and during prolonged exercise in a warm environment (2,5,32). In the absence of clear metabolic differences between trials, it is possible that these effects of CHO may reflect a central effect of CHO ingestion, mediated through CHO-sensing receptors found in the mouth (6) or elsewhere.
In a similar manner to the findings of the cool study, the ingestion of CHO did not produce a measurable change to the physiological response to exercise in the heat; there were no detectable differences in HR, core temperature, or rates of substrate use. There was a tendency for the sweat losses incurred during exercise to be higher with the increasing CHO content of the drink, but this was caused by the increased exercise duration; the calculated sweat rate was not different between trials (P = 0.688). Fatigue during prolonged exercise in the heat has been attributed to the attainment of a critical core temperature, resulting in a loss of drive and motivation to continue exercise. This has been suggested to act as a safety mechanism, limiting heat production and preserving the integrity of the organism in the presence of heat stress. It seems that this view is perhaps overly simplistic, with feedback to the CNS arising from several sources including skin temperature receptors, HR, and central venous pressure resulting in a deterioration of CNS drive with exercise-induced hyperthermia. A key role for skin temperature in influencing performance in the heat has been proposed (7). Weighted mean skin temperature was not monitored in the present study, but it seems unlikely that any differences between trials would have been apparent.
The validity, reliability, and sensitivity of tests commonly used as a measure of exercise performance have generated considerable controversy in recent years. Constant-power tests to volitional exhaustion have been used to examine the influence of various interventions on performance, but this method of testing has been criticized for a lack of ecological validity and poor test–retest reliability. This view is supported by the findings of Jeukendrup et al. (24), who found a large day-to-day variability (coefficient of variation = 27%) in TTE tests and a much smaller variability in a time trial protocol (<4%). Although some continue to voice concerns over a lack of ecological validity, data from our research group report more consistent performance in time-to-fatigue tests (28) (coefficient of variation = 6%), and recent reports have highlighted similar errors of measurement when changes in performance are normalized across tests (21). A key factor to consider when selecting an appropriate exercise test is its sensitivity and the smallest worthwhile effect that can be reliably detected (14). Amann et al. (1) demonstrated that TTE and time trial protocols display a similar sensitivity to the effects of hypoxia and hyperoxia on performance and suggest that this finding will extend to other factors influencing performance. This is brought about by larger effects on performance in response to an intervention with constant power tests than are typically observed in time trial protocols; this compensates for the larger test–retest variability, resulting in a very similar signal-to-noise ratio to that seen with time trial protocols (1,14). In some research situations, the obvious limitation of a time trial test is a difficulty in comparing the effect of an intervention on the physiological response to exercise because at any given time, one volunteer’s relative workload may vary greatly from that of other participants. However, this can be overcome through the addition of a period of constant-load exercise undertaken before the time trial, as described by Jeukendrup et al. (24).
In conclusion, compared with a CHO-free placebo, ingestion of solutions containing 4% and 6% CHO improved exercise performance in a cool environment, whereas a 6% CHO beverage resulted in a significant increase in TTE at 30°C. Discussion of the responses observed in the cool and warm studies has been kept largely separate because of the use of two groups of volunteers and different exercise intensities, but some clear comparisons can be drawn. In particular, the magnitude of change observed between the 0% and 6% trials in each environmental condition was remarkably similar (∼19%), given the apparently different mechanisms of fatigue thought to be in operation. Despite the differences in exercise performance, there were few alterations to the physiological responses to the exercise in both environmental conditions. This is perhaps due to the relatively small quantity of CHO ingested, compared with some previous investigations.
The authors thank David Ferguson, Astin Ewington, Floor van Langen, and John Quigley for their technical assistance during this investigation.
The study was carried out in relation to the product Powerade and was funded in part by the Coca-Cola Company.
The authors report no conflicts of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2012The American College of Sports Medicine
PROLONGED EXERCISE; TIME TO EXHAUSTION; HEAT STRESS; SPORTS DRINKS