Double blind carbohydrate ingestion does not improve exercise duration in warm humid conditions

Summary

The positive effects of carbohydrate (CHO) supplementation on endurance exercise are well documented but the placebo (PLA_c) effect can make the ergogenic qualities of substances more difficult to determine. Therefore, this study tested the effect of double blind ingestion of PLA_c and CHO_c in capsules versus known capsule (CHO_k) ingestion on prolonged exercise heat stress. Nine well trained male volunteers (mean ± S.D.: 23 ± 3 years; 62.4 ± 6.5 kg and 65.8 ± 5.2 mL kg⁻¹ min⁻¹ peak oxygen consumption) exercised at 60% of maximum power output until volitional exhaustion (TTE) in the three different conditions. Capsules were ingested with 252 ± 39 mL of water. Blood glucose in CHO_c and CHO_k was similar but higher (p < 0.05) than PLA_c from 45 min to end of exercise. There were no differences in TTE between PLA_c (125.2 ± 37.1 min) or CHO_c (138.8 ± 47.0 min) or between CHO_c and CHO_k (155.8 ± 54.2 min). Time to volitional exhaustion was different between PLA_c and CHO_k (p < 0.05). Increased TTE resulted when participants and researchers knew the capsule content, but not in the double blind condition. The difference could be related to a combined effect of CHO ingestion and knowledge of what was ingested possibly acting as a potent psychological motivator.

Introduction

The positive effects of carbohydrate (CHO) supplementation on exercise duration are well documented. Most of these studies have predominantly used beverages ranging from 2 to 7% CHO.1, 2, 3, 4 Conversely, other studies suggest that CHO supplementation is not as effective as previously thought and exercise performance is not enhanced.5, 6, 7 The mechanism/mechanisms by which carbohydrate ingestion might improve exercise performance remains debatable partly because of the various environmental conditions, as well as a variety of experimental designs and placebo (PLA) controls. For instance, some studies have focused on warm environments although controversy regarding the benefits of ingestion of CHO in these conditions still exists.⁸

Although some researchers have questioned the methods used in CHO studies with respect to the control of the placebo group,9, 10 others have recently confirmed the placebo effect as a potent factor that can have a significant effect on performance or on the sense of effort.¹¹ Therefore, it would be prudent to account for this possibility when studying the ingestion of carbohydrates as the expectations of participants could determine the intrinsic feedback as shown in the classic study of Arile and Saville.¹² As researchers are interested in distinguishing between the effect and the real responses of substances being tested, the placebo effect can make the ergogenic qualities of different substances more difficult to determine. For example, Clark et al.¹⁰ showed an improved performance when participants believed they were ingesting CHO but were actually consuming a PLA. In this particular study, 43 participants completed 2 × 40 km time trials; one trial to establish baseline performance and a second trial where the participants ingested a CHO or a PLA fluid. These two groups were further subdivided into three groups where participants either knew they were ingesting CHO or they knew they were ingesting a PLA or to a group not knowing either. Interestingly, the group that ingested PLA but was told to be having CHO had a better performance than the group that was told and actually had ingested CHO.

In a further attempt to examine the effect of CHO versus a PLA beverage, Timmons et al.¹³ using gelatin capsules for the ingestion of CHO, exercised athletes at 100% $V_{{\text{O}}_{2\text{peak}}}$. These authors did not observe an enhancement in anaerobic performance, although exercise duration was ∼105.4 s in the CHO trial compared to ∼103.2 s in the PLA trial. Furthermore, in a study where glucose (CHO) and saline (PLA) infusions were used to asses 1 h cycle time-trial performance in a thermoneutral environment, no improvement in performance was reported.¹⁴ This group¹⁵ also investigated the possible effect of CHO receptors in the mouth on performance, and concluded that an enhancement in performance is more related to an increase in central drive or motivation than a metabolic effect. This study shows the importance in developing methods that do not allow participants to recognize what is being administered since four of the nine participants in this study detected they were ingesting CHO, and of these four, three performed better when ingesting CHO. Therefore, infusion and capsules might be a better method to verify the real effects of a substance and discriminate between the effect of CHO and PLA on exercise performance.

The purpose of this study was to examine the effect of ingesting capsules as either 6% CHO or PLA with the added effect of participants knowing in advance what they were ingesting. A further aim was to evaluate the CHO–PLA effect on exercise duration in heat stress conditions given that controversy still exists as to the role of CHO ingestion in these conditions.