The present study examined the effect of mouth rinsing with a carbohydrate solution on executive function after sustained moderately high-intensity exercise. The main result of the present study was that the reaction time significantly increased after exercise in CON but not in CHO, which resulted in a significant interaction (condition × time) on the reaction time in the incongruent Stroop test. This finding indicates that mouth rinsing with a carbohydrate solution attenuated the expected decline in executive function due to the sustained moderately high-intensity exercise, which supports our hypothesis.
An increased reaction time was observed after exercise in CON, but no such increase was observed in CHO. This result seems to indicate that mouth rinsing with the carbohydrate solution attenuated the exercise-induced decline in executive function. Mouth rinsing with glucose has been reported to improve the reaction time in incongruent trials in a resting condition . Such a positive effect on the reaction time seems to have helped minimize the increase in reaction time after sustained moderately high-intensity exercise in CHO, preserving executive function.
The concentrations of plasma epinephrine and NE tended to be different between mouth-rinsing conditions, with a higher value in CON than in CHO after exercise. Such changes in stress hormones may be a possible reason for the positive effect of mouth rinsing with a carbohydrate solution on the decline in executive function. The Pearson correlation coefficient revealed a significant correlation in the changes between the plasma NE level and the reaction time in CON (r = 0.874, P = 0.010). The plasma epinephrine and NE directly affect synthesis and release of NE in the brain through the vagus nerve, and high levels of NE release are presumed to disrupt executive function [8, 33, 34]. The plasma NE level observed in the present study was comparable to the level reported in a previous study, which demonstrated that cognitive function was impaired after prolonged cycling at ~66% V̇O2max . Therefore, executive function seems to be impaired by the exercise-induced increase in the epinephrine and NE levels in CON, and the preservation of executive function in CHO may be attributed, at least in part, to the inhibition of the excessive release of such stress hormones.
Mouth rinsing with a carbohydrate solution did not change the subjective perceptions of fatigue, arousal, and pleasure, indicating no significant interactions and no main effects of condition, although there were significant main effects of time. In a previous study, glucose intake enhanced executive function in comparison with the effects of an inert sucralose placebo, without differences in perceived fatigue and time availability, and in the number of perceived mistakes between conditions . Therefore, the glucose-induced enhancement in executive function occurs separately from the subjective perceptions of fatigue, arousal, and pleasure during exercise.
The positive effect on executive function of mouth rinsing with a carbohydrate solution seems to be associated with a nonmetabolic pathway and to be unrelated to the glucose metabolic pathway. The blood glucose concentration showed neither significant interactions nor main effects of condition; however, a significant main effect of condition on the RPE was observed in this study. This indicates that the perceived exertion was reduced through a nonmetabolic pathway. A previous study demonstrated that mouth rinsing with a carbohydrate solution improved running performance because carbohydrate lowered the perception of effort at a given workload, as a result of activation of the reward center of the brain . In support of this finding, it has been reported that the potential effect of mouth rinsing with carbohydrate is activation of reward-related brain regions, which include the anterior cingulate cortex and the striatum, and the dorsolateral prefrontal cortex . The dorsolateral prefrontal cortex play a critical role during the incongruent condition of the Stroop test . Thus, the positive effect of mouth rinsing with a carbohydrate solution on executive function could be particularly associated with reward-related brain regions. Furthermore, there is a possibility that the vagus nerve contributed to the preservation of executive function as another nonenergetic pathway . Oral exposure to carbohydrate was found to increase heart rate variability, which was associated with prefrontal cortical activity through the vagus nerve, and persons with greater heart rate variability were observed to perform better on tasks related to the executive function . Hence, the results of the present study suggest that non-energetic mechanisms such as the motivational reward pathways and the vagus nerve could possibly contribute to the preservation of executive function without increasing the metabolic energy level. Future studies are needed to determine whether nonmetabolic pathways would be associated with attenuation of the decline in executive function after sustained moderately high-intensity exercise.
One limitation of this study is its small sample size. A loss of significant interaction and main effects on some of the outcome measures, such as blood hormone concentrations and subjective perceptions, might have been due to the small sample size. However, the sample size of this study was determined from the preliminary results of the changes in the reaction time of the incongruent Stroop Color and Word Test after sustained moderately high-intensity exercise, which was the primary outcome measure of this study. On the basis of the means and variances of those data, seven participants were needed to detect changes in the reaction time to achieve a power of (1 – β) = 0.90 at P < 0.05. Thus, the sample size seems to be enough to detect changes in the reaction time. Four male and four female participants were included in the present study. Although some previous studies examined the effects of exercise or mouth rinsing with a carbohydrate solution on cognitive performance [12, 22, 38, 39] in the mixed gender samples, it remains unclear whether the effect of carbohydrate solution mouth rinsing on executive function in exercise varies according to gender. The performance of cognitive tasks could have been affected by the phases of menstrual cycle of the female subjects. However, the experiment was conducted during the follicular phase of their menstrual cycles; thus, the influence of the menstrual cycle, if at all, should be minimal. Further studies would be needed to elucidate gender differences in the effect of the carbohydrate solution mouth rinsing on executive function since hormonal responses could be different between males and females.