The effect of carbohydrate mouth rinse on performance, biochemical and psychophysiological variables during a cycling time trial: a crossover randomized trial

The present study investigated the effect of CMR on biochemical and psychophysiological variables of cyclists in a fed state. There was no significant improvement of performance or change in metabolic response after CMR compared to the ad libitum strategy and the placebo. However, there was an attenuation on psychophysical responses after mouth rinse interventions (PMR and CMR) compared to ad libitum intake. Ad libitum water ingestion also led to higher blood lactate concentrations during the trial.

Recent studies indicate that the nutritional status prior to interventions is one of the factors determining improvement in cycling performance. Several studies report their benefit in 4 h fasting situations [12, 1618, 29]. One possible explanation is that fasting can increase muscle metabolism in trained cyclists [32], and carbohydrates mouth rinse may enhance exercise intensity and promote more rapid muscular adaptations [33]. Nevertheless, the practice of competing while fasting is not common among athletes, and the relevance of this strategy is questionable [23]. In our study, the time trial was performed around 2 h after participants’ last meal, and there was no statistical change in performance, considering time for completion of test, heart rate, power and average speed. Similarly, researchers found no effect on performance when carbohydrate mouth rinse was used by athletes in a fed state [2123]. It is important to note that athletes’ eating patterns did not vary during interventions on different days, which ensures that changes in the variables can be attributed to the strategies tested.

Regarding psychophysiological responses, no significant difference was observed between CMR and PMR, but both presented difference from ad libitum intake. RPE was higher in last 7.5 km and affective response was lower at 22.5 km in the DAL strategy. These results corroborates the higher lactate level found in the ad libitum intake. In contemporary theoretical models of fatigue, perceived exertion appears as an important indicator in the regulation of physical performance [34], which is used to monitor and to evaluate exercise tolerance and the level of effort, associating with the physiological stress [35]. The central governor theory proposes the existence of a conscious regulation in order to ensure the maintenance of the physiological homeostasis and prevent catastrophic changes. In this sense, previous study has found an association between increased blood lactate concentration and increased perceived exertion, suggesting a possible increment in muscle activity during increasing intensity of exercise [36]. Thus, a raise of perceived exertion may modulate performance by means of self-determined fluctuations in external workload, in order to prevent an unsustainable effort that would be deleterious to performance [34, 37, 38]. In our study is possible that participants under ad libitum intervention may have made a greater effort to maintain the same workload [12, 39], since no significant changes were found in power and performance of these individuals.

Previous studies did not find differences in perceptual responses between CMR and PMR interventions [12, 17, 23]. Backhouse et al. observed lower values of RPE when subjects ingested a carbohydrate solution compared to a placebo solution, but they did not report attenuation of perceptual responses when the solution is not ingested [40]. In our study, the use of artificial sweetener in order to reduce the difference of taste among solutions may have influenced the results, since the interaction of such sweeteners with taste receptors could mask the effect of carbohydrate solution [41, 42].

Cramer et al. believed that CMR would attenuate perceptual responses in the heat, however, they concluded that the high temperature may have counteracted any possible ergogenic effect of this strategy, since in these situations the high thermal and cardiovascular strain contribute more to performance than the presence of carbohydrates [43]. Studies previous have shown that the hydration status is not the main factor influencing physical performance and causing perceptual changes in this type of exercise [9, 10, 38, 44, 45]. In our study, subjects did not reach dehydration in any of the interventions, and fluid ingestion or rinsing did not influence the results. Goulet et al. suggested that exercise intensity and duration have a greater impact on performance [44].

In the pioneering study of Carter et al., suggested that CMR strategy could increase blood glucose and insulin concentrations, improving performance through enhanced glucose uptake into the active muscles and maintaining carbohydrate oxidation rates compared with water rinsing [12]. This hypothesis is based on the parasympathetic reflex and the cephalic phase of insulin release that are triggered by contact of sweet substances with taste receptors. However, the activation of theses mechanisms without the use of sweet substances has not been described [46]. Recently, Murray et al. [47] observed that CMR improved performance in 40 km cycling time-trial without altering plasma insulin concentration. Plasma insulin was collected at 5 km intervals throughout the first 25 km, and glucose samples were collected at 5 km intervals throughout the exercise bout. No change in plasma insulin was detected between conditions (p = 0.638, ES < 0.03). In our study, the carbohydrate used for mouth rinsing was a maltodextrin without flavor solution that was compared to rinsing with a placebo and ad libitum water intake, but no statistical differences were observed in glucose and insulin concentrations. Other studies also did not show significant changes in glucose concentrations without ingestion of carbohydrate solutions [17, 21, 29, 41].

Ali et al. compared strategies of mouth rinse and ingestion with carbohydrate and placebo, and they observed an increase in blood glucose, lactate and insulin concentrations only after carbohydrate intake [33]. Our study also suggested a lack of changes in metabolic responses under CMR, since we did not observe significant changes in plasma cortisol levels after exercise under the influence of all interventions. Bakhouse et al. reported a decrease in cortisol levels when carbohydrate was ingested, compared to placebo intake [40]. There are no other studies evaluating the effect of CMR on cortisol or other glucoregulatory hormones. Similarly, blood CK concentrations did not differ among interventions. These results support the hypothesis that the mechanism responsible for performance improvement observed in some studies is probably related to the central nervous system [2, 14, 18, 19]. Like most studies in the literature that evaluate the response of glucoregulatory hormones to exercise, we did not control plasma shifts accounted. In fact, we do not know whether these hormones are altered by possible plasma volume alterations induced exercise, so we suggest that future studies control this variable.

It is important to emphasize that the inclusion of the DAL strategy is a novelty of our study, comparing the ergogenic effects of CMR with the traditional fluid intake recommendations for this type of exercise [911]. Most studies compared CMR to placebo or water rinsing. Although we have not used scales of thirst sensation and gastrointestinal discomfort, participants reported that they preferred trials without rinsing, since they did not have the habit of using it. The sensation of thirst was expressed more after rinsing interventions, and there was no report of gastrointestinal discomfort in any of the interventions.