Amid the research of nutritional practices to enhance exercise performance, the ingestion of carbohydrates (CHO) has arguably the most support. There is a wealth of research investigating the effects of consuming CHO before, during, and after exercise to support both performance and recovery, with a number of narrative reviews available (6,8,23,25,38). Typically, the ergogenic benefit afforded by CHO is stronger when exercise protocols are ≥2 hours in length, supposedly due to the metabolic demand of exercising for this duration. It has also been shown to help performance of shorter protocols of approximately 1 hour when intensity is sufficiently high (26,41). However, some have demonstrated that it can enhance performance in protocols of much shorter durations when metabolic demand is likely to be met by endogenous CHO stores, and therefore may not necessarily warrant exogenous feeding of CHO. In response to this, several authors, Carter et al. (7) being the first, have provided evidence that simply rinsing the mouth with CHO (carbohydrate mouth rinsing [CMR]) without ingesting it can influence performance. As the CHO is not ingested, it is not possible that it supports the endogenous stores of CHO, and it has been proposed that oral receptors in the mouth may modulate central nervous system responses. Findings from Chambers et al. (9) support this theory as they reported rinsing the mouth with glucose and maltodextrin separately stimulated areas of the brain associated with motor output.
Although this nutritional strategy seems promising, and may be of interest to those who struggle with the possible gastrointestinal discomfort associated with the ingestion of CHO (8), the literature to support it is still in its relative infancy. De Ataide e Silva et al. (13) performed a systematic review of research available up to May 2013 and concluded that mouth rinsing with CHO “seems to improve performance,” reporting an average improvement of 5.05 W (95% confidence interval [CI] = 0.90–9.20). However, although such reviews are useful as they combine and synthesize findings from a number of different papers, and it is very difficult to consider all factors in the analysis, this particular review suffers from some limitations. De Ataide e Silva et al. (13) only quantified the findings from studies where power output was the main performance outcome despite the fact that studies report a number of variables including time to completion, time to exhaustion, peak power, and average power. Furthermore, De Ataide e Silva et al. (13) report mean difference in power output between conditions and not effect size (ES). The reporting of average change in power is useful in a practical sense to help practitioners understand the extent of change, but the use of this statistic alone is vague and may not provide a sufficient understanding of the efficacy of CMR on performance. The aim of this study was to review the existing literature to quantify the effect of CHO mouth rinse on exercise performance.
A database (SPORTDiscus, PubMed) search for relevant peer-reviewed articles (excluding abstracts and unpublished theses/dissertations) was performed in September 2016. An original search term of “carbohydrate OR glucose OR maltodextrin OR dextrose AND mouth rinse AND mouth wash OR exercise OR sport OR performance OR run* OR cyc*” returned 1,075,933 and 1,492,935 entries in SPORTDiscus and PubMed respectively. A shorter search term of “mouth rinse OR mouth wash AND exercise” returned a more manageable 57 and 80 entries in SPORTDiscus and PubMed, respectively. Further searches consisted of entering various combinations of the following key words into Google Scholar; “carbohydrate,” “mouth rinse,” “mouth wash,” “sport performance,” “sport,” “exercise,” “running,” and “cycling.” A manual cross-reference of relevant articles and review articles was also performed. Identified studies were included on the basis that they were performed on humans under normothermic conditions, clearly stated the type of CHO in the mouth rinse, used a placebo-controlled repeated measures design, the mouth rinse was tested using a single exercise, and the relevant raw data were available to calculate ES (i.e., mean and SD or standard error). The following studies were excluded from the analysis; Beaven et al. (3) because raw data were not available for the placebo condition (an attempt was made to contact the author), Rollo et al. (39) because the performance outcome was self-selected running speed which is not in itself a performance measure per se that could be compared with the outcomes of other studies in the same subanalysis, Rollo et al. (40) because CMR was not compared with a placebo mouth rinse, Rollo et al. (38) because it was a review article, and 3 studies were excluded because the mouth rinse efficacy could have been influenced by a previous exercise (1,30,36). An overview of the search strategy is outlined in Figure 1.
The effectiveness of the mouth rinsing was quantified by determining the ES for each variable, which can be categorized as small (0.2), moderate (0.5), or high (0.8). This was calculated using the following equation (this equation was reversed in the case of those studies using performance time as the performance measure, as a lower number is beneficial): ES = (mean of CHO − mean of placebo)/SD of placebo.
Some studies reported the standard error of the mean rather than the SD. SD was calculated from these studies using the following equation:
A weighted ES was then calculated to account for changes in individual sample sizes as used by Matson and Tran (31) and Peart et al. (33):
The most common exercise protocol of a ∼1-hour cycling time trial with ∼6% CHO was used by 8/25 studies; therefore, a subanalysis on these studies was conducted to allow a comparison of findings from a similar exercise. As both power and time to completion were used as performance measures for the ∼1-hour cycling time trials, a further subanalysis on these was performed. Percentage changes in performance were analyzed in this further subanalysis and interpreted as recommended by Hopkins (20).
Table 1 describes the 25 included articles that allowed the analysis of 56 ES (Table 2). The overall ES for the influence of CMR on performance exclusive of other factors was 0.18 (weighted = 0.18), and the small ES for exercises that lasted longer than 25 minutes (0.25) was on average higher than the trivial ES for shorter exercises lasting under 3 minutes (0.06). No statistical comparison was made between the groups because of the differing sample sizes, but of note is that the upper 95% CI for shorter exercise was almost identical to the lower 95% CI for longer exercises, suggesting a possible difference. There was an average negative ES for resistance exercises, with the majority of the 95% CI lower than null. The most common exercise protocol of a 1-hour cycling time trial with ∼6% CHO was used by 8/25 studies; therefore, a subanalysis of these studies was conducted to allow a comparison of findings from a similar exercise (Table 2). The overall ES of these studies was 0.20, and the upper 95% CI approached moderate and reached moderate-large ES for power output and time to completion, respectively.
The average ES reported in this study can be classified as trivial-small, and some of the lower 95% CI marginally cross 0, suggesting a trivial chance of a negative impact on performance. However, it must also be noted that some of the upper 95% CI reach 0.64 suggesting that there may be a moderate benefit for some individuals. Table 2 identifies that the higher ES are typically in exercises lasting 25 minutes or greater, and there has been a particular focus on cycling time trials of approximately 1 hour administering ∼6% CHO. A number of studies implementing this protocol have reported small-moderate ES of 0.3–0.5 (7,9,19,29,35); however, the average ES is only small (Table 2). This may be influenced by the small ES reported by Beelen et al. (4), but is more than likely due to the small negative ES shown by Ispoglou et al. (21). In fact, if Ispoglou et al. (21) are removed from the analysis, the mean ES for time trial performance increases from 0.31 to 0.41, demonstrating the impact that this study has on the final ES. There are some methodological differences between these studies such as preparticipation fasting times. The low ES reported by Beelen et al. (4) was attributed to participants being in a fed state, and other authors have shown that ES are higher when CMR is used in a fasted state (17,29). However, this cannot explain the negative ES from Ispoglou et al. (21) as participants performed the trial after a 3-hour fast, similar to the 2–4 hours fast used in other studies (7,19,29). Unfortunately, the number of differing fasting protocols and relatively small number of studies in resulting subgroups did not allow for a subanalysis for the effect of fasting on CMR efficacy. There is also some disparity between studies for duration of the rinse (typically 5 or 10 seconds). However, this also cannot explain the much lower effect in the Ispoglou et al. (21) study as the 5-second rinse used was comparable with Carter et al. (7) and Gam et al. (19). Another consideration is that the true effect of the CMR in many of these studies is unclear as very few compare CMR to a control condition. Gam et al. (19) argued that a control condition is essential in future work as although they reported a significant difference between CMR and a placebo, they found that a control was just as beneficial compared with the placebo (their ES reduces from 0.33 to 0.17 when CMR is compared with control rather than a placebo). It could be the case that the action of rinsing the mouth may impact on performance by interrupting the participant, and this small decline in performance may be off-set by the CHO content i.e., performance returned to control conditions as opposed to being improved. Further work is needed to compare CMR to a control, perhaps in more ecologically valid settings where small interruptions may have a greater practical consequence.
Although ES were on average trivial-small, it should be considered that at the elite level a small effect may be practically significant in competition. Although some studies observed “physically active” participants, a strength of the current body of research is that most studies observed participants specifically trained for the task (Table 1). In fact, all of the studies in the ∼1-hour cycling time trial subgroup analysis apart from Devenney et al. (15) recruited cyclists or triathletes, and although the ES was small the average improvement in time to completion was 2.48% (Table 2). To put this into context, Hopkins (20) suggests that the smallest worthwhile change for cycling time trial time to completion is 1.3%, and other authors have reported less substantial improvements in performance of 0.16% (14) and 2.34% (24) for the same task when CHO was ingested. Moreover, Pottier et al. (35) actually observed significant improvements in 1-hour cycling time trial performance when using a mouth rinse, but not when ingesting the CHO.
Resistance exercises had on average a negative ES with most of the 95% CI range being below 0, suggesting a potentially adverse effect on performance. However, it should be considered that this has been based on only 2 studies (11,32), and the performances between conditions in these studies were almost identical (Table 1). The average negative ES is likely due to participants performing one less repetition in a repetition to fatigue exercise in Clarke et al. (11), which was within the reported normal variation for the outcome. Therefore, although there is no evidence to support CMR for resistance exercise, there is also not enough evidence to portray it as being detrimental.
It is evident that more work is needed in the area to standardize testing procedures to facilitate practitioner and athlete interpretation of findings. In particular, it would be of benefit to see more comparisons to a control condition. The current evidence suggests that CMR is not ergogenic for very short duration exercises lasting less than 3 minutes, and that CMR may be more beneficial for exercises of approximately 1 hour but the ES are variable (see CIs in Table 2). It is worth noting that although the ES are on average trivial-small, they may be higher than the smallest worthwhile change for elite performers.
The evidence reviewed in this article suggests that the average performance enhancement afforded by CMR is trivial-small, but may be greater than the smallest worthwhile effects for elite athletes. It is unclear if the small effects would provide any meaningful benefit to subelite performers; however, it should be considered that the average effect is not necessarily true for every athlete and the broad CIs suggest that CMR may be worth experimenting with on an individual basis. Athletes whose events last 25–60 minutes may be more likely to observe an ergogenic effect than those taking part in shorter more anaerobic events or resistance exercises. For events and activities lasting more than 60 minutes, the cost-benefit relationship to withholding the ingestion of CHO should be considered. Finally, although not included as a subanalysis in this review, some authors have suggested that CMR is more effective when in a fasted state. Once again the cost-benefit relationship should be considered for competing in a fasted state, but practitioners may wish to consider CMR if training in a low glycogen state.
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