The performance-enhancing effects of carbohydrate (CHO) ingestion during prolonged ([greater than or equal to] 1 hour) exercise are well documented (Jeukendrup, 2004). Mechanisms behind these effects have traditionally been attributed to metabolic influences such as sparing of endogenous muscle glycogen stores (Stellingwerff et al., 2007), and maintenance of blood glucose concentration as well as the rate of CHO oxidation in the later stages of exercise (Wilber and Moffatt, 1992). However, studies showing the efficacy of CHO ingestion during shorter duration exercise ([less than or equal to] 1 hour; Anantaraman et al., 1995; Ball et al., 1995) indicate that CHO may also exert its effects via central mechanisms (Pottier et al., 2010), as endogenous CHO availability is generally not a limiting metabolic factor during exercise of this duration (Rollo et al., 2008).
The use of a CHO mouth rinse (rinsing a small volume of CHO solution around the oral cavity before expectorating it) has been shown to enhance performance during running and cycling lasting ~30-60 minutes (Carter et al., 2004; Chambers et al., 2009; Rollo et al., 2010). Functional magnetic resonance imagery (fMRI) studies demonstrate that introducing sweet and non-sweet carbohydrate into the oral cavity activates the primary and putative secondary taste cortices in the orbitofrontal cortex (Chambers et al., 2009; O'Doherty et al., 2001). Stimulation of these regions may also activate the dorsolateral prefrontal cortex, anterior cingulate cortex, ventral striatum, and anterior insula/frontal operculum (Chambers et al., 2009). These brain regions may control behavioural and autonomic responses to rewarding stimuli (Chambers et al., 2009; Rolls, 2007). In particular, the dorsolateral prefrontal cortex and the ventral striatum are thought to have a role in cognitive and attentional processing and motivation, respectively (Chambers et al., 2009; Kelley et al., 2002). These findings are suggestive of a non-metabolic mechanism of CHO efficacy, as CHO is not made systemically available when using a mouth rinse (Rollo et al., 2010). A non-metabolic mechanism is supported in a more ecologically valid way by observation of a significant increase in motor output and muscle force production immediately following the introduction of CHO into the oral cavity (Gant et al., 2010). Together, this work suggests that the presence of CHO in the mouth may stimulate oral CHO receptors that facilitate increased central drive and/or motivation, potentially improving exercise performance (Chambers et al., 2009). However, while oral taste receptors for sweet stimuli such as glucose have been identified (Rollo and Williams, 2011), potential receptors that may detect nonsweet CHO such as maltodextrin have not yet been documented (Chambers et al., 2009).
Increased central drive and/or motivation associated with CHO mouth rinses suggests their use may be beneficial in sports requiring high levels of central activation and motivation over a short time, such as sprinting. The only study to investigate the influence of a CHO mouth rinse on single sprint performance reported no significant influence of mouth rinsing on performance or metabolic responses to a 30 second cycle sprint (Chong et al., 2011). However, only a single administration of the mouth rinse was used immediately prior to the sprint, resulting in the oral cavity being exposed to the mouth rinse for ~10 seconds. Gant et al. (2010) reported significant increases in motor output and force production when the oral cavity was exposed to a CHO mouth rinse for 1560 seconds. Other work reported a significant increase in plasma insulin concentration with an oral exposure time of 45 seconds (Just et al., 2008). Therefore, the duration of oral exposure to a CHO mouth rinse may significantly influence its efficacy (Rollo et al., 2010), perhaps by increasing stimulation of oral receptors (Sinclair et al., 2013). The influence of oral exposure time on CHO mouth rinse efficacy is supported by recent work suggesting a dose-response relationship to the duration of CHO mouth rinse exposure on 30 minutes self-paced cycling performance (Sinclair et al., 2013).
The aim of this study is to investigate the influence of serial administration of a CHO mouth rinse (cumulative oral exposure time 40 seconds) on performance, metabolic and perceptual responses to a 30 second cycle sprint. We hypothesized that serial administration of a CHO mouth rinse would significantly improve sprint performance compared with a placebo (PLA) mouth rinse.
Twelve physically active males volunteered for the study (mean ([+ or -] SD) age: 23.1 (3.0) years, height: 1.83 (0.07) m, body mass (BM): 86.3 (13.5) kg). Participants were physically active and regularly undertook cycle ergometer sprinting as part of their normal physical activity routine. Participants were informed of the experimental procedures prior to providing written consent. The Abertay University Research Ethics Committee granted ethical approval, in line with the Helsinki Declaration.
The study employed a repeated measures, counterbalanced, cross-over design with simple randomization of trial orders and double-blind administration of mouth rinses. Each participant attended the laboratory on 3 occasions separated by 3-7 days. Sessions took place between 9-11 am. The first visit was a familiarization of the full protocol, in line with recommendations for improving the reliability of performance testing (Hopkins et al., 2001). The session began by describing and explaining the rating of perceived exertion (RPE), felt arousal (FA; Svebak and Murgatroyd, 1985), and nausea scales (Chong et al., 2011), and anchoring each end of those scales. Thereafter, BM in kg (Seca Scales, Hamburg, Germany) wearing shorts only and height in m (Seca Stadiometer, Hamburg, Germany) were recorded. Participants were then fitted with a heart rate (HR) monitor (Polar S610i, Finland) and sat quietly for 10 minutes. Halfway through this period FA and nausea ratings were taken along with a capillary blood sample from the index finger of the right hand for the immediate quantification of blood lactate (LactatePro, Arkray Factory Inc, Shiga, Japan) and glucose (Freestyle, Warwickshire, UK) concentrations. At 6 minutes, participants were provided with 25 ml of water and asked to rinse it around their mouth for 5 seconds, and then expectorate it into a beaker. Subsequently, every 2 minutes until the beginning of the cycle sprint participants repeated the mouth rinse procedure (6, 8, 10, 12, 14, 16, 18, and 20 minutes; 200 ml water).
At the end of the 10 minute seated period RPE, FA, and nausea were recorded and a capillary blood sample taken. The participant then mounted the cycle ergometer (Monark Ergomedic 894E, Sweden) for a standardised warm up of 5 minutes at 60 rpm against a 1.5 kg resistance. This was immediately followed by three practice starts where the investigator provided a 3 second countdown after which the participant cycled maximally. The participant was given ~2 seconds to overcome the inertia on the flywheel before the investigator introduced a resistance equivalent to 0.075 g x [kg.sup.-1] of pre-exercise BM and the participant continued to cycle maximally for 3 seconds. Immediately after, the load was removed and the participant cycled at 60 rpm for 45 seconds. This practice start was repeated two more times. Administration of the mouth rinse continued every...