The following section will outline the scope of menthol mouth swilling in specific training and competitive scenarios. This discussion will be limited to the context in which menthol mouth swilling will be employed, as opposed to the practicalities of delivering within these environments, which are discussed subsequently.
10.2.1.1 Menthol as a supporting strategy during heat acclimation
Previous research has shown menthol mouth swilling to acutely improve time to exhaustion (Mündel and D.A. Jones, 2009; Flood, Waldron and Jeffries, 2017; Jeffries, Goldsmith and Waldron, 2018) and time trial performance (Riera et al., 2014; Tran Trong et al., 2015; Stevens, Bennett, et al., 2016; Stevens, Thoseby, et al., 2016; Riera et al., 2016). Similarly, pre-and percooling strategies may also improve exercise performance in the heat (Stevens, Taylor and Dascombe, 2016; Best, Payton, et al., 2018) when applied for a single (Marsh and Sleivert, 1999; Quod et al., 2008; Duffield et al., 2010) or between bouts of exercise (Tran Trong et al., 2015; Galpin et al., 2016; Chan et al., 2017). Little is known, however, about how these strategies can be implemented in training alongside a heat acclimation protocol or training camp. The use of pre-and percooling strategies to support key, but not all, sessions within a heat acclimation protocol or training camp may allow for either a greater training duration or intensity, or a greater time at an elevated core temperature. This extended exposure to heat stress may provide further adaptations that would otherwise not have been attainable by the athlete, as sufficient time under heat stress may not have been attained, or may have come with an increased cost e.g. a compensatory adjustment in subsequent training sessions or the onset of heat illness.
Ecologically, the use of menthol mouth swilling in a laboratory-based acclimation programme would be easy to administer, and monitor the effects of administration upon performance, underpinning physiological adaptations and subjective outcomes. Variation in training session stimulus throughout the acclimation protocol would also allow for further exploration into the efficacy of menthol mouth swilling in the heat. Likewise, if one were to employ menthol mouth swilling over the course of a heat training camp, it would be prudent to target long runs, where increased heat storage is a product of exercise duration i.e. greater opportunity for heat accumulation. Interval sessions that allow for regular, repeated aliquots of the mouth swill to be utilised such as in Chapters 7 and 8 would also be sensible choices. Within these sessions an increase in heat storage is likely due to increased metabolic heat production (Mora-Rodriguez, Del Coso and Estevez, 2008), as exercise intensity and anaerobic contribution is greater (Febbraio et al., 1996), coupled with a decrease in evaporative heat loss (Kenny et al., 2009; Kenny and Jay, 2013), and lowered skin blood flow (González Alonso, Crandall and Johnson, 2008;
Crandall and González-Alonso, 2010). Interval sessions would also account for the potential for menthol to attenuate sensations of breathlessness, or increased RPElung
brought about by high intensity running, as per Chapter 7.
Menthol mouth swilling may also be a useful addition to adjunct heat acclimation strategies such as hot water immersion (Zurawlew et al., 2016; Zurawlew, Mee and Walsh, 2018), or the wearing of additional clothing (Stevens, 2018), where the treatment elicits either a heat maintenance (hot water immersion) or heat storage (additional clothing) response. In these instances, as is often reported during exercise, menthol may serve to ameliorate thermal comfort and sensation, again extending the time for which an athlete is exposed to the adaptive stimulus. Whether using menthol directly to support heat acclimation, or to support adjunct strategies, care must be taken to monitor athletes for signs of heat illness, or increased physiological strain that does not return to normal upon sufficient recovery, as the possible negative effects of extended or repeated heat exposure following perceptual cooling (i.e. menthol) is not currently known (Stevens and Best, 2017; Best, Payton, et al., 2018).
10.2.1.2 Ultra-endurance activity and gastrointestinal upset
Approximately 30-50% of endurance athletes experience gastrointestinal issues (de Oliveira, Burini and Jeukendrup, 2014), and typical estimates in ultra-endurance athletes are >60% (Gaskell, Snipe and Costa, 2019), with incidences as high as 93% reported (Jeukendrup et al., 2000) following an Ironman™ triathlon. There is also variability of prevalence reported between training and competition (Pugh et al., 2018), and the potential for differences in onset of gastrointestinal symptoms between exercise modalities (Pfeiffer, Stellingwerff and Hodgson, 2012; Costa et al., 2017). Prevalence also increases with exercise intensity and duration (Pfeiffer et al., 2012; Costa et al., 2016; 2017), and may be further exacerbated by the heat (Glace, Murphy and McHugh, 2013; Guy and Vincent, 2018), due to factors such as translocation of lipopolysachardies potentially inducing endotoxemia (Guy et al., 2016). Attenuating these symptoms through mentholated product ingestion would likely improve training time and intensity and reduce race attrition. Menthol mouth swilling may also allow for a welcome break from carbohydrate ingestion or swilling towards the end of non-ultra-endurance events, or respite from carbohydrate dominant strategies in ultra-endurance activity, where flavour fatigue may be considered a limiting factor (Costa et al., 2014; Costa, Hoffman and Stellingwerff, 2018).
Menthol ingestion, in some form, may also warrant consideration in long or hot events where gastrointestinal injury is a likely side effect of participation. Peppermint is commonly used to treat gastrointestinal symptoms, and has been shown to be effective in the alleviation of symptoms related to irritable bowel syndrome (Kline et al., 2001; Ford
et al., 2008; Alammar et al., 2019), and to a lesser extent reduce nausea (Tate, 1997; B.
Lane et al., 2012), colonic tension (Shavakhi et al., 2012), and flatulence (Kline et al., 2001). Peppermint may also improve rate of gastric emptying (Inamori et al., 2007). These findings suggest that the administration of menthol, which is a natural derivative of peppermint oil, may have potential therapeutic benefits that may be useful for athletes that suffer with gastrointestinal issues (Holzer, 2011), and extend beyond the stimulation of TRPM8 receptors as a mediator of cold temperature detection (Bautista et al., 2007; Gavva et al., 2012). Ginger has previously been used to mitigate gastrointestinal issues in elite ultra-marathon runners (Stellingwerff, 2016), suggesting a consumer appetite for natural or plant based strategies in this population.