There are several practical implications elucidated by the research conducted in this thesis, that should be borne in mind if menthol mouth swilling is to be implemented outside of the confines of a laboratory by practitioners, or to further strengthen experimental design.
10.2.2.1 Individual variation in menthol preference
Individual variation in menthol preference may be a predictor in assessing an athlete’s response to menthol mouth swilling. In Chapter 5, no statistically significant differences between participants’ ratings of menthol concentrations were found, but small standardised differences in means were noted. Visually, elevations in VAS rating seem to appear at low, medium and high menthol swill concentrations (see Figure 5.1A), hinting at the possibility of thresholds for menthol liking; these thresholds may have been more apparent had fewer solutions been assessed. Similarly, the addendum to Chapter 5 highlights that participants respond to the solution that causes the least irritation and has the most pleasant mouth-feel.
Physiological and cultural differences are two plausible explanations for the emergence of menthol liking thresholds within the data set. Physiologically, this variation may be explained in part by genetic factors relating to the expression of TRPM8 receptors (Morgan, Sadofsky and Morice, 2015), the sensitivity of their trigeminal nerve (Viana, 2011; Frasnelli et al., 2011; Michlig et al., 2016), and one’s ability to differentiate between trigeminal stimuli (Cliff and Green, 1996; Frasnelli et al., 2011), as well as the thickness of the stratum corneum in the area under menthol exposure (H.R. Watson et al., 1978).
Whereas cultural differences may dictate factors such as menthol concentration within products, and in doing so expose an individual to a higher or lower concentration of
menthol acutely or chronically if one is a habitual consumer, which in itself can alter sensitivity to menthol (Kalantzis, Robinson and Loescher, 2007; Patel, Ishiuji and Yosipovitch, 2007; Klein et al., 2010; Botonis et al., 2016). The role and time course of habituation to menthol at an individual level has practical implications upon an athlete, or practitioner, aiming to assess menthol mouth swilling in training, with a view to implementing in competition. This may mean purposefully withholding menthol containing stimuli from their diet, or not using menthol mouth swilling in selected sessions to deliberately increase subjective factors such as thermal discomfort and sensation.
10.2.2.2 Interfering with hydration and thirst
In Chapter 6, it was found that menthol may mimic the effects of swilling water around the oral cavity in as much as it can reduce thirst, whilst concomitantly improving measures of thermal comfort and thermal sensation. These small to moderate effects remain when dietary carbohydrate intake is controlled for, which may affect the effectiveness of carbohydrate mouth swilling during exercise trials in some instances (Fares and Kayser, 2011; S. C. Lane et al., 2013; Che Muhamed et al., 2014), but is not entirely supported (Ali et al., 2016). Chapter 6 served as a proof of concept of Eccles’ model ((Eccles, 2000;
Eccles et al., 2013) Figures 10.2 and 10.4), in that thirst was reduced as a result of menthol stimulation of the oral cavity.
Whilst thirst is not performance limiting per se (Cheuvront et al., 2010; Sawka, Cheuvront and Kenefick, 2015; Kenefick, 2018), the opportunity to implement a strategy that attenuates thirst at a lower weight cost than swilling from a water bottle may confer an energetic (W/kcal saved due to weight minimised) and resultant performance (time trial performance) advantage to a rider or runner.
Exercise durations of approximately one hour e.g. 40km time trial, may be a useful starting point for implementation, as they can be performed competitively with minimal need for exogenous fuelling strategies due to not entirely depleting endogenous carbohydrate stores (Fares and Kayser, 2011), and represent a relevant performance test which displays a low coefficient of variation in lab and outdoor settings (Smith et al., 2001).
A natural extension of this work would be to assess the effectiveness of menthol mouth swilling upon exercise performance following graded levels of hypohydration, as this further acts as a driver of thirst (when expressed as a change in blood osmolality), as per Figures 10.2 and 10.4 (adapted from (Eccles, 2000; Eccles et al., 2013). A hypohydration range of 2-4% is representative of those typically attained during training or competition
and so is recommended. This has relevance for not only elite athletes, but also recreational exercisers who have professions which may predispose them to becoming hypohdrated.
10.2.2.3 Flavour fatigue and habituation: two sides of the same coin
Flavour or taste fatigue is a concern for athletes during prolonged endurance activity, and ultra-endurance competitions. It is brought about by repetition of a flavour stimulus which in effect acutely desensitises taste receptors to a previously pleasant or tolerable flavour.
In endurance sports, flavour fatigue is commonly associated with carbohydrate containing products as they are sweet, so a contrasting stimulus that, in the case of menthol, is cooling may serve to counter this, as these stimuli target and are transduced by different TRP receptors than sweet flavours (TRPM5; (Liu and Liman, 2003)). Severity and consequence of flavour fatigue can vary greatly, from a practitioner having to alter a feeding strategy to an inability for an athlete to tolerate foodstuffs, potentially leading to race withdrawal if prolonged and severe. This is also seen in menthol’s counterirritant capsaicin (Karrer and Bartoshuk, 1995; Cliff and Green, 1996; Kalantzis, Robinson and Loescher, 2007).
Similarly, given menthol’s increasing status as a potential ergogenic aid we must remain open to the possibility that one may become habituated to the strategy and the ergogenic potential of the stimulus diminishing, accordingly. This may be able to be predicted to a certain extent by simple questions that outline an athlete’s use and preference for menthol containing or mint flavoured products. These questions may elucidate an athlete’s acceptability and typical usage of menthol products; if usage remains consistent despite the introduction of menthol mouth swilling in training or competition and the athlete is still demonstrating improvements in perceptual or performance outcomes, it may be safe to assume habituation has not taken place. Conversely, if the athlete is avoiding or reducing their consumption/use of other menthol containing products, or reporting lower perceptual responses to the intervention independent of accompanying improvements in physiological markers (e.g. Tcore, sweat rate), it may be assumed that habituation has occurred.
Habituation of thermal sensation has been observed when a moderate menthol concentration of 0.2% was applied topically over the course of a week (Gillis et al., 2015).
This habituation response was attributed to a pathway specific to thermal sensation, as it occurred independent of other physiological or perceptual responses (Gillis et al., 2015).
Similarly, habituation to sweet stimuli have been reported (Leterme et al., 2008), and are attributed to gustative habituation to sweet taste, as opposed to a reduction in pleasure derived from exposure to sweet stimuli. Conversely this response is yet to be observed in