Acceptability and efficacy of a menthol mouth swill amongst elite rugby players

Abstract

Background: Many sporting events take place in hot and humid environments such as the Tokyo Olympics 2021, the FIFA World Cup in Qatar 2022, and Super Rugby in New Zealand and Australia during late summer. Such conditions challenge the body’s ability to thermoregulate, often resulting in compromised performance. Therefore, it is common practice for athletes to implement cooling strategies as means of attenuating the rise in core temperature and prolonging the onset of fatigue. The cooling properties of menthol have shown to be effective in reducing thermal sensation and enhancing performance during exercise in hot climates, however, there is little work that explores the efficacy of menthol in elite athletes during high-intensity team-based sport. Objective: To explore the effect of a 0.1% menthol mouth swill on thermal sensation and measures of fluid balance in elite rugby union players during training in the heat. To assess the acceptability of a 0.1% menthol mouth swill in elite rugby players. Design: A randomised cross-over design involving 27 participants (24.5 years ± 3.1) from a Super Rugby franchise in New Zealand. Participants were randomly assigned a 0.1% menthol mouth swill, or a placebo (water) mouth swill, which was swilled a total of four times during blocks of high-intensity, rugby-specific training drills in an outdoor environment during summer. Both thermal sensation and acceptability were assessed using spoken questionnaires. Thermal sensation was measured using a nine-point scale ranging from “very cold” to “very hot”. Acceptability was assessed using a questionnaire that prompted participants to rank taste, flavour, sensation, aftertaste, and overall pleasantness on a Likert scale from “dislike extremely” to “like extremely”. Performance was measured using Global Positioning System (GPS) metrics. Anthropometric measurements and fluid intakes were also assessed, which were used to calculate measures of fluid balance. Results: Presented as mean ± standard deviation, there was no significant difference in fluid intake (1522.2 ± 609.8 mL and 1429.5 ± 581.5 mL for menthol and placebo respectively, p = 0.24), nor percentage of dehydration (1.2 ± 0.6% compared to 1.1 ± 0.6% for menthol and placebo respectively, p = 0.21). Thermal sensation scores were significantly lower at the end of the training session following the menthol intervention (1.6 ± 1.8 compared to 2.4 ± 1.4 for menthol and placebo respectively, p = 0.02). This did not translate to any improvements in performance. The sensation of the menthol mouth swill was rated more favourable compared to the placebo, both before and after the trials (p < 0.01, and p = 0.03 respectively). Other acceptability indices did not differ significantly. Conclusion: A 0.1% menthol mouth swill is well accepted among elite rugby players and has no meaningful influence on fluid balance. Furthermore, it is effective in reducing thermal sensation at the completion of exercise yet produces no improvements in rugby performance. Individual variation among the results warrants an individualised approach when considering menthol application in rugby. Additional research in menthol application at the individual level is required to further understand the acceptability and efficacy of a menthol mouth swill