The effects of swilling an l(−)-menthol solution during exercise in the heat

We have previously demonstrated that provision of a cold fluid (4°C) during exercise in the heat increases fluid intake and improves exercise capacity when compared to a control fluid (19°C). The present study investigated whether these positive effects could simply be replicated with a cooling agent, menthol. Nine healthy, non-acclimatised males (25 ± 7 years; \( \dot{V} \)O_2max: 54 ± 5 ml kg⁻¹ min⁻¹) cycled to exhaustion at 65% of their peak aerobic power output at 34°C, swilling 25 ml of either an l(−)-menthol (0.01%) or orange-flavoured placebo solution every 10 min, whilst water was available ad libitum; all fluids were kept at 19°C. Eight out of nine subjects cycled for longer whilst swilling with menthol and this resulted in a 9 ± 12% improvement in endurance capacity. Rectal temperatures rose by 1.7°C during exercise with the same time course in both conditions, whilst skin temperature remained largely unchanged. Swilling with menthol resulted in hyperventilation by 8 ± 10 L min⁻¹ and reduced central (cardiopulmonary) ratings of perceived exertion by 15 ± 14%. No differences between trials were observed for heart rate, oxygen uptake or carbon dioxide production, blood concentrations of glucose or lactate, sweat rate or volume of water ingested. We conclude that a change in the sensation of oropharyngeal temperature during exercise in the heat significantly affects endurance capacity, ventilation and the (central) sense of effort.