Determination of the optimal dose and dosing strategy for effective L-menthol oral rinsing during exercise in hot environments: optimising oral L-menthol administration

Purpose
This multi-study programme investigated the optimal concentration of L-menthol delivered as an oral mouth rinse to modulate thermo-behaviour during exercise in a hot environment (35 oC).

Method
In study 1, 38 participants completed a survey to establish an effective and tolerable range of L-menthol concentration. 31 participants completed an RPE-protocol examining 1. the dose-response effect of L-menthol mouth rinse on exercise performance (n = 16) and 2. the temporal effectiveness of administering L-menthol in an incremental and decremental dosing pattern (n = 15). Power output, heart rate, body core temperature and thermal sensation were reported throughout.

Results
The optimal menthol concentration for peak power was between 0.01 & 0.1% (~6 % increase, P < 0.05) and 0.5% (~9 % increase, P < 0.05) with respect to control. Work completed was increased at 0.01% (~5 %, P < 0.05), at 0.1% (~3 %, P < 0.05) and had a detrimental effect at 0.5% (-10% decrease, P < 0.05). There were no differences between an ascending dose protocol (0.01 to 0.5 %), descending dose protocol (0.5 to 0.01 %) or a constant 0.01% dose protocol. There were no reported differences in body core temperature or heart rate across trials (P > 0.05).

Conclusion
The optimal dose of L-menthol when delivered via oral rinsing is between 0.01 – 0.1 %. At lower concentrations, L-menthol appears to be less effective and at higher concentrations (>0.5%), L-menthol appears to elicit greater irritation and may not positively modulate thermo-behaviour during exercise in a hot environment.