Thermoregulation physiology and physiopathology of ultraendurance runners

Background: During physical exercise, only ~25% of the energy supplied by the substrates is converted into mechanical muscle work. The remaining ~85% is released as heat. In temperate environments, the metabolic heat is dissipated by several physical phenomena (radiation, conduction, convection, and evaporation). However, if the dissipation capacity is lower than the production of metabolic heat, the body temperature gradually increases until exercise is stopped, or heat-related pathologies develop (Exertional Heat Illness). Exercise-induced heatstroke is the second most common cause of death in athletes after cardiac disease.

Several recommendations concerning training and competitions taking place in hot environments have been published. However, the results of the epidemiological studies and the recommendations resulting mainly from laboratory studies are limited to events whose distance does not exceed that of the marathon. They therefore do not consider ultra-endurance disciplines.

These disciplines, defined by durations of effort longer than 6 hours, have specific characteristics (length of the events, intensity, high difference in elevation, exotic destinations, extreme environments, prevalence of digestive disorders and organ suffering) which mean that extrapolation of knowledge on the physiology of thermoregulation from "classic" endurance events, such as marathons, to ultra-endurance events is hazardous.

Aims:

- Describe the physiology and pathophysiology of thermoregulation processes in ultraendurance runners during an ultra-trail (165 km) in a tropical environment: le Grand Raid de La Réunion.

- Compare these thermoregulation processes between local runners (subjects considered with heat acclimated phenotype) and runners living in temperate climate (subjects considered without heat acclimated phenotype).

- Estimate the cumulative incidence of Exertional Heat Illness in ultraendurance runner population.