Physiological response to exercise in the heat: Implications for risk mitigation and adaptation.

Purpose: Recent field studies of physical exertion in challenging environmental conditions have reported dissociation between elevation in body core temperature (T_c) and successful task completion. This prompted us to further examine physiological mechanisms that might underlie variability in the response to exertional heat exposure. We hypothesized that, in response to exercise in the heat, systematic differences in central and peripheral physiological variables would be apparent between participants who successfully completed the task, versus those who became hyperthermic or symptomatic.

Methods: Thirty-eight healthy participants attempted a 120-min walk (5 km/h, 2% grade) in a climate-controlled chamber (40°C, 50%RH). At rest and at regular intervals during the walk, measures of physiological heat strain were assessed. Twenty-seven participants were Completers, seven were stopped because their T_c exceeded 39°C (Hyperthermics), and four became Symptomatic (e.g. lightheaded, headache, dizzy) and did not complete the walk.

Results: Visceral adipose tissue was higher in those who became Hyperthermic, compared to the Completers (437 ± 183 vs 245 ± 268 g; p = 0.034), despite similar height and body mass. Hyperthermics also had higher heart rate (p = 0.009), and lower end-diastolic volume (p = 0.031), and stroke volume (p = 0.031) during the early stages of walking, compared to the Completers. None of the Symptomatics reached a T_c >39°C (symptoms occurred at 38.1 ± 0.4°C), and none of the Hyperthermics reported symptoms.

Conclusions: During exertional heat exposure, adiposity and exaggerated early-stage hemodynamic responses were related to T_c elevation, but hyperthermia was not related to the development of symptoms, and baseline parameters relating to body composition and fitness were not related to symptom development.