Introduction: Exercise in the heat can impose significant physiological strain and may result in incapacity, illness and death from exertional heat stroke (EHS). The adrenocortical response to exercise is known to be amplified with concurrent heat stress, suggesting the potential utility of cortisol measurement in dynamic surveillance for heat intolerance. In laboratory conditions, a standard Heat Tolerance Assessment (HTA) may be used to determine thermal tolerance to exercise-heat stress. How cortisol responses vary with HTA and whether thermal responses can be discriminated by their measurement has not been investigated.
Methods: During morning hours, 16 healthy military volunteers with no prior history of heat illness underwent blood and saliva sampling at rested baseline (PRE) and immediately after maximal oxygen uptake (VO2max) testing (POST1) and subsequent HTA (POST2). HTA was conducted at 60% of VO2max in a climate-controlled chamber (ambient temperature 34°C, relative humidity 40%). Core temperature (Tc) was measured by rectally-sited temperature probe. Cortisol was assayed by radioimmunoassay.
Results: There was a significant (P<0.01) association between salivary and serum cortisol measures PRE (r=0.95), POST1 (r=0.8), POST2 (r=0.92) and across pooled timepoints (r=0.85). Volunteers with Tc response >39.0°C (n=5) vs. <39.0°C (n=11) during HTA had significantly higher salivary cortisol on completion of exercise (26.31 [30.81, 16.36] vs. 3.97 [3.45, 4.28] nmol.l−1). Salivary cortisol >8.94 nmol.l−1 showed 100% sensitivity and 91% specificity for thermal response >39.0°C.
Conclusions: Salivary cortisol correlates highly with serum cortisol in populations exposed occupationally to exercise in the heat. The strength of association is maintained with high intensity (VO2max testing) and more moderate intensity (HTA) exercise bouts. Discriminating Tc response by salivary cortisol measurement could provide a non-invasive method of risk stratifying and dynamically assessing for EHS in both laboratory and field settings, including athletic competition, recreational activities and military training.