Endocrine Abstracts

Aims: Exertional hyperthermia stimulates the hypothalamic-pituitary adrenal axis, increasing the availability of free cortisol in body fluids and opening a window onto physical stress for novel biosensing technologies. This study aimed to determine the impact of prolonged endurance exercise on relationships described for shorter activity bouts, by characterising interactions between salivary indices of glucocorticoid activity, serum total cortisol response and thermal strain.

Methods: Core body temperature (Tc) was measured 5 min^-1 in 32 recreational runners completing Brighton Marathon 2022 (4 female, 28 male). Peak Tc (Tcpeak) was determined and cumulative hyperthermia calculated as the area under the temperature-time curve for Tc>38 °C (AUCTc38). Saliva and blood were sampled at rested baseline and within 30 min of finishing, for later analysis by commercial radioimmunoassay and in-house LCMS/MS.

Results: Runners finished the marathon in 240 ± 41 min, attaining Tcpeak 39.3 ± 0.45 °C. There was significant (P<0.05) elevation in serum cortisol (397 ± 137 vs 1109 ± 278 nmol.L^-1) and salivary cortisol (12.3 ± 10.9 nmol.L^-1 vs 52.0 ± 39.2 nmol.L^-1), cortisone (15.5 ± 11.0 vs 64.7 ± 39.3 nmol.L^-1) and 11-dehydrocorticosterone. Salivary analytes mutually correlated to a high degree (r>0.75, baseline and post-marathon), whereas serum cortisol correlated less robustly at baseline and showed weakening linear associations with salivary analytes post-run: only the relationship to salivary cortisol remained significant (r=0.57). At higher thermal strain, changes in the ratio of salivary cortisol:cortisone, but not serum cortisol, varied positively with Tcpeak and AUCTc38 (r>0.45).

Conclusions: Prolonged exercise disrupted close relationships between serum and salivary cortisol and Tc response, which are otherwise preserved in studies of shorter exercise duration and lower cumulative thermal stress. Increasing tissue temperature may skew the balance of salivary glucocorticoids towards cortisol, with production appearing to exceed inactivation. This could potentially confound attempts to monitor physical performance with biosensors according to the site and fluid sampled.