The Effect of High-Intensity Exercise on Changes in Salivary and Serum Cortisol Proportion Dynamics

Typically, salivary cortisol is reported as 5–10% of total cortisol, but the stability of this proportion and the effect of exercise on the 24-h profile is unclear. Therefore, this study investigated the circadian rhythm of the proportion of serum cortisol represented by salivary cortisol, and the impact of acute high-intensity exercise. Recreationally trained males (n = 8, age = 25.7 ± 2.4 years, height = 174.7 ± 7.8 cm, mass = 69.8 ± 12.1 kg) completed two 24-h profiles (rest and exercise conditions) for serum (Q60) and salivary (Q120) cortisol. Exercise consisted of 5 × 30 s sprinting intervals on the cycle ergometer. Cortisol was assessed using commercially available assays. The proportion (Cprop) of serum cortisol (Cser) represented by salivary cortisol (Csal) was calculated as [Cprop = Csal/ Cser × 100]. Multilevel growth models tested for trends across the 24-h profile. The highest relation between Cser and Csal was observed at 08:00 AM (r = 0.90). The average Cprop was 5.95% and demonstrated a circadian profile characterized by a cubic model. Acute exercise did not alter Cser, Csal, or Cprop. Thus, the proportion of Cser represented by Csal changes across a 24-h period and should be accounted for if using salivary cortisol to reflect circadian output of cortisol.