Abstract
Salivary cortisol (Csal) represents the free cortisol concentration of serum cortisol (Cser). It has been suggested that Csal is approximately 5-10% of total Cser, however, the impact of diurnal variation in C, and how this pattern affects the proportion of Cser and Csal (Cprop), has yet to be explored. PURPOSE: Therefore, the purpose of this study was to assess the diurnal changes of Cprop, and the extent to which a high-intensity exercise bout may impact this relationship. METHODS: Male (n = 7) college-aged students (26.3±2.8 yrs, 176.5±8.1 cm, 73.6±12.6 kg, 9.9±3.2 BF(%), VO2max: 68.9±9.5 ml.kg-1.min-1) completed two 24-hour (rest and exercise) cortisol profiles. Subjects had a catheter inserted at 0600hr and blood and saliva samples were collected simultaneously every 120 mins. During the exercise condition, subjects performed 5x30s sprinting intervals on the cycle ergometer, at a resistance of 7.5% of body mass. Subjects were permitted 3-min of passive recovery between bouts. Csal and Cser were analyzed via competitive-binding assay. Cprop was calculated as proportion of Csal relative to Cser at each time point. Multilevel growth models with varying fixed/random coefficients were tested and compared (AIC). The final cubic growth model controlled for condition and freely estimated the intercept, velocity, and acceleration terms while the initial trajectory of Cprop was fixed across all individuals. RESULTS: The IQR of Cprop was 3.56-6.29%. The greatest Cprop values were consistently observed during nighttime sampling with the magnitude of Cprop being greatest at 2300hr. The cubic growth models were determined to best-represent 24-hr changes in Cprop. The final model showed a significant effect for exercise (β = -1.37, p = 0.036; AIC = 1030.781). CONCLUSION: The greatest Cprop at 2300hr appears to be driven primarily by elevations in Csal. This shift may be influenced by night time cortisol secretion interactions with binding proteins, alterations in salivary flow rate, or salivary gland enzymatic activity. Exercise appears to influence Cprop dynamics, especially during late afternoon and nighttime hours. It is therefore recommended that cortisol profiles be constructed from both total and free Cser for the most accurate monitoring of the HPA-axis, especially within an exercise context.
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