RESUMEN
Background and Objectives: Cortisol is a valuable marker for assessing the body's response to any form of stress. We conducted this study in order to evaluate the variations of salivary and serum cortisol levels in professional football players in relation to cardiopulmonary exercise testing (CPET) and their significance in potentially evaluating overtraining in athletes. Also, the question of whether salivary cortisol determination could fully substitute serum sampling was addressed. Materials and Methods: A total of 19 male professional football players were evaluated by measuring serum cortisol levels at rest (T0) and immediately after a CPET (T1) and salivary cortisol levels at rest (T0), 10 min after a CPET (T2), and 30 min after a CPET (T3). Results: T0 serum cortisol showed a statistically significant correlation with the oxygen uptake at the anaerobic threshold divided by the body weight (VO2-AT/weight), as did the T2 salivary cortisol with the maximum oxygen uptake at the anaerobic threshold (VO2-AT) and VO2-AT/weight. T0 salivary cortisol was significantly correlated with the subjects' height and the predicted O2 pulse. Conclusions: While some correlations were discovered, they are insufficient to recommend cortisol as a routine biomarker in athletes' evaluation. However, significant correlations were established between salivary and serum determinations, meaning that the non-invasive procedure could substitute venous blood sampling.