Serum free cortisol responses to a standard exercise test among elite triathletes

Eighteen elite male triathletes completed an incremental treadmill run to volitional exhaustion, followed by two maximal cycle ergometer sprints. Venous blood samples were obtained at rest (R), immediately post- (P) and one hour postexercise (H). Serum was analysed for total cortisol (TC) by a radioimmunoassay, and free cortisol (FC) by a temperature and time dependent ultrafiltration/dialysis method. Mean (+SE) resting cortisol levels were 627 + or - 23 (TC) and 12.5 + or - 2.5 nmol 1(-1)(FC). The mean percentage increase in TC (P vs. R) was 49.0 + 73%, accompanied by a 344 + or - 47% increase in FC. Significantly divergent (p<0.001) total cortisol responses (H vs. P) were observed within the group, and the athletes were divided into two subgroups (A and B) on this basis. In group A, both TC and FC returned to resting levels. In group B, there was a further slight increase in TC, accompanied by a significant (p<0.01) increase in FC reaching 851 + or - 268% of resting levels. These data suggest that exercise-induced increases in free cortisol, and hence its catabolic effect, are of much greater relative magnitude than TC changes. The relationship between TC and FC was best fitted to an exponential regression, with a clearcut change in slope at 700 nmol 1(-1)(TC) suggesting saturation of the binding protein. We suggest that the measurement of cortisol in athletes be reassessed, favouring determination of FC in the future.