Automated Measurements of Salivary Cortisol Levels Within Circadian Rhythms Detect Differences in Exercise-induced Stress Response Between Two Altitude Training Camps

Background: There are conflicting reports with regard to the superiority of cortisol as a stress marker in high-altitude training due to the influence of the circadian rhythm. This study aimed to determine if the automated measurement of salivary cortisol concentration via sequential sampling could detect the differences in acclimatization and exercise stress between two altitude camps. Methods: A total of 12 elite female long-distance runners living near sea levels were enrolled in this study. For the first camp simulating higher altitude, the runners lived at 1800 m and trained at 1700 m for 7 days. For the second camp simulating lower altitude, they lived at 1550 m and trained at 1300 m for 7 days. Their saliva was sequentially collected on the last 2 days during each camp involving different intensity exercise in the morning and afternoon. The salivary cortisol concentrations were measured using electrochemiluminescence immunoassay. Results: The sequential saliva collection and automated measurement were able to detect the basal levels and exercise-induced changes in the salivary cortisol within the runners' circadian rhythms in each altitude camp. Before dinner, the basal salivary cortisol concentrations were significantly higher in the higher-altitude camp. The rate of change in the salivary cortisol concentration after the afternoon exercise on day 2 of the higher-altitude camp with higher exercise intensity was significantly higher than that in the lower-altitude camp (p = 0.003). The rate of change in the salivary cortisol concentration during the morning exercise on day 2 was significantly higher in the higher-altitude camp than in lower-altitude camp (p = 0.028) despite the same exercise programs and intensities. Conclusions: Measurement of the salivary cortisol levels during the circadian rhythms could detect the differences in acclimatization and exercise stress between two altitude camps. Automated measurements of salivary cortisol concentrations via sequential saliva sampling may be useful for assessing adaptation disorders and excessive exercise stress and also help develop adequate high-altitude training programs for athletes.