Cardiorespiratory fitness mediates cortisol and lactate responses to winter and summer marches

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Background: The influence of homeostatically regulated physiological processes, including cardiorespiratory fitness (VO2max), on the response to physical stressors such as acclimatisation and marching, remains understudied. We aimed to investigate the effects of summer and winter acclimatisation and marching on cortisol levels and blood lactate, to gain insight into the role of these physiological processes in the stress response. Methods: Two groups of young Europeans, classified as poor (PCF; n=9) and good physical condition (GCF; n=21), based on a VO2MAX threshold of 40 mL O2/ kg/min, underwent 2-h March (6-7 km/h) in winter (5˚C) and summer (32˚C). Commercial tests, UniCel DxI Access Cortisol assay and EKF Biosen Clinic/GP assay were used for cortisol and lactate blood measurements (morning samples and those taken immediately after marches), respectively. Results: Basal cortisol levels were significantly higher at 5°C than at 32°C (PCF group: P=0.0079; cortisol dropped after the March at 5°C in both groups, but increased at 32°C only in PCF. Basal lactate levels were higher at 32°C only in the GCF group (compared to PCF, P=0.0014) and post-marching (compared to basal values, P=0.0002). Conclusion: Cold exposure elicits a more significant stress response, based on higher basal cortisol levels, in individuals with worse physical fitness. Anaerobe metabolism prevails in participants with better physical fitness exposed to high ambient temperature based on increased basal and post-marching lactate levels. The study is significant for individual training/performance optimisation as it indicates the association between physiological stress responses and individual physical fitness levels.

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