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Introduction: Winter warfare training (WWT) is a critical component of military training that trains warfighters to operate effectively in extreme environments impacted by snow and mountainous terrain. These environmental factors can exacerbate the disruption to the hormone milieu associated with operating in multi-stressor settings. To date, there is limited research on the physiological responses and adaptations that occur in elite military populations training in arduous environments. The purpose of this study was to quantify hormone responses and adaptations in operators throughout WWT., Materials and Methods: Participants engaged in baseline laboratory metrics at their home station, Fort Carson, located in Colorado (CO) prior to WWT, for one week in Montana (MT) and one week in Alaska (AK). WWT periods were separated by approximately one month. Blood was collected upon wake at baseline (CO) and on the first and last day of WWT at each location (MT and AK). Plasma was analyzed for stress, metabolic, and growth-related hormones via enzyme-linked immunoassay (ELISA). Sleep quality was assessed via the Pittsburg Sleep Quality Index (PSQI) at baseline (CO) and on the first day of training in MT and AK. Cognitive function was evaluated using the Defense Automated Neurobehavioral Assessment (DANA) at baseline (CO) and on the first and last day of WWT in both MT and AK., Results: Fourteen US Army operators in 10th Special Forces Group (SFG) Operational Detachment participated in winter warfare training (WWT; age: 31.5 years; 95%CI[28.1, 34.3]; height: 180.6 cm; 95%CI[177.3, 183.4]; weight: 87.4 kg.; 95%CI[80.6, 97.7]; body fat: 18.9%; 95%CI[13.7, 23.1]; male: n=13; female: n=1). Plasma adrenocorticotropic hormone (ACTH) levels increased from baseline (19.9 pg/mL; 95%CI[8.6, 24.2])  to pre-WWT (26.9 pg/mL; 95%CI [16.2, 37]; p=0.004), decreased from pre-  (26.9 pg/mL; 95%CI [16.2, 37]) to post-WWT in MT (22.3 pg/mL; 95% CI [8, 23.7]; p=0.004;), and increased from pre-  (25 pg/mL; 95%CI[ 28.4) to post-WWT (36.6 pg/mL; 95%CI [17.9, 48.9]) in AK (p=0.005). Plasma cortisol levels decreased from pre- (174 ng/mL; 95%CI[106.2, 233.6])  to post-WWT (94.5 ng/mL; 95%CI[54.8, 101.7]) in MT (p=0.001) and, conversely, increased from pre- (123.1 ng/mL; 95%CI[97.5, 143.9]) to post-WWT  (162.8 ng/mL; 95%CI[128, 216.7]) in AK (p<0.001). Alterations in growth-related hormones (insulin-like growth factor 1 [IGF-1], insulin-like growth factor binding protein 3 [IGFBP-3],  and sex hormone binding globulin [SHBG]) were observed throughout WWT (p<0.05). The Total Testosterone / Cortisol ratio (TT / CORT; molar ratio) was lower pre-WWT in MT (0.04; 95%CI[0.01,0.04) compared to baseline in CO (0.07; 95%CI[0.04, 0.07]; p=0.042). Triiodothyronine (T3) levels increased from pre-  (101.7 ng/dL; 95%CI[93.7, 110.4]) to post-WWT  (117.8 ng/dL; 95%CI[105.1, 129.4]) in MT (p=0.042). No differences in sleep quality were reported between locations (CO, MT, and AK). Alterations in cognitive function were exhibited between locations and during WWT in both MT and AK (p<0.05)., Conclusions: Over the course of WWT, elite operators experienced alterations in stress, metabolic, and growth-related hormones, as well as cognitive performance. The increase in stress hormones (i.e., ACTH and cortisol) and reduction in cognitive performance following training in AK are suggestive of heightened physiological strain, despite similarities in physical workload, self-reported sleep quality, and access to nutrition. The variation in hormone levels documented between MT and AK may stem from differences in environmental factors, such as lower temperatures and harsh terrain. Further research is warranted to provide more information on the combined effects of military training in extreme environments on operator health and performance., (Published by Oxford University Press on behalf of the Association of Military Surgeons of the United States 2024. This work is written by (a) US Government employee(s) and is in the public domain in the US.)