Physiological markers in females and males after multiple bouts of caloric deprivation during arduous military training

The U.S. Army Ranger Training Course (RTC) is a multi-stressor environment including cognitive and physical stress coupled with repeated sleep and energy deficits over 61+ days. Previous investigations of physiological responses to RTC operational stressors were conducted exclusively in males, leaving female responses uncharacterized. Therefore, this study aimed to identify changes in metabolic, growth, and sex hormones, inflammatory and iron markers in males and females undergoing RTC. Blood samples and body composition assessments (DXA) were collected at the start of RTC (Baseline (BL)) and at the end of each phase (Phase (P)1 - general field, P2 - mountains, P3 - swamp). During each phase, daily urine sampling and measures of energy balance (doubly labeled water method) occurred. Females experienced physiological changes primarily in P2, with reductions in estradiol (mean difference [95% Confidence Interval]: -79.29 [-115.32, -43.26]), progesterone (-1.48 [-2.70, -0.26]), testosterone (-0.74 [-1.43, -0.04]), free testosterone: -12.70 [-23.23, -2.17]), thyroid stimulating hormone (TSH; -0.54 [-1.03, -0.05]), and insulin-like growth factor 1 (IGF-1; -92.58 [-121.28, -63.88]) compared to BL. Males demonstrated significant decreases in free testosterone, TSH, triiodothyronine (T 3 ), IGF-1, hemoglobin, and hematocrit across all three phases compared to BL (all, p < 0.05), with the largest changes in P3 for TSH (-0.66, [-1.21, -0.11]), T 3 (-0.30 [-0.41, -0.20]), IGF-1 (-104.70 [-116.52, -92.88]), hemoglobin (-1.32 [-1.71, -0.94]), and hematocrit (-3.89 [-5.02, -2.75]). Ferritin increased across all phases compared to BL regardless of sex ( p < 0.05). These findings indicate that multi-stressor environments may disproportionately affect male physiological markers, whereas females exhibited fewer, phase-specific changes.