Baseline Testosterone Stratification Reveals Divergent Hormonal Adaptations in Men and Women Undergoing Basic Combat Training

This study examined whether hormonal and body composition responses to 10-weeks of U.S. Army Basic Combat Training (BCT) differ by baseline testosterone and sex. Methods: Male (n=97) and female (n=105) trainees were stratified at baseline as having LOW (men <10.4 nmol/L; women <0.82 nmol/L) or NORMAL baseline testosterone. We measured serum total and free testosterone, sex hormone binding globulin (SHBG), luteinizing hormone (LH) and estradiol (E2) at weeks 0, 2, 4, 6, 8, and 9 and cortisol weekly; dual-energy X-ray absorptiometry (DXA) assessed body composition at week 0 and 9. Linear mixed models tested time and group effects; generalized linear models related baseline testosterone status and testosterone area under the curve with changes in body composition. Results: In LOW women, testosterone increased at each timepoint (all p<0.01), reaching concentrations 53% higher at week 9 compared to week 0 (30.2± 7.8 vs 19.7± 3.2 ng/dl). In men, testosterone increased (LOW: +76%; NORMAL: +23%) across BCT (all p<0.05), except at week 2. Among LOW men, concentrations stabilized by week 4 (344.8± 81.4 ng/dl) and remained normal. Cortisol was highest at baseline and declined in all men and in NORMAL women. LH and E2 remained stable in both sexes, while SHBG declined only in women, independent of baseline testosterone. Neither baseline testosterone nor cumulative exposure were associated with changes in lean or fat mass. Conclusions: SHBG and cortisol trajectories differed by sex and baseline testosterone status. Testosterone changes were sex specific, reflecting an anabolic training environment in men and modest increased androgen availability in LOW women.