Impact of United States Marine Corps Recruit Training on Stress Physiology and the Hypothalamic Pituitary Gonadal Axis in Male Marine Recruits.

INTRODUCTION

Limited data exist on the physiological effects of entry-level military training. This study quantified stress hormones, sex hormones, and sex drive in Marine recruits during Recruit Training (RT) at the Marine Corps Recruit Depot, San Diego.

MATERIALS AND

METHODS

Sixty male recruits (age: 19.0 ± 1.0 years; height: 174.6 ± 7.2 cm; weight: 71.5 ± 10.4 kg; body fat: 12.5% ± 4.9%) -completed pre- and post-RT reproductive health surveys assessing self-reported sex. Saliva samples collected at pre-RT, weeks 1, 4, 7, 10, and post-RT were analyzed for salivary α-amylase (sAA), cortisol, and testosterone. Linear mixed-effects models assessed biomarker changes over time, and the Stuart-Maxwell test evaluated shifts in -sex-drive categories. Logistic regression and LOESS were used to examine whether pre-RT sex drive and testosterone predicted dropout probability, respectively. Statistical significance was set at P < .05. The study protocol was approved by the NHRC Institutional Review Board in compliance with all applicable federal regulations governing the protection of subjects NHRC.2020.0008.

RESULTS

Pre-RT sAA levels exceeded weeks 1 (P = .03), 4 (P < .0001), 7 (P < .0001), and 10 (P = .0002). sAA declined through mid-training and rose post-RT (P < .001 vs. weeks 4-7). Cortisol increased early (weeks 1-4; P < .01), decreased by week 10 (P = .0024), and rebounded post-RT (P < .001). Testosterone peaked at week 4 (P < .05 vs. weeks 1, 10, post-RT) and dropped below pre-RT levels by week 10 (P = .0003). The testosterone-to-cortisol (T: C) ratio was lowest at week 1 (P = .04) and highest at week 10 (P = .0019), then fell post-RT (P = .04).

CONCLUSIONS

Marine recruits experienced elevated physiological strain early in RT, followed by partial adaptation indicated by mid- to late-training stabilization in stress biomarkers. These findings highlight dynamic hypothalamic-pituitary-gonadal axis and stress responses during military training. Further research on proxies of hypothalamic-pituitary-gonadal axis function, such as sex drive and testosterone, as predictive measures of training completion are warranted to elucidate the role reproductive health has in military training attrition.