The Association Between Energy Balance and Body Composition Changes in Recruits During Marine Corps Basic Training

Background/Objectives: United States Marine Corps (USMC) recruit training (“boot camp”) is a physically demanding environment in which recruits often experience significant changes in body composition. Energy balance (EB) plays a critical role in shaping these outcomes, yet how EB is associated with longitudinal fat and muscle changes remains unclear. This study evaluated the relationship between periodic EB and changes in body composition in male and female recruits. Methods: One hundred eighty-two USMC recruits (122 females, 60 males) volunteered; 158 recruits (106 females, 52 males) were included in the analysis. Energy intake was assessed on 10 distinct sampling days, energy expenditure was tracked via wearable devices, and body composition was obtained using bioelectrical impedance analysis. Statistical analyses included t-tests to compare sex differences in EB and general linear models to assess associations between EB, sex, and changes in body composition. Results: Across seven sampled days, females experienced periods of greater (more negative) daily energy deficit (−649 ± 775 kcal/day, 17%) compared to males (−388 ± 673 kcal/day, 8%, bootstrap p = 0.035). More negative estimated EB was associated with greater fat mass loss, with no evidence that this association differed by sex. In the 7-day model, the association between estimated EB and skeletal muscle mass differed by sex (sex × EB interaction, bootstrap p = 0.015); this interaction was not significant in the 10-day model that included the Crucible (bootstrap p = 0.792). Conclusions: USMC recruits experienced periods of low-moderate energy deficits during boot camp. Estimated EB during sampled training days was associated with fat mass change, whereas the association with skeletal muscle mass change differed by sex only in the 7-day model.