Sex-differences In Body Composition Changes During Marine Corps Officer Candidates School

Individuals participating in military training are subject to physical training that can affect body mass and composition, which may be dependent on sex, and relate to risk for musculoskeletal injury (MSKI) and/or attrition. PURPOSE: To examine changes in body composition between sexes during Marine Corps Officer Candidates School (OCS) and potential associations with MSKI and attrition. METHODS: 28 male (25.9 ± 0.7 yr; 25.7 ± 0.4 kg/m2) and 27 female (25.3 ± 0.6 yr; 23.5 ± 0.4 kg/m2) candidates completed bioelectrical impedance analysis (BIA) at pre-testing; 17 males and 11 females completed BIA after 10-weeks of OCS. Two-way repeated measures ANOVAs were performed on body mass (BM), lean mass (LM), percent body fat (%BF), fat mass (FM), fat mass index (FMI), and lean mass index (LMI). Independent t-tests compared baseline measures between those that completed training (n = 35) vs. not (n = 20), and those that reported an MSKI (n = 25) vs. no-MSKI (n = 28); ANCOVAs adjusted for sex. Data are mean ± SEM; α = 0.05. RESULTS: BM decreased (main effect time: p = 0.008; 74.4 ± 1.6, 72.3 ± 1.5 kg) during OCS. Similarly, FM (14.0 ± 0.7, 9.5 ± 0.4 kg, p < 0.001), and FMI decreased over time (4.8 ± 0.3, 3.3 ± 0.2, p < 0.001). An interaction effect was observed for %BF (p = 0.008), which decreased in males (15.1 ± 1.1, 11.1 ± 0.8%, p < 0.001) and females (23.6 ± 1.4, 15.9 ± 0.9%, p < 0.001). Interaction effects were observed for LM (p = 0.002), which increased in females (50.0 ± 2.1, 54.3 ± 2.2 kg, p < 0.001) but not males (70.6 ± 1.7, 71.3 ± 1.8 kg, p = 0.336), and LMI (p < 0.001), which increased in females (18.1 ± 0.5, 19.4 ± 0.5 kg/m2, p < 0.001), but not males (22.3 ± 0.4, 22.5 ± 0.4 kg/m2, p = 0.418). BM (75.9 ± 1.9, 66.1 ± 2.6 kg, p = 0.003), LM (62.6 ± 2.0, 52.6 ± 2.5 kg, p = 0.003) and LMI (20.5 ± 0.4, 18.9 ± 0.4 kg/m2, p = 0.015) were greater in those that completed training vs. those that did not. LM was lower in those that reported an MSKI vs. those that did not (54.7 ± 2.3, 61.8 ± 2.3 kg, p = 0.037). Differences between MSKI and attrition groups were not significant after adjusting for sex. CONCLUSION: Despite similar reductions in body mass and fat mass, lean mass only increased in women. These results are a model of body composition changes following early career military training and aligns with the Marine Corps’ efforts to incorporate advanced methodologies for assessing body composition. ONR N00014-21-1-2725