Limited Sex Differences in Energy Balance and Body Composition during 61+D of US Army Ranger Training

Introduction: Negative energy balance (Ebal) and changes in body composition are common in Soldiers during combat training with increased total daily energy expenditure (TDEE) and restricted food intake. Whether sex differences exist in those effects is unknown. Methods: Ebal and body composition (dual-energy X-ray absorptiometry; body mass (BM), fat mass (FM), fat-free mass (FFM)) of Soldiers (10 females:27 males) completing US Army Ranger training course (RTC) were compared by sex before baseline and after each training phase (P1-P3). Doubly labeled water was used to estimate TDEE, food wrappers collection to assess dietary energy intake, and daily activity energy expenditure was calculated. Results: Baseline BM and FFM were lower ( p < 0.01) in females than males, while baseline FM was higher in females than males ( p = 0.01). Body composition differences remained at each phase, except at P3 when no sex difference in FM ( p = 0.15) was detected. Males experienced a greater loss in BM from baseline than females at each phase (all, p ≤ 0.05). Females maintained their FFM to a greater extent from baseline than males at each phase (all, p ≤ 0.01). TDEE was higher in males compared to females in P1 and P2 (both, p < 0.01), but not in P3 ( p = 0.37). Females showed a greater increase in dietary intake from P1 to P2 ( p = 0.004) compared to males. No differences in the change from P1 to P2 or P1 to P3 were detected for TDEE, Ebal, resting energy expenditure or activity energy expenditure. Conclusions: Despite differences in absolute Ebal, the magnitude of change in Ebal did not differ by sex across RTC. Males lost more BM throughout the long-term course while females preserved FFM to a greater extent than males.