Sex Differences In 61d Sustained Exercise And Food Deprivation: Negative Energy Balance In Female And Male U.S. Army Rangers

Background: The U.S. Army Ranger Training Course (RTC) is a 61d physically and mentally challenging training that introduces three distinct field training environments (P1: general field; P2: mountains; P3: swamp) and as such provides a controlled environment to assess long-term negative energy balance across sex. Purpose: To quantify total energy expenditure (TEE), energy intake (EI), energy balance (Ebal) and changes in body composition in male and female Soldiers during RTC training. METHODS: TEE (kcal/d; measured by doubly labeled water), EI (kcal/d; ration food wrapper collection and ration-specific nutrient database), Ebal (mean difference, F-M; TEE-EI) and body composition (dual-energy X-ray absorptiometry) were measured in 37 RTC graduates (mean ± SD; 10F, 27 ± 3y, 69.3 ± 6.5 kg; 27 M, 25 ± 4y, 82.7 ± 9.0 kg). Linear regression models adjusted for age and baseline body mass (BM) assessed sex differences within phase and linear mixed models adjusted for sex, age, and BM assessed overall and sex differences by phase. An interaction term between phase and sex assessed sex differences over time with multiple imputation when appropriate. RESULTS: Over the 61d RTC all Soldiers were in absolute negative energy balance (mean ± SE; -342 ± 225) and lost BM (kg) (-3.7 ± 0.4 F; -5.4 ± 0.5 M; p < 0.05). Adjusted TEE differed by sex in P1 (-822 ± -322; p = 0.02) but not across RTC (P1-3; p = 0.08). Differences by sex in EI were observed at P2 (331 ± 117; p = 0.01) and P3 (223 ± 106; p = 0.04), however not across RTC (p = 0.4). Ebal differed by sex at P2 (1089 ± 460; p = 0.03) with no sex difference across RTC (342 ± 225; p = 0.1). Changes in body composition across RTC differed by sex for fat-free mass, males lost more than females by phase (p ≤ 0.01) and across RTC (p = 0.01). Fat mass loss did not differ by sex across phase or RTC. CONCLUSION: Female RTC graduates maintained a similar negative energy balance to male counterparts despite differences in energy flux and body composition. Disclaimer: Funded by USAMRDC; Authors views do not represent the Army or DoD.