Energy Expenditure During Military Operations in A Hot, Dry Environment

Quantifying the energy cost of operational tasks is a pre-requisite for setting an appropriate calorific value provided by operational ration packs (ORP). Despite this fact, the energy requirements of free-living individuals undertaking extended, physically demanding tasks under arduous or extreme environmental conditions remains to be fully quantified. PURPOSE To measure energy expenditure during arduous work in a hot, dry environment. METHODS Sixteen healthy men participated in a 7-day Military Collective Training Exercise (CTX) in Kenya, consisting of 2 phases, CTX 1 (days 1–3) and CTX 2 (days 4–7). Total daily energy expenditure (TDEE) during the CTX was measured from daily urine samples using the doubly-labelled water method (2H218O; DLW (n = 8)). Indices of hydration status (urine osmolality, urine specific gravity (Usg), urine colour) and body mass (BM) were assessed daily (n = 12). Local environmental heat stress (Wet Bulb Globe Temperature (WBGT)) was recorded hourly. Uniaxial accelerometers were used to monitor physical activity levels. Data were analysed using repeated measures ANOVA. Significance was accepted at the alpha = 0.05 level. RESULTS Mean (1 SD) daily WBGT during the CTX was 23.9 (2.3) oC. Mean drybulb temperature was 28.1 (6.9) oC. Mean globe temperature was 41.5 (3.8) oC. TDEE was 16.1 (2.0) MJ · day-1 (3833 (470) kcal · day-1). During CTX 2, TDEE was 27.5 (3.5) MJ · day-1 (6547 (829) kcal · day-1), which approaches the energetic ceiling proposed by Westerterp et al. (1986). Subjects were physically active for 48 (11) % of the period of the CTX, with a mean activity count of 334 (156) counts · min−1. Mean total activity was during CTX 2 was significantly greater than during CTX 1 (P <0.05). Urinary markers of hydration status did not change during the CTX. However, Usg was 0.128 at the start of the CTX, which is indicative of hypohydration. No changes in BM were found. CONCLUSIONS Simulated military operations in a hot, dry environment were associated with a moderate to high TDEE and physical activity count. TDEE was less than the energy content provided by In-Service UK Forces hot climate ORP. This work was funded by the Human Sciences Domain of the UK Ministry of Defence Scientic Research Programme.