Adequacy of food rations in soldiers during exercise in hot, day-time conditions assessed by doubly labelled water and energy balance methods

Abstract

The energy requirements of people doing physical work in hot climates are not clearly understood. In particular, we know little about the combined effects of heat stress and muscular work on energy requirements. During military exercises in the African bush soldiers are supplied with standard rations, the adequacy of which is unknown. We have now assessed the adequacy of these food and water rations in 12 male Zimbabwean soldiers during 12 days of strenuous, heat-stress exercise in the field. We used two methods to measure energy expenditure: the double-labelled water method (DLW) and the energy balance method (i.e. comparing dietary energy with changes, if any, in body energy stores). Two groups were studied: one group (eight subjects) carried out field exercises; the control group consisted of four soldiers doing normal work. Mean daily energy expenditure as assessed by the DLW method was [mean (SE)] 23 (1.5) MJ · day⁻¹ for the field group and 14 (0.5) MJ · day⁻¹ for the control group (P<0.001). By the energy balance method, daily energy expenditure was calculated to be 26 (0.7) MJ · day⁻¹ and 15.5 (0.4) MJ · day⁻¹ for the field group and control group, respectively. Body mass loss was 3 (0.1) kg [4.6 (0.3)% of body mass] for the field group, but the control group gained 1.1 (0.1) kg. Mean daily fluid intake was 11 (0.5) 1 · day⁻¹, suggesting that the standard ration supplied was inadequate. Body mass loss was caused by both the energy deficit and total body water loss. These results suggest strenuous work in hot, dry field conditions imposes extra energy requirements.