Water Intake and Urine Output During a 194-Kilometre Unsupported Desert March
Authors: V.R. Nevola, M.A. Stroud OBE, Lt Col. J.J. Turner RE and W.R. Withey OBE
British troops are required to deploy rapidly to environments that expose them to high levels of heat stress, with limited natural sources of water upon which to survive. Potable water must therefore be supplied by military logistics to reduce the risk of heat illness and degraded physical and mental performance caused by dehydration. Mathematical models can be used to predict water requirements for desert operations, but they often provide estimates that exceed military guidelines (15 litres per man per day) [1] [2]. This study measured the amount of water consumed during a simulated military operation in a hot, desert environment to compare the need with military guidelines [1] [3]. Methods: 5 men (mean (1 SD) aged 41.8 (9.1) years; body mass 81.1 (6.8) kg; body fat (bioimpedance) 17.8 (3.1) %) consented to participate in this Ethics Committee approved study. A 194-km desert march was conducted along the length of Qatar. Mean hourly dry-bulb temperature during the march was 25 (4)°C, and relative humidity averaged 48 (17) %. All water, food (high-energy bars and High5 carbohydrate-electrolyte energy source) and equipment was transported in two, 2-wheeled carts (total load including carts was 320 kg). The rate of oxygen uptake when pulling the carts in temperate conditions in the UK was measured using a portable expired gas analyser (Cosmed K4b2). This rate was then used during cycle ergometer exercise in controlled conditions (dry-bulb and globe temperatures 40°C; relative humidity 50%; airspeed 1ms-1) to estimate the amount of water needed for the expected 60 hour march. A 50 litre per person water budget was allowed. Following a 6-day heat acclimatisation phase in Qatar, subjects began the march hydrated. Body mass was recorded every 6 hours. The mass of water and food intake, urine and faeces output was recorded throughout the march. Other measurements were: heart rate (HR) (every 15s, from which HRreserve was calculated); gastrointestinal temperature (Tgi) (every 10s); and ankle and wrist activity (every 2s). Total body water (TBW) and body fat (%) were recorded by bioimpedance immediately pre- and post-march. Paired data were compared using a t-test. Results: Four of five subjects completed the march in 78 hours. Physical activity (mean relative intensity 58 (7) % of HRreserve) was recorded during 74 of the 78 hours. A total of 46.0 (10.5) MJ were consumed. Water intake was 34.7 (6.4) litres; 2.9 (0.6) litres of urine were excreted. TBW increased by 0.3 (0.4) litres (range of 0.8 to 1.5%) compared with pre-march values (p<0.05). Mean Tgi was 38.1 (0.6)°C; occasionally it rose above 39.3°C. Fat mass declined (p<0.05) by 3.4 (1.0) kg, accounting for the loss in total body mass of 3.3 (1.6) kg. Conclusions: Daily consumption of up to 11 litres of water per man was sufficient to maintain hydration when working at up to 58% HRreserve during this prolonged, self-sufficient desert march. These data support the existing military guidelines for the provision of potable water [18].