Protection of total body water content and absence of hyperthermia despite 2% body mass loss (‘voluntary dehydration’) in soldiers drinking ad libitum during prolonged exercise in cool environmental conditions

The extent to which humans need to replace fluid losses during exercise remains contentious despite years of focused research. The primary objective was to evaluate ad libitum drinking on hydration status to determine whether body mass loss can be used as an accurate surrogate for changes in total body water (TBW) during exercise. Data were collected during a 14.6-km route march (wet bulb globe temperature of 14.1°C ). 18 subjects with an average age of 26±2.5 (SD) years participated. Their mean ad libitum total fluid intake was 2.1±1.4 litres during the exercise. Predicted sweat rate was 1.289±0.530 l/h. There were no significant changes (p>0.05) in TBW, urine specific gravity or urine osmolality despite an average body mass loss (p