Ad Libitum Fluid Replacement in Military Personnel during a 4-h Route March

Introduction: Opportunities to determine optimal rates of fluid ingestion could reduce the mass soldiers might need to carry on military missions. Purpose: The first objective was to evaluate the effects of an ad libitum fluid replacement strategy on total body water (TBW), core temperature, serum sodium concentrations [Na+], and plasma osmolality (POsm). The second objective was to determine if an ad libitum water intake was sufficient to maintain these variables during exercise. A third objective was to determine if changes in body mass are an accurate measure of changes in TBW. Methods: A field study was conducted with 15 soldiers performing a 16.4-km route march. The average age of 15 subjects was 27 yr (SD = 4.6 yr). Results: Their mean hourly ad libitum fluid intake was 383 mL (SD = 150 mL). Predicted sweat rate was 626 ± 122 mL·h-1. Despite an average body mass loss of 1.0 kg (SD = 0.50 kg) TBW, POsm and serum [Na+] did not change significantly during exercise. There was a significant (P < 0.05) linear relationship with a negative slope between postexercise serum [Na+] and changes in both body mass and percentage of TBW. Postexercise POsm and serum [Na+] were significantly related (P < 0.05). Higher postexercise percentage of TBW was associated with lower postexercise POsm and serum [Na+] levels. There was no relation between percent body mass loss and postexercise core temperature (38.1°C ± 0.6°C). Conclusions: A mean ad libitum water intake of 383 mL·h-1, replacing approximately 61% of body mass losses during 4 h of exercise, maintained TBW, core temperature, POsm, and serum [Na+] despite a 1.4% body mass loss. A reduction in body mass of 1.4% (1.0 kg) was not associated with a reduction in TBW.