Ad libitum vs. restricted fluid replacement on hydration and performance of military tasks

The primary objective was to evaluate the effect of ad libitum vs. restricted fluid replacement protocol on hydration markers and performance in selected military tasks. The secondary objective was to determine if 300 ml x h(-1) could be considered a safe minimum fluid intake under the experimental conditions.

Data were collected simulating a route march over 16 km. There were 57 subjects who participated in the study.

The mean pre-exercise body mass of the ad libitum group was 70.4 +/- 13.3 (SD) kg compared to 69.3 +/- 8.9 kg in the restricted group. The mean total fluid intake of the ad libitum group was 2.1 +/- 0.9 L compared to 1.2 +/- 0.0 L in the restricted group. The ad libitum and restricted intake groups, respectively, lost a mean of 1.05 kg +/- 0.77 (1.5%) and 1.34 kg +/- 0.37 (1.9%). Calculated sweat rate was 608 +/- 93 ml x h(-1) compared to 762 +/- 162 ml x h(-1) in the ad libitum group.

There were no significant differences for either urine specific gravity (USG) or urine osmolality (UOsm) before or after the exercise. It is not clear whether fluid intake and calculated sweat rates are causally related or explained by their codependence on a third variable; for example, the exercising metabolic rate. Thus, 300 ml x h(-1) intake could be considered a current safe minimum water intake for soldiers of similar mass under similar experimental conditions, namely similar exercise durations at equivalent exercise intensities in a moderate, dry climate.