Change in body mass accurately and reliably predicts change in body water after endurance exercise

This study tested the hypothesis that the change in body mass (Delta BM) accurately reflects the change in total body water (Delta TBW) after prolonged exercise. Subjects (4 men, 4 women; 22-36 year; 66 +/- A 10 kg) completed 2 h of interval running (70% VO2max) in the heat (30A degrees C), followed by a run to exhaustion (85% VO2max), and then sat for a 1 h recovery period. During exercise and recovery, subjects drank fluid or no fluid to maintain their BM, increase BM by 2%, or decrease BM by 2 or 4% in separate trials. Pre- and post-experiment TBW were determined using the deuterium oxide (D2O) dilution technique and corrected for D2O lost in urine, sweat, breath vapor, and nonaqueous hydrogen exchange. The average difference between Delta BM and Delta TBW was 0.07 +/- A 1.07 kg (paired t test, P = 0.29). The slope and intercept of the relation between Delta BM and Delta TBW were not significantly different from 1 and 0, respectively. The intraclass correlation coefficient between Delta BM and Delta TBW was 0.76, which is indicative of excellent reliability between methods. Measuring pre- to post-exercise Delta BM is an accurate and reliable method to assess the Delta TBW.