Change in body mass accurately and reliably predicts change in body water after endurance exercise
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This study tested the hypothesis that the change in body mass (ΔBM) accurately reflects the change in total body water (ΔTBW) after prolonged exercise. Subjects (4 men, 4 women; 22–36 year; 66 ± 10 kg) completed 2 h of interval running (70% VO2max) in the heat (30°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 ΔBM and ΔTBW was 0.07 ± 1.07 kg (paired t test, P = 0.29). The slope and intercept of the relation between ΔBM and ΔTBW were not significantly different from 1 and 0, respectively. The intraclass correlation coefficient between ΔBM and ΔTBW was 0.76, which is indicative of excellent reliability between methods. Measuring pre- to post-exercise ΔBM is an accurate and reliable method to assess the ΔTBW.
KeywordsHydration status Prolonged exercise Fluid replacement Deuterium oxide Fourier transform infrared spectroscopy
The authors are grateful to the subjects for their participation in this study. Additionally, we thank Cynthia Bartok for her advice on TBW procedures and calculations, Josh Stapleton for his assistance with FTIR procedures, Mosuk Chow and Dennis Passe for their statistical consultation, Jane Pierzga, John Jennings, Matt Kenney, Jose Flores, Ben Miller, Doug Johnson, and Randy McCullough for their technical assistance, and the General Clinical Research Center nursing staff for their medical support. Support for this study was provided by the Gatorade Sports Science Institute and the General Clinical Research Center Grant MO1 RR010732.
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