Fluid shifts resulting from exercise in rats as detected by bioelectrical impedance

Purpose: This study was designed to investigate the immediate effect of exe rcise intensity and duration on body fluid volumes in rats throughout a 3-w k exercise program. Methods: Changes in the extracellular water (ECW) and t otal body water (TBW) volumes of rats were measured preexercise and postexe rcise using multiple frequency bioelectrical impedance analysis. Groups of rats were exercised at two intensities (6 m.min(-1) and 12 m.min(-1)) for t wo exercise times (60 min and 90 min) 5 d.wk(-1) during a 3-wk period. Chan ges in plasma electrolytes, glucose, and lactate resulting from the exercis e were also measured on 3 d of each week. Results: Each group of animals sh owed significant losses in ECW and TBW as a direct result of daily exercise . The magnitude of fluid loss was directly related to the intensity of the exercise, bur not to exercise duration; although the magnitude of daily flu id loss at the higher intensity exercise (12 m.min(-1)) decreased as the st udy progressed, possibly indicating a training effect. Conclusion: At low-i ntensity exercise, there is a small bur significant loss in both TBW and EC W fluids, and the magnitude of these losses does not change throughout a 3- wk exercise program. At moderate levels of exercise intensity, there is a g reater loss of both TBW and ECW fluids. However, the magnitudes of these lo sses decrease significantly during the 3-wk exercise program, thus demonstr ating a training effect.