The effect of acute exercise on body density, body volume, and plasma volume

In order to determine the relationship between reductions in body weight, body volume, and plasma volume, 22 track runners underwent exercises of various intensities and durations. The average mileage performed in the distance runs was 8.4 miles. The average caloric cost was 822 and 288 Cal for the distance and sprint activities, respectively. The overall range of energy expenditure was 100 to 1000 Cal. Before and after the workbout, body volume and density were determined by hydrostatic weighing on each subject. In addition, venous blood samples were analyzed for hematocrit (Hct) and hemoglobin (Hb), and the percent change in plasma volume (% delta PV) was calculated. Body weight and body volume decreases occurred in all subjects. Body density, percent body fat, and plasma volume were significantly altered only in the group of athletes undergoing distance runs. Body weight loss ranged from 0.3 kg to 2.0 kg and was correlated with body volume decreases (r = 0.91) and energy expended (r = 0.73). Body volume changes were related to the extent of density increases (r = 0.58), changes in plasma volume (r = 0.53), and energy enpended (r = 0.75). The % delta PV was significantly related only to changes in body volume. The results of this study indicate that acute exercise does affect body volume and thus the density and % fat determinations. These changes do not parallel closely the changes in plasma volume. The data suggest that for absolute accuracy, the state of hydration of the subject should be controlled prior to hydrostatic weighing. However, even in dehydration caused by prolonged distance running, the density measurement error is small and nearly within the methodological error of the hydrostatic weighing technique.