Body weight, the heart rate, and the plasma volume during a high-intesity, long-duration run

Abstract

Objetive: to determine the amount of ad libitum intake of three rehydrating beverages of different osmolalities, and to establish their effects on the percentage of body weight loss, the increase of heart rate and the reduction of plasma volume, in nine subjects with aerobic training, during a highintensity,long-duration run.

Methodology: the experiment was carried out on a treadmill with 1% of inclination that was kept constant throughout the procedure; it started with 9 minutes of warm-up at a speed equivalent to 59% of the reserve heart rate; then, 88 minutes of running divided in three intervals (29, 29, and 30 minutes) at 80% of the reserve heart rate; finally, 90 minutes of recovery in three intervals of 30 minutes each. During the “dehydrated treatment” no fluid replacement was done; during the “Hydrating treatments” (H-I, H-II, H-III) equivalent volumes were drunk of the aforementioned rehydrating beverages.

Results: during the exercise period the fluid loss was 1.440 ml/h. In each one of the treatments there was a loss of body weight (p<0.001), an increase of heart rate (<0.05) and, except for HII and H-III, a reduction of plasma volume (p<0.05). There were no differences between the treatments in these three parameters.

Conclusions: ad libitum intake was less than the amount internationally recommended; probably that explains that the effects of osmolality of the rehydrating beverages on the analyzed parameters could not be determined. These findings ought to motivate a review of the hydrating practices of our athletes because, seemingly, they are not ingesting an adequate amount of fluids to obtain the benefits attributed to hydration.