Fluid-regulating factors during rehydration with glycerol: Effects on subsequent exercise performance in a hot $\rm (36\sp\circ C)$ environment

The purpose of this investigation was to examine the effect of rehydration with glycerol and to study the cardiovascular, thermoregulatory and endocrine responses during exercise following the rehydration. Eight endurance-trained male cyclists participated in the study (age: 24 $\pm$ 1 yr, height: 181 $\pm$ 2 cm, weight: 70.1 $\pm$ 1.0 kg, and VO$\sb2$peak: 61.4 $\pm$ 0.8 ml$\rm\cdot kg\sp{-1}\cdot min\sp{-1}).$ Each subject completed three dehydration-rehydration experimental trials which differed only with regard to the rehydration treatment. Prior to each experimental day, subjects dehydrated to $-$4% of their body weight. The rehydration treatments were: (a) no fluid (NF); (b) glycerol (G) bolus (1g/kg), followed by water; and (c) water alone (W). The fluid administration (3% body weight) was given over an 80-min period. Thirty minutes after the rehydration subjects cycled (74% VO$\sb2$peak) to exhaustion in a hot (36.8$\sp\circ$C) environment. Blood samples were taken before and after the rehydration treatment as well as at the end of the exercise test for analyses of fluid-regulating and stress hormones. Vasopressin and plasma renin activity decreased during rehydration in the W and G trials, but no differences were found between trials. Plasma osmolality was increased during the G trial compared to the W trial throughout the experiment. For the G trial plasma volume expanded (p $\u3c$ 0.05) during rehydration and it was remained elevated (p $\u3c$ 0.05) during exercise. Exercise time to exhaustion was longest (p $\u3c$ 0.05) during the G trial (32.5 $\pm$ 3.8 min) compared to both W (27.1 $\pm$ 3.3 min) and NF (18.9 $\pm$ 2.7 min) trials. Rehydration with glycerol did not significantly alter fluid balance. Cutaneous vascular conductance was found to be significantly elevated (p $\u3c$ 0.05) during the G trial, but glycerol administration had no other thermoregulatory or cardiovascular benefit compared to the W. All fluid-regulating and stress hormones were increased during exercise (with the exception of AP), but no differences were found between the G and W trials. These data suggest that glycerol may not have any direct effect on fluid regulating or stress hormones during rehydration or exercise. Additionally, rehydration with the glycerol solution improved exercise time to exhaustion, which was probably driven by the maintenance of greater plasma volume during exercise.