Intravenous versus oral rehydration during a brief period: Stress hormone responses to subsequent exhaustive exercise in the heat

The purpose of this study was to determine if intravenous fluid rehydration, versus oral rehydration, during a brief period (20 min) differentially affects plasma ACTH, cortisol, and norepinephrine concentrations during subsequent exhaustive exercise in the heat. Following dehydration (DHY) to -4% of body weight, 8 nonacclimated highly trained males (age = 23.5 ± 1.2 years, V̇O_2peak = 61.4 ± 0.8 ml · kg · min^-1, % body fat = 13.5 ± 0.6%) cycled to exhaustion at 74% V̇O_2peak in 36.8 °C on three different occasions. These included: (a) no fluid (NF), where no fluid was provided during the rehydration period; (b) DRINK, where oral rehydration (0.45% NaCl) was provided equal to 50% of the prior DHY; and (c) IV, where intravenous infusion (0.45% NaCl) was provided equal to 50% of the prior DHY. Exercise time to exhaustion was not different (p = .07) between the DRINK (34.86 ± 4.01) and IV (29.48 ± 3.50) trials, but both were significantly (p < .05) longer than the NF (18.95 ± 2.73) trial. No differences (p < .05) were found for any of the hormone measures among trials. The endocrine responses at exhaustion were similar regardless of hydration state and mode of rehydration, but rehydration prolonged the exercise time to exhaustion.