Purpose: This study sought to determine the effect of rapid intravenous (IV) versus oral (ORAL) rehydration immediately after dehydration, on cardiovascular, thermoregulatory, and perceptual responses during subsequent exercise in the heat.
Methods: Eight males (21.4 ± 0.7 yr; 176.2 ± 1.6 cm; 75.2 ± 3.7 kg; 63.7 ± 3.6 mL·kg−1·min−1 V˙O2max, 9.0 ± 1.7% fat) participated in three randomized trials. Each trial consisted of a 75-min dehydration phase (36°C; 42.5% rh, 47 ± 0.9% V˙O2max) where subjects lost 1.7 L (IV and no-fluid (NF) trials) to 1.8 L of fluid (ORAL trial). In the heat, fluid lost was matched with 0.45% saline in 20 min by either IV or ORAL rehydration; no fluid was given in the NF trial. Subjects then performed a heat-tolerance test (HTT; 37.0°C, 45% rh, treadmill speed of 2.4 m·s−1, 2.3% grade) for 75 min or until exhaustion (Tre of 39.5°C). During the HTT, thermal and thirst sensations, RPE, rectal temperature (Tre), heart rate (HR), and mean weighted skin temperature (Tsk) were measured.
Results: Plasma volume in the IV treatment was greater (P < 0.05) after rehydration compared with ORAL and NF. However, during the HTT there were no overall differences (P > 0.05) in HR, Tre, Tsk, RPE, thermal sensations, or HTT time (ORAL, 71 ± 8 min; IV, 73 ± 5 min; NF, 39 ± 29 min) between the ORAL and IV treatments. Sensations of thirst were lower (P < 0.05) in ORAL compared with IV and NF, likely because of oropharyngeal stimuli.
Conclusions: Despite a more rapid restoration of plasma volume, IV rehydration was not advantageous over ORAL rehydration in regards to physiological strain, heat tolerance, RPE, or thermal sensations.