Fluid ingestion is more effective in preventing hyperthermia in aerobically trained than untrained individuals during exercise in the heat

Abstract

It is unclear if fluid ingestion during exercise in the heat alleviates the thermoregulatory and cardiovascular strain similarly in aerobically trained and untrained individuals. It is also unknown at what exercise intensity the effects of rehydration are greater. Ten aerobically trained (T) and 10 healthy untrained (UT) subjects ([Formula: see text]O(2peak), 60 ± 6 vs. 44 ± 3 mL O(2)·kg(-1)·min(-1), respectively; P < 0.05) pedalled in a hot, dry environment (36 ± 1 °C; 25% ± 2% relative humidity; airflow, 2.5 m·s(-1)) at 40%, 60%, and 80% [Formula: see text]O(2peak) while ingesting fluids (Fluid). The results were compared with those from our previous study [Mora-Rodriguez et al., Eur. J. Appl. Physiol. 109(5): 973-981 (2010)] with no fluid ingestion (No Fluid). Subjects were not heat-acclimated. At 40% [Formula: see text]O(2peak), Fluid reduced rectal temperature (T(RE)) in T and UT (0.31 ± 0.08 and 0.32 ± 0.07 °C; respectively). At 60% [Formula: see text]O(2peak), Fluid reduced T(RE) in T more than in UT (0.30 ± 0.10 °C vs. 0.18 ± 0.10 °C; P < 0.05) but had no effect at 80% [Formula: see text]O(2peak) in any group. At similar relative intensity, heart rates (HR) were similar between groups. Fluid lowered heart rate (i.e., HR) similarly in the T and UT at 40% and 60% [Formula: see text]O(2peak) (11% and 6%, respectively; P < 0.05) but not at 80% [Formula: see text]O(2peak) (P > 0.05). At similar metabolic heat production (i.e., 60% for T vs. 80% [Formula: see text]O(2peak) for UT), Fluid lowered T(RE) only in the T individuals (P < 0.05). In summary, rehydration during low- and moderate-intensity exercise reduces T(RE) and HR more than during high-intensity exercise (80% [Formula: see text]O(2peak)) in T and UT subjects. Fluid replacement is more effective on preventing the rise in T(RE) in T than in UT individuals during moderate-intensity exercise (60% [Formula: see text]O(2peak)), as well as when exercising at a similar heat production rate.