Precooling via immersion in CO 2 -enriched water at 25°C decreased core body temperature but did not improve 10-km cycling time trial in the heat.

This study compared the effects of precooling via whole-body immersion in 25°C CO 2 -enriched water (CO 2 WI), 25°C unenriched water (WI) or no cooling (CON) on 10-km cycling time trial (TT) performance. After 30 min of precooling (CO 2 WI, CON, WI) in a randomized, crossover manner, 11 male cyclists/triathletes completed 30-min submaximal cycling (65%VO 2peak ), followed by 10-km TT in the heat (35°C, 65% relative humidity). Average power output and performance time during TT were similar between conditions ( p  = 0.387 to 0.833). Decreases in core temperature (T core ) were greater in CO 2 WI (-0.54 ± 0.25°C) than in CON (-0.32 ± 0.09°C) and WI (-0.29 ± 0.20°C, p  = 0.011 to 0.022). Lower T core in CO 2 WI versus CON was observed at 15 th min of exercise ( p  = 0.050). Skin temperature was lower in CO 2 WI and WI than in CON during the exercise ( p  < 0.001 to 0.031). Only CO 2 WI (1029 ± 305 mL) decreased whole-body sweat loss compared with CON (1304 ± 246 mL, p  = 0.029). Muscle oxygenation by near-infrared spectroscopy (NIRS), thermal sensation, and thermal comfort were lower in CO 2 WI and WI versus CON only during precooling ( p  < 0.001 to 0.041). NIRS-derived blood volume was significantly lower in CO 2 WI and WI versus CON during exercise ( p  < 0.001 to 0.022). Heart rate ( p  = 0.998) and rating of perceived exertion ( p  = 0.924) did not differ between conditions throughout the experiment. These results suggested that CO 2 WI maybe more effective than WI for enhanced core body cooling and minimized sweat losses.