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ENVIRONMENTAL PHYSIOLOGY

No effect of nutritional adenosine receptor antagonists on exercise performance in the heat

¹U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts; ²Ernest Mario School of Pharmacy, Rutgers-The State University of New Jersey, Piscataway, New Jersey; and ³Pennington Biomedical Research Center, Baton Rouge, Louisiana

Submitted 6 October 2008 ; accepted in final form 18 November 2008

Nutritional adenosine receptor antagonists can enhance endurance exercise performance in temperate environments, but their efficacy during heat stress is not well understood. This double-blinded, placebo-controlled study compared the effects of an acute dose of caffeine or quercetin on endurance exercise performance during compensable heat stress (40°C, 20–30% rh). On each of three occasions, 10 healthy men each performed 30-min of cycle ergometry at 50% O_2peak followed by a 15-min performance time trial after receiving either placebo (Group P), caffeine (Group C; 9 mg/kg), or quercetin (Group Q; 2,000 mg). Serial blood samples, physiological (heart rate, rectal, and mean skin body temperatures), perceptual (ratings of perceived exertion, pain, thermal comfort, motivation), and exercise performance measures (total work and pacing strategy) were made. Supplementation with caffeine and quercetin increased preexercise blood concentrations of caffeine (55.62 ± 4.77 µM) and quercetin (4.76 ± 2.56 µM) above their in vitro inhibition constants for adenosine receptors. No treatment effects were observed for any physiological or perceptual measures, with the exception of elevated rectal body temperatures (0.20–0.30°C; P < 0.05) for Group C vs. Groups Q and P. Supplementation did not affect total work performed (Groups P: 153.5 ± 28.3, C: 157.3 ± 28.9, and Q: 151.1 ± 31.6 kJ; P > 0.05) or the self-selected pacing strategy employed. These findings indicate that the nutritional adenosine receptor antagonists caffeine and quercetin do not enhance endurance exercise performance during compensable heat stress.

caffeine; quercetin; thermoregulation; central fatigue; dietary supplements

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