METABOLIC, CATECHOLAMINE, AND EXERCISE PERFORMANCE RESPONSES TO VARIOUS DOSES OF CAFFEINE

This study examined the exercise responses of well-trained endurance a thletes to various doses of caffeine to evaluate the impact of the dru g on exercise metabolism and endurance capacity. Subjects (n = 8) with drew from all dietary sources of caffeine for 48 h before each of four tests. One hour before exercise they ingested capsules of placebo or caffeine (3, 6, or 9 mg/kg), rested quietly, and then ran at 85% of ma ximal O-2 consumption to voluntary exhaustion. Blood samples for methy lxanthine, catecholamine, glucose, lactate, free fatty acid, and glyce rol analyses were taken every 15 min. Plasma caffeine concentration in creased with each dose (P < 0.05). Its major metabolite, paraxanthine, did not increase between the 6 and 9 mg/kg doses, suggesting that hep atic caffeine metabolism was saturated. Endurance was enhanced with bo th 3 and 6 mg/kg of caffeine (increases of 22 +/- 9 and 22 +/- 7%, res pectively; both P < 0.05) over the placebo time of 49.4 +/- 4.2 min, w hereas there was no significant effect with 9 mg/kg of caffeine. In co ntrast, plasma epinephrine was not increased with 3 mg/kg of caffeine but was greater with the higher doses (P < 0.05). Similarly only the h ighest dose of caffeine resulted in increases in glycerol and free fat ty acids (P < 0.05). Thus the highest dose had the greatest effect on epinephrine and blood-borne metabolites yet had the least effect on pe rformance. The lowest dose had little or no effect on epinephrine and metabolites but did have an ergogenic effect. These results are not co mpatible with the traditional theory that caffeine mediates its ergoge nic effect via enhanced catecholamines.