Substrate Metabolism and Exercise Performance with Caffeine and Carbohydrate Intake

1) To investigate the effect of caffeine on exogenous carbohydrate (CHO) oxidation and glucose kinetics during exercise; and 2) to determine whether combined ingestion of caffeine and CHO enhanced cycling performance compared with CHO alone. Ten endurance-trained cyclists performed three experimental trials consisting of 105 min steady-state (SS) cycling at 62% VO2max followed by a time trial (TT) lasting approximately 45 min. During exercise, subjects ingested either of the following: a 6.4% glucose solution (GLU), a 6.4% glucose plus caffeine solution providing 5.3 mg kg(-1) of caffeine (GLU + CAF), or a placebo (PLA). Glucose solutions contained a trace amount of [U-C]glucose, and eight subjects received a primed continuous [6,6-H2]glucose infusion. Peak exogenous CHO oxidation rates were not significantly different between GLU and GLU + CAF trials (52.6 +/- 2.7 and 49.1 +/- 2.1 micromol kg.min(-1), respectively). Rates of appearance (Ra) and disappearance (Rd) of glucose were significantly higher with CHO ingestion than PLA (P < 0.01) but were not significantly different between GLU and GLU + CAF trials. Performance times were 43.45 +/- 0.86, 45.45 +/- 1.07, and 47.40 +/- 1.30 min for GLU + CAF, GLU, and PLA, respectively. Therefore, GLU + CAF ingestion enhanced TT performance by 4.6% (P < 0.05) compared with GLU and 9% (P < 0.05) compared with PLA. The coingestion of caffeine (5.3 mg kg(-1)) with CHO during exercise enhanced TT performance by 4.6% compared with CHO and 9.0% compared with water placebo. However, caffeine did not influence exogenous CHO oxidation or glucose kinetics during SS exercise.