Purpose: 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. Methods: 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-13C]glucose, and eight subjects received a primed continuous [6,6-2H2]glucose infusion. Results: Peak exogenous CHO oxidation rates were not significantly different between GLU and GLU + CAF trials (52.6 ± 2.7 and 49.1 ± 2.1 µmol/kg·min, 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. Conclusion: The coingestion of caffeine (5.3 mg/kg) 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.
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