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% V(dot)dot;O2max 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−1·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.
Conclusion: 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.
School of Sport and Exercise Sciences, University of Birmingham, Birmingham, UNITED KINGDOM
Address for correspondence: Asker E. Jeukendrup, Ph.D., School of Sport and Exercise Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom; E-mail: firstname.lastname@example.org.
Submitted for publication February 2008.
Accepted for publication June 2008.