Substrate metabolism and exercise performance with caffeine and carbohydrate intake

Med Sci Sports Exerc. 2008 Dec;40(12):2096-104. doi: 10.1249/MSS.0b013e318182a9c7.


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-C]glucose, and eight subjects received a primed continuous [6,6-H2]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 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.

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.

Publication types

  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Bicycling / physiology*
  • Caffeine / administration & dosage
  • Caffeine / pharmacology*
  • Central Nervous System Stimulants / pharmacology*
  • Dietary Carbohydrates / metabolism
  • Gas Chromatography-Mass Spectrometry
  • Glucose / administration & dosage
  • Glucose / metabolism*
  • Humans
  • Male
  • Oxidation-Reduction
  • Placebos
  • Task Performance and Analysis


  • Central Nervous System Stimulants
  • Dietary Carbohydrates
  • Placebos
  • Caffeine
  • Glucose