title: Substrate metabolism and exercise performance with caffeine and carbohydrate intake.
abstract: OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: School of Sport and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom.
authors.forename: Carl J
authors.initials: CJ
authors.lastname: Hulston
authors.forename: Asker E
authors.initials: AE
authors.lastname: Jeukendrup
mesh_terms: D000328:Adult; D001642:Bicycling / Q000502:physiology*; D002110:Caffeine / Q000008:administration & dosage / Q000494:pharmacology*; D000697:Central Nervous System Stimulants / Q000494:pharmacology*; D004040:Dietary Carbohydrates / Q000378:metabolism; D008401:Gas Chromatography-Mass Spectrometry; D005947:Glucose / Q000008:administration & dosage / Q000378:metabolism*; D006801:Humans; D008297:Male; D010084:Oxidation-Reduction; D010919:Placebos; D013647:Task Performance and Analysis
chemical_list: D000697:Central Nervous System Stimulants; D004040:Dietary Carbohydrates; D010919:Placebos; D002110:Caffeine; D005947:Glucose