Effects of carbohydrate feedings on plasma free tryptophan and branched-chain amino acids during prolonged cycling

Eur J Appl Physiol Occup Physiol. 1992;65(6):513-9. doi: 10.1007/BF00602357.


Brain serotonin (5-hydroxytryptamine, 5-HT) has been suggested to be involved in central fatigue during prolonged exercise. Changes in the ratio of plasma free tryptophan (free Trp) to branched-chain amino acids (BCAA) are associated with altered brain 5-HT synthesis. The purposes of this study were to describe systematically the effects of prolonged exercise on changes in plasma free Trp and BCAA and to examine the effects of carbohydrate (CHO) feedings on these same variables. Eight well-trained men [VO2max = 57.8 (SE 4.1) ml kg-1 min-1] cycled for up to 255 min at a power output corresponding to VO2 at lactate threshold (approximately 68% VO2max) on three occasions separated by at least 1 week. Subjects drank 5 ml kg-1 body wt-1 of either a water placebo, or a liquid beverage containing a moderate (6% CHO) or high (12% CHO) concentration of carbohydrate beginning at min 14 of exercise and every 30 min thereafter. Exercise time to fatigue was shorter in subjects receiving placebo [190 (SE 4) min] as compared to 6% CHO [235 (SE 10) min] and 12% CHO [234 (SE 9) min] (P < 0.05). Glucose and insulin decreased in the placebo group, and free Trp, free-Trp/BCAA, and free fatty acids increased approximately five- to sevenfold (P < 0.05). These changes were attenuated in a dose-related manner by the carbohydrate drinks.(ABSTRACT TRUNCATED AT 250 WORDS)

Publication types

  • Clinical Trial
  • Controlled Clinical Trial

MeSH terms

  • Adult
  • Amino Acids, Branched-Chain / blood*
  • Bicycling*
  • Blood Glucose / metabolism
  • Body Temperature / physiology
  • Dietary Carbohydrates / pharmacology*
  • Dose-Response Relationship, Drug
  • Exercise / physiology*
  • Exercise Test
  • Fatty Acids, Nonesterified / blood
  • Heart Rate / physiology
  • Humans
  • Insulin / blood
  • Lactates / blood
  • Male
  • Oxygen Consumption / physiology
  • Pulmonary Gas Exchange / physiology
  • Tryptophan / blood*


  • Amino Acids, Branched-Chain
  • Blood Glucose
  • Dietary Carbohydrates
  • Fatty Acids, Nonesterified
  • Insulin
  • Lactates
  • Tryptophan