Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans

J Physiol. 2005 Apr 15;564(Pt 2):649-60. doi: 10.1113/jphysiol.2005.083170. Epub 2005 Feb 10.


The effects were compared of exercise in the fasted state and exercise with a high rate of carbohydrate intake on intramyocellular triglyceride (IMTG) and glycogen content of human muscle. Using a randomized crossover study design, nine young healthy volunteers participated in two experimental sessions with an interval of 3 weeks. In each session subjects performed 2 h of constant-load bicycle exercise ( approximately 75% ), followed by 4 h of controlled recovery. On one occasion they exercised after an overnight fast (F), and on the other (CHO) they received carbohydrates before ( approximately 150 g) and during (1 g (kg bw)(-1) h(-1)) exercise. In both conditions, subjects ingested 5 g carbohydrates per kg body weight during recovery. Fibre type-specific relative IMTG content was determined by Oil red O staining in needle biopsies from m. vastus lateralis before, immediately after and 4 h after exercise. During F but not during CHO, the exercise bout decreased IMTG content in type I fibres from 18 +/- 2% to 6 +/- 2% (P = 0.007) area lipid staining. Conversely, during recovery, IMTG in type I fibres decreased from 15 +/- 2% to 10 +/- 2% in CHO, but did not change in F. Neither exercise nor recovery changed IMTG in type IIa fibres in any experimental condition. Exercise-induced net glycogen breakdown was similar in F and CHO. However, compared with CHO (11.0 +/- 7.8 mmol kg(-1) h(-1)), mean rate of postexercise muscle glycogen resynthesis was 3-fold greater in F (32.9 +/- 2.7 mmol kg(-1) h(-1), P = 0.01). Furthermore, oral glucose loading during recovery increased plasma insulin markedly more in F (+46.80 microU ml(-1)) than in CHO (+14.63 microU ml(-1), P = 0.02). We conclude that IMTG breakdown during prolonged submaximal exercise in the fasted state takes place predominantly in type I fibres and that this breakdown is prevented in the CHO-fed state. Furthermore, facilitated glucose-induced insulin secretion may contribute to enhanced muscle glycogen resynthesis following exercise in the fasted state.

Publication types

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

MeSH terms

  • Adult
  • Cross-Over Studies
  • Dietary Carbohydrates / metabolism
  • Exercise / physiology*
  • Fasting / blood
  • Fasting / metabolism*
  • Glycogen / blood
  • Glycogen / metabolism*
  • Humans
  • Male
  • Muscle Fibers, Skeletal / metabolism*
  • Muscle Fibers, Slow-Twitch / metabolism
  • Triglycerides / blood
  • Triglycerides / metabolism*


  • Dietary Carbohydrates
  • Triglycerides
  • Glycogen