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Glycogen availability does not affect the TCA cycle or TAN pools during prolonged, fatiguing exercise.

Jacinta Baldwin1, Rodney J Snow, Martin J Gibala

  • 1Department of Physiology, University of Melbourne, Parkville 3010, Victoria, Australia.

Journal of Applied Physiology (Bethesda, Md. : 1985)
|May 9, 2003
PubMed
Summary

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This study found that low muscle glycogen does not limit the tricarboxylic acid cycle intermediates (TCAI) or cause fatigue during prolonged exercise. Exercise capacity is not solely dependent on glycogen levels for aerobic energy production.

Area of Science:

  • Exercise Physiology
  • Biochemistry
  • Sports Science

Background:

  • Prolonged exercise fatigue is often attributed to depleted intramuscular glycogen.
  • This depletion is hypothesized to limit tricarboxylic acid cycle activity via reduced tricarboxylic acid cycle intermediates (TCAI).

Purpose of the Study:

  • To test the hypothesis that insufficient intramuscular glycogen limits TCA cycle activity and causes fatigue during prolonged exercise.
  • To investigate the relationship between glycogen availability, TCAI pool size, and exercise performance.

Main Methods:

  • Seven endurance-trained men performed cycling exercise to exhaustion at ~70% peak oxygen uptake.
  • Two conditions were used: low (LG) and high (HG) pre-exercise intramuscular glycogen content.
  • Muscle biopsies were analyzed for glycogen, TCAI, total adenine nucleotide pool (TAN), IMP, and creatine phosphate.

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Main Results:

  • Muscle glycogen was lower at fatigue than at rest in both LG and HG trials.
  • TCAI pool expansion and content at fatigue were similar between LG and HG conditions.
  • Exercise duration was longer in the HG trial, but no further changes in TCAI were observed despite marked glycogen reduction.
  • TAN, IMP, and creatine phosphate levels remained stable and were not affected by glycogen availability.

Conclusions:

  • Altered glycogen availability does not impair TCAI pool expansion during prolonged exercise.
  • Glycogen depletion does not affect energy-providing metabolites like TAN, IMP, or creatine phosphate.
  • The findings do not support the theory that reduced TCAI is a primary cause of fatigue in prolonged exercise by limiting aerobic energy provision.