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Fuel metabolism during ultra-endurance exercise
H G Rauch1, J A Hawley, T D Noakes
1MRC/UCT Bioenergetics of Exercise Research Unit, University of Cape Town Medical School, Sports Science Institute of South Africa, PO Box 115, Newlands, 7725, South Africa.
Pflugers Archiv : European Journal of Physiology
|June 24, 1998
Summary
During prolonged cycling, glucose and lactate oxidation remained high, even as muscle glycogen stores depleted. This suggests lactate transfer between muscle fibers fuels sustained carbohydrate use during endurance exercise.
Area of Science:
- Exercise Physiology
- Human Metabolism
- Sports Science
Background:
- Sustaining high-intensity exercise requires continuous energy supply.
- Carbohydrate oxidation, including glucose and lactate, is crucial for endurance performance.
- Muscle glycogen is a primary fuel source, but its depletion can limit exercise duration.
Purpose of the Study:
- To investigate the contribution of glucose and lactate oxidation to energy production during prolonged cycling.
- To determine the fate of muscle glycogen and plasma lactate during sustained exercise.
- To explore potential mechanisms for sustained carbohydrate utilization when muscle glycogen is depleted.
Main Methods:
- Cyclists performed 6-hour rides at 55% VO2max.
- Participants ingested labeled glucose or received labeled lactate infusions.
- Euglycemia was maintained to control hormonal and substrate levels.
- Carbohydrate oxidation rates (glucose, lactate, muscle glycogen) were quantified using isotopic tracers.
Main Results:
- Glucose and lactate oxidation remained elevated throughout the exercise period.
- Muscle glycogen oxidation decreased but remained significant.
- Total muscle glycogen oxidized exceeded initial stores, suggesting lactate inter-tissue transfer.
- Plasma lactate oxidation contributed substantially to energy production.
Conclusions:
- Sustained high rates of carbohydrate oxidation are maintained by both glucose and lactate utilization during prolonged exercise.
- Significant inter-muscle fiber lactate transport likely replenishes glycogen and fuels oxidation in active muscles.
- Lactate plays a critical role beyond being a mere byproduct, acting as an important energy substrate during endurance exercise.