Fuel metabolism during ultra-endurance exercise

Cyclists oxidized approximately 530 g of muscle glycogen during prolonged exercise while maintaining euglycaemia.

• Maintenance of euglycaemia limited increases in free fatty acids, noradrenaline, and adrenaline levels.
• Muscle glycogen oxidation decreased from 100 to 71 micromol/min/kg in the final three hours of exercise.
• Glucose and lactate oxidation rates remained relatively stable at about 60 and 50 micromol/min/kg, respectively.
• A total of approximately 270 g of glucose and 130 g of plasma lactate were oxidized.
• The oxidation of muscle glycogen exceeded the predicted initial glycogen content, suggesting lactate diffusion from inactive to active muscle fibers.

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