Glucose kinetics during prolonged exercise in euglycaemic and hyperglycaemic subjects

Area of Science:

• Exercise Physiology
• Metabolic Regulation
• Nutrient Metabolism

Background:

• Skeletal muscle's capacity to oxidize exogenous glucose is crucial for sustained exercise performance.
• Understanding fuel substrate utilization during exercise under varying glycemic conditions is essential for optimizing athletic potential.

Purpose of the Study:

• To investigate the maximal oxidation rate of exogenous glucose by skeletal muscle during prolonged exercise.
• To compare fuel substrate kinetics under conditions of euglycaemia (normal blood glucose) and hyperglycaemia (high blood glucose).

Main Methods:

• Trained subjects (n=12) performed 2-hour cycling at 70% VO2 max.
• Plasma glucose levels were maintained at 5 mM (euglycaemic trial, ET) or 10 mM (hyperglycaemic trial, HT) using glucose infusion.
• Labeled (U-14C-glucose) and traced (2-3H-glucose) isotopes were used to measure glucose oxidation (Rox) and endogenous glucose production (Ra).

Main Results:

• Endogenous glucose production (Ra) was suppressed during both ET and completely abolished during HT.
• Exogenous glucose oxidation (Rox) significantly increased in HT compared to ET.
• Hyperglycaemia and hyperinsulinaemia markedly increased total carbohydrate oxidation and suppressed fat oxidation.

Conclusions:

• Skeletal muscle can oxidize exogenous glucose at high rates during exercise when blood glucose is elevated.
• Hyperglycaemia shifts fuel preference towards carbohydrates, limiting endogenous glucose supply and fat utilization.
• These findings highlight the regulatory role of plasma glucose concentration in modulating fuel metabolism during endurance exercise.