Carbohydrate ingestion during prolonged exercise: effects on metabolism and performance

Area of Science:

• Exercise Physiology
• Sports Nutrition
• Metabolic Regulation

Background:

• Carbohydrate (CHO) oxidation is vital for prolonged strenuous exercise (>60% VO2 max).
• Muscle glycogen was historically considered the primary CHO source, potentially overlooking blood glucose's role.
• As exercise duration increases, muscle glycogen depletes, necessitating greater reliance on blood glucose.

Purpose of the Study:

• To investigate the relative contributions of muscle glycogen and blood glucose to energy production during prolonged strenuous exercise.
• To determine the efficacy of carbohydrate ingestion in maintaining blood glucose oxidation and exercise performance.
• To explore optimal timing and strategies for carbohydrate supplementation during various exercise intensities and durations.

Main Methods:

• The study theorizes the equal contribution of blood glucose and muscle glycogen over 3-4 hours of strenuous exercise.
• It examines the impact of carbohydrate availability on blood glucose concentration and oxidation rates.
• It analyzes the effects of pre-exercise and during-exercise carbohydrate feeding on exercise tolerance and fatigue onset.

Main Results:

• Blood glucose uptake and oxidation increase progressively as muscle glycogen depletes during prolonged exercise.
• Carbohydrate ingestion during exercise maintains blood glucose levels and CHO oxidation, delaying fatigue by approximately 45 minutes.
• Supplementing blood glucose at ~1 g/min late in exercise is effective; pre-emptive feeding (30 min before fatigue) is as effective as continuous ingestion.
• Carbohydrate feeding does not alter muscle glycogen utilization but may benefit intermittent or low-intensity exercise through glycogen synthesis.

Conclusions:

• Blood glucose is a critical energy substrate during prolonged strenuous exercise, potentially as important as muscle glycogen.
• Maintaining blood glucose availability through carbohydrate ingestion is essential for sustained CHO oxidation and delaying fatigue.
• Optimal carbohydrate supplementation strategies vary by exercise intensity and duration, with pre-emptive feeding being advantageous.