Metabolic availability of glucose ingested 3 h before prolonged exercise in humans

Area of Science:

• Exercise Physiology
• Nutritional Biochemistry

Background:

• Understanding fuel utilization during exercise is crucial for optimizing athletic performance and health.
• The availability of ingested carbohydrates as an energy source during prolonged physical activity requires further investigation.

Purpose of the Study:

• To determine if orally ingested glucose, consumed 3 hours prior to exercise, serves as a readily accessible energy source during a 4-hour moderate-intensity exercise bout.
• To quantify the oxidation of exogenous glucose during rest and exercise.

Main Methods:

• Five healthy males ingested 100g of 13C-labeled glucose, rested for 3 hours, then exercised for 4 hours at 45% max O2 consumption.
• Total glucose oxidation was calculated using non-protein respiratory quotient and 13C-tracer methodology.
• Hormonal (insulin, glucagon) and metabolic (blood glucose, free fatty acids, glycerol) markers were monitored.

Main Results:

• Total carbohydrate oxidation averaged 285g over 7 hours, with 253g oxidized during the 4-hour exercise period.
• Exogenous glucose oxidation was 11.3g during the 3-hour rest and significantly increased to 18.9g/30min during early exercise, totaling 67.5g over 4 hours.
• Blood glucose remained stable (3.5-4.0 mmol/l), while insulin levels dropped significantly and free fatty acids/glycerol increased during exercise.

Conclusions:

• Oral glucose ingested 3 hours before exercise is substantially oxidized during subsequent moderate-intensity activity.
• This exogenous glucose contributes significantly to energy provision during prolonged exercise, supporting stable blood glucose levels.
• The study highlights the importance of pre-exercise carbohydrate intake for sustained energy availability during endurance activities.