Glucose metabolism during leg exercise in man

Area of Science:

• Exercise Physiology
• Human Metabolism
• Biochemistry

Background:

• Understanding substrate utilization during exercise is crucial for optimizing athletic performance and managing metabolic health.
• The interplay between glucose uptake, production, and hormonal regulation during physical exertion requires detailed investigation.

Purpose of the Study:

• To quantify arterial concentrations and net substrate exchange (glucose, lactate, pyruvate, glycerol) in the leg and splanchnic bed during rest and exercise.
• To determine the contribution of blood glucose to overall energy expenditure in exercising muscles.
• To elucidate the role of hepatic glucose production and gluconeogenesis in maintaining glucose homeostasis during exercise.

Main Methods:

• Measurements of arterial and venous substrate concentrations (glucose, lactate, pyruvate, glycerol) at rest and during graded bicycle ergometer exercise.
• Calculation of net substrate exchange across the leg and splanchnic vascular beds.
• Estimation of glucose uptake and total glucose turnover using Fick principle and splanchnic balance methods.
• Assessment of hepatic gluconeogenesis based on splanchnic substrate removal.

Main Results:

• Arterial glucose levels rose, and plasma insulin decreased slightly during heavy exercise.
• Leg glucose uptake increased significantly with exercise intensity and duration, contributing substantially to muscle carbohydrate oxidation.
• Splanchnic glucose production rose 3-5 fold during heavy exercise, closely matching estimated total glucose turnover.
• Hepatic gluconeogenesis contributed minimally (6-11%) to splanchnic glucose production during moderate to heavy exercise.

Conclusions:

• Blood glucose becomes a critical substrate for muscle oxidation during prolonged exercise.
• Peripheral glucose utilization by muscles increases during exercise, independent of circulating insulin levels.
• Augmented hepatic glycogenolysis is the primary mechanism for maintaining blood glucose homeostasis and supplying fuel to exercising muscles.