Oct.22 2pm

Introduction

  • The discussion begins with a presentation slide and animation aimed at helping students understand carbohydrate metabolism especially in relation to exercise.

Carbohydrate Sources

  • The main focus is on identifying the sources of carbohydrates in the body.

    • Muscle Glycogen

    • Stores about 80% of total glycogen.

    • Liver Glycogen

    • Holds a smaller amount of glycogen compared to muscle.

    • Blood Glucose

      • Sourced from several pathways:

      • Breakdown of liver glycogen.

      • Gluconeogenesis (production of glucose from non-carbohydrate sources such as amino acids).

Glycogen Storage and Diet Influence

  • Storage Determinants:

    • The amount of glycogen stored in the liver and muscles is influenced by carbohydrate intake.

    • Consuming carbohydrates regularly is necessary to maintain high glycogen levels, especially for regular exercisers.

    • After fasting (e.g., for 20 hours), glycogen stores in liver and muscle become significantly depleted.

    • Exercise Impact: After intense exercise, glycogen stores are lower and can be nearly depleted.

    • Individuals with greater aerobic training capacity can store more glycogen as it is an adaptation from consistent training.

Importance of Carbohydrate Intake for Athletes

  • Athletes must manage carbohydrate intake pre-, during, and post-exercise to optimize glycogen stores.

  • Importance of carbohydrate consumption during sports events is highlighted (e.g., marathon runners, halftime snacks).

Glycogen Metabolism

  • Glycogenolysis Explained:

    • Breakdown of glycogen to release glucose.

    • Glycogen -> Glucose-6-Phosphate or Glucose.

  • Comparison with Glycolysis:

    • Glycolysis starts with Glucose-6-Phosphate and ends with Pyruvate.

Key Enzyme: Glycogen Phosphorylase

  • This is the rate-limiting enzyme for glycogenolysis that catalyzes the cleavage of glucose from glycogen.

Energy Expenditure & Substrate Use During Exercise

  • Energy expenditures vary with exercise intensity (40%, 55%, and 75% max) compared to resting energy, which consists mainly of circulating glucose and fatty acids.

  • As exercise intensity increases, the reliance on muscle glycogen increases because:

    • Muscle glycogen is immediately available for energy production to meet high ATP demands during intense activities.

Factors Influencing Muscle Glycogen Use

  • **Exercise Intensity:

    • At 60% of VO2 max, muscle glycogen becomes the major substrate.

  • **Exercise Duration:

    • Muscle glycogen use typically declines due to lower availability as it gets consumed.

    • As duration increases, intensity often decreases, contributing to lower reliance on glycogen due to reduced intensity.

Impact of Diet on Glycogen Usage

  • Consuming carbohydrates, especially for athletes, enhances muscle glycogen use whereas a ketogenic diet results in increased fat metabolism instead.

Endurance Training Adaptations

  • Endurance trained individuals:

    • Store more glycogen in muscle and use less glycogen at the same intensity.

    • Higher mitochondrial density allows for more efficient ATP production from glucose, reducing glycogen utilization, leading to the phenomenon of glycogen sparing.

Glycogen Sparing Concept

  • Trained individuals can use glycogen more efficiently due to:

    • Increased mitochondrial biogenesis, allowing pyruvate to undergo aerobic metabolism rather than fermenting to lactate.

    • The distinction is made that trained individuals can spare their glycogen compared to untrained individuals due to greater fat oxidation.

Hormonal Response and Glycogen Regulation

  • Glycogen and Blood Glucose Dynamics:

    • As muscle uptake of glucose rises during exercise, liver glycogen is mobilized to maintain blood glucose levels.

    • Key Hormones:

    • Glucagon: Released when blood glucose levels drop, stimulates liver glycogen breakdown.

    • Catecholamines (epinephrine, norepinephrine): Increase glycogenolysis and fatty acid mobilization under stress, stimulating energy release during high-intensity exercise.

Insulin and its Contrasting Effects

  • During rest, carbohydrate consumption leads to an increase in insulin, which facilitates glucose uptake into muscles but also:

    • Stimulates glycogen, protein, and fat synthesis.

    • Counterproductive during exercise when breakdown, not synthesis, is desired.

Summary

  • Regular carbohydrate intake and strategic consumption before, during, and after exercise is critical for maintaining and replenishing glycogen stores, which significantly influences performance in endurance and high-intensity activities.