3.1.1 - Introduction to Carbohydrates
Role of Carbohydrates:
Major fuel source for energy or ATP generation needed by contracting muscle.
Explored in Module 2, key energy substrate in high-intensity sprint-type exercises, and important for prolonged moderate to high-intensity exercise.
3.1.2 - Carbohydrate Stores
Stores:
Predominantly stored as glycogen in the liver and skeletal muscle.
Glycogen particles can contain up to 30,000 individual glucose molecules linked together by the core enzyme glycogenin.
Distribution:
Approximately 80% (~400g) of total body carbohydrates are stored as glycogen in skeletal muscle.
Glycogen concentrations in skeletal muscle range from 50-500 mmol/kg of dry muscle weight, influenced by training status, prior exercise, and dietary carbohydrate intake.
The liver contains more glycogen than muscle in concentration but due to its smaller mass (~1.5 kg relative to skeletal muscle, which makes up 40-50% of body weight), liver glycogen is around 100g (10-15% of total carbohydrate stores).
Remaining carbohydrates circulate in blood as glucose (~5g).
Liver glycogen can be mobilized to maintain blood glucose levels when needed.
3.1.3 - Carbohydrate Use with Exercise Intensity and Duration
Utilization Mechanisms:
Muscle glycogen and blood glucose are primary sources for ATP generation during exercise.
Source of carbohydrate used depends on exercise intensity and duration, primarily affecting glycolysis and oxidative phosphorylation (aerobic metabolism).
High Intensity (95-100% VO2max):
Muscle glycogen is preferentially utilized for ATP production.
Low Intensity (25% VO2max):
Minimal glycogen breakdown occurs; total contribution from glycogen and glucose is 10-15% of the energy source.
Increasing Intensity:
As intensity surpasses ~85% VO2max, reliance on glucose and glycogen increases, contributing 70% or more to total energy production.
Prolonged Exercise at Moderate to High Intensity
Energy Contribution:
Carbohydrates account for nearly 50% of total energy expenditure.
Initial Carbohydrate Utilization:
At onset, muscle glycogen is the primary fuel source.
Muscle glycogen stores can deplete by 40-60% within the first 90-120 minutes.
As duration increases, reliance shifts towards blood glucose.
3.2.1 - Carbohydrate and Exercise Performance
Historical Context:
Observations from the 1920s suggested increased dietary carbohydrate intake enhances performance and prevents fatigue during endurance events.
Muscle biopsy techniques in the 1960s confirmed that glycogen content directly affects endurance and exercise capacity.
Early studies justified dietary carbohydrate loading strategies (“glycogen super compensation”) for athletes.
Impact of Low Glycogen Levels:
Starting exercise with low muscle glycogen can negatively affect performance.
Endurance Events and Carbohydrate Supply
Exogenous Carbohydrate Supply:
Endurance or prolonged exercise often exceeds body’s capacity to store or replenish glycogen, necessitating carbohydrate ingestion during events.
3.3.1 - Muscle Glycogen Breakdown during Exercise
Glycogen Degradation Rates:
Breakdown rates depend on exercise intensity:
Low intensity shows slow breakdown rates; negligible decrease in muscle glycogen after 120 min.
High intensity leads to quicker depletion of glycogen within 60 minutes, with fatigue occurring as muscle glycogen depletes.
3.3.2 - Regulation of Muscle Glycogen Breakdown during Exercise
Glycogenolysis:
Process for glycogen breakdown regulated by glycogen phosphorylase.
Enzyme Forms:
Exists as inactive ‘b’ form and active ‘a’ form; transformation occurs due to increased calcium levels from muscle contraction, and hormonal stimulation by adrenaline mediating through beta-adrenergic receptor activation which raises cAMP.
Allosteric Regulation:
Increased activity through allosteric modulators such as ADP, AMP, IMP, and Pi reflecting ATP demand.
3.4.1 - Muscle Glucose Uptake during Exercise
Importance of Glucose:
Glucose's critical role as a carbohydrate source; uptake rates increase with intensity and duration.
Regulation Sites for Glucose Uptake:
Extracellular factors such as blood glucose concentration and muscle blood flow.
Membrane factors regulating glucose transport (GLUT4).
Intracellular mechanisms controlling glucose metabolism and uptake through glycolysis and glycogenolysis.
3.4.2 - Muscle Glucose Supply
Glucose Supply Equation:
Muscle blood flow can increase 20-fold during intense exercise, aiding glucose delivery.
3.4.3 - Muscle Glucose Transport
Mechanism of Transport:
Glucose enters via specialized transporter GLUT4, facilitating diffusion into muscle cells.
GLUT4 translocates to the cell surface in response to insulin and muscle contraction.
Insulin release promotes GLUT4 translocation, while its levels decrease during exercise, prompting alternate contraction-induced GLUT4 traffic to sustain glucose uptake.
3.4.4 - Muscle Glucose Metabolism
Initial Phosphorylation:
Upon entry into muscle cells, glucose is quickly phosphorylated by Hexokinase to create glucose-6-phosphate (G6P), which is a critical intermediate metabolite influencing glucose uptake and metabolism.
Feedback Mechanism:
Elevated G6P levels can inhibit hexokinase, affecting glucose transport into the muscle.
Inversely relates muscle glycogen and glucose uptake; exercised induced glycogen breakdown diminishes G6P levels, leading to increased glucose uptake.
3.5.1 - Glycolysis
Overview:
Glycolysis is the first metabolic pathway in skeletal muscle for ATP production. Initial substrate is either glycogen or glucose, converting to G6P, anaerobic ATP generation occurs, leading to the production of pyruvate.
3.5.2 - Oxidative Phosphorylation
Mitochondrial Process:
Oxidative phosphorylation utilizes NADH and FADH2 to produce ATP from pyruvate breakdown into carbon dioxide and water.
3.5.3 - Pyruvate Dehydrogenase Role
Key Enzyme:
The enzymatic conversion of pyruvate to acetyl-CoA is crucial for aerobic ATP production; influences glucose and fat metabolism interactions.
Activation Dynamics:
Activated rapidly at exercise initiation; its activity decreases with prolonged exercise due to lowered carbohydrate availability.
3.6 - Liver Glucose Production during Exercise
Importance:
Liver is critical for glucose production to maintain blood glucose levels during physical activity, counteracting glucose depletion from muscle uptake.
3.6.2 - Exercise Intensity Effects
Liver Glucose Production Dynamics:
Increases with exercise intensity, with production escalating notably during strenuous activity.
3.6.3 - Glycogenolysis versus Gluconeogenesis
Processes Explained:
Glycogenolysis: Release of glucose from liver glycogen stores.
Gluconeogenesis: Conversion of non-glucose precursors to glucose (lactate, pyruvate, glycerol, amino acids).
3.6.4 - Regulation During Exercise
Hormonal Influence:
Plasma insulin decreases whereas glucagon and adrenaline increase to stimulate liver glucose output.
Blood glucose levels also feedback to influence liver glucose production; increased availability can inhibit liver glucose output.
3.7.1 - Carbohydrate Ingestion Effects
Study Insights:
Ingesting carbohydrates enhances performance by stabilizing blood glucose and delaying fatigue during exercise.
Results from Classic Study:
Placebo resulted in significant glucose decline and fatigue, while carbohydrate ingestion maintained plasma glucose levels, enabling longer exercise before fatigue onset.
3.8 Effect of Training on Carbohydrate Metabolism
Endurance Training Effects:
Reduces reliance on carbohydrates during prolonged exercise; decreases both muscle glycogen usage and circulating glucose demand post-training.
3.9 - Effect of Heat on Carbohydrate Metabolism
Heat Impact Summary:
Exercising in elevated temperatures shifts metabolism towards carbohydrate utilization, affecting both anaerobic and aerobic pathways, influenced by adrenaline, muscle temperature, and regulatory enzyme status.