3.1.1 - Introduction to Carbohydrates

• Carbohydrates are critical as a fuel source for energy or ATP generation vital for muscle contractions.
• Key energy substrate during:
  • High intensity sprint-type exercises.
  • Prolonged, moderate to high-intensity exercises.

3.1.2 - Carbohydrate Stores

• The body’s endogenous carbohydrate stores are insufficient despite their importance as a fuel source during exercise.
• Main storage locations:
  • Liver: Higher concentration compared to muscle, but smaller mass (approximately 1.5 kg).
  • Skeletal Muscle: 80% (~400g of total carbohydrate) is stored as glycogen.
  • Glycogen concentrations range from 50-500 mmol/kg of dry muscle weight.
  • Dependent factors: training status, previous exercise, dietary carbohydrate intake.
• Glycogen Structure:
  • A glycogen particle can contain up to 30,000 glucose molecules linked together by glycogenin.
• Total body carbohydrate stores information:
  • Muscle glycogen: 80%
  • Liver glycogen: Approximately 100g or 10-15% of total stores.
  • Remaining glucose circulates in blood (approximately 5g).
• Glycogen’s role:
  • Maintains blood glucose levels by breaking down liver glycogen when required.

3.1.3 - Carbohydrate Use with Exercise Intensity and Duration

• Muscle glycogen and blood glucose are the primary sources for ATP generation in contracting muscles.
• Factors affecting carbohydrate use:
  • Intensity and duration of exercise.
• Energy systems involved:
  • Glycolysis: Previously explored in Module 2.
  • Oxidative phosphorylation/aerobic metabolism: Explored further in this Module.

High Intensity vs Low Intensity Exercise

• High Intensity (95-100% VO2max):
  • Muscle preferentially uses muscle glycogen for ATP production.
• Low Intensity (25% VO2max):
  • Minimal glycogen breakdown occurs, total contribution of glycogen/glucose is only 10-15%.
• As intensity increases, contribution from glucose and glycogen rises:
  • Up to 70% during ~85% VO2max exercises.

Prolonged Exercise

• During prolonged exercise (>90-120min):
  • Muscle glycogen is the primary source initially, and can deplete by 40-60%.
  • Increased reliance on blood glucose as exercise duration increases.

3.2.1 - Carbohydrate and Exercise Performance

• Historical findings:
  • 1920s: Recognition of increasing dietary carbohydrates before marathons prevents fatigue.
  • 1960s: Link confirmed between dietary carbohydrate, muscle glycogen, and exercise capacity via muscle biopsy technique.
• Impact of low glycogen on performance:
  • Initially acknowledged; low glycogen negatively affects endurance performance.
• Introduction of glycogen super compensation:
  • A strategy for athletes to enhance performance.

3.3.1 - Muscle Glycogen Breakdown during Exercise

• Muscle glycogen significantly declines with exercise, mainly influenced by intensity:
  • Low Intensity: Slow breakdown, little reduction after ~120 minutes.
  • Heavy Exercise: Much steeper decline, nearly depleted after 60 minutes.
• Relationship between glycogen depletion and fatigue:
  • Fatigue occurs at depletion since ATP production cannot maintain exercise intensity.

Summary of Glycogen Utilization

• Breakdown is usually rapid in early stages of exercise but declines as duration increases due to reduced availability of glycogen.
• Blood glucose compensates to maintain total carbohydrate oxidation.

3.3.2 - Regulation of Muscle Glycogen Breakdown during Exercise

• Process: Glycogenolysis via glycogen phosphorylase.
  • Primary rate-limiting factor regulated by:
  • Activation of b-form to a-form through phosphorylation (increased calcium levels and hormonal stimulation by adrenaline).
• Allosteric regulation:
  • Increased activity via ADP, AMP, IMP, and Pi from ATP use in muscle contractions ensuring glycogen breakdown aligns with ATP demand.

3.4.1 - Muscle Glucose Uptake during Exercise

• Rate of glucose uptake by muscle increases with exercise intensity and duration.

Regulation of Muscle Glucose Uptake

• Steps:
  • Extracellular: Blood glucose concentration and blood flow impact.
  • Membrane: Mainly by activity of GLUT4 transporters.
  • Intracellular: Metabolism processes related to glucose utilization.

3.4.2 - Muscle Glucose Supply

• Expressed as:
  • $ext{Glucose supply} = ext{Blood flow} imes ext{Blood glucose concentration}$
• Blood flow to muscles increases up to 20-fold during intense exercise.

3.4.3 - Muscle Glucose Transport

• Glucose enters muscle cells via GLUT4, utilizing facilitated diffusion.
• Insulin's Role:
  • Stimulates GLUT4 translocation from storage to plasma membrane, enhancing glucose uptake post-feeding.
• During exercise, insulin levels drop, however:
  • Muscle contraction facilitates GLUT4 translocation independently of insulin.

3.4.4 - Muscle Glucose Metabolism

• Upon entry, glucose is phosphorylated by Hexokinase to form glucose-6-phosphate (G6P).
• G6P plays a dual role:
  • Drives glycolysis.
  • Can inhibit hexokinase, creating feedback regulation limiting glucose transport.

3.5.1 - Glycolysis

• Process starts with glucose or glycogen, converting to G6P.
• End product: Pyruvate.

Glycolysis and Further Metabolism

• Produces ATP and results in necessary electron carriers (NADH).

3.5.2 - Oxidative Phosphorylation

• Key Components within mitochondria:
  • Citric Acid Cycle and Electron Transport Chain.
• Converts substrates to carbon dioxide and water, producing ATP.

3.5.3 - Pyruvate Dehydrogenase

• Converts pyruvate to acetyl-CoA; regulates aerobic ATP production rate.

Regulation of Pyruvate Dehydrogenase (PDH)

• Also regulated by method of phosphorylation, converting it to the active state (PDHa).
• Factors influencing PDH activation:
  • Increased Calcium (Ca2+), ADP, and pyruvate.

3.6.1 - Liver Glucose Production Importance

• Critical for preventing hypoglycemia during exercise by maintaining blood glucose levels.

3.6.2 - Liver Glucose Production Response to Exercise

• Exercise intensity and duration directly influence liver glucose production:
  • Increases up to 4-fold with strenous exercise.

3.6.3 - Liver Glucose Production Mechanisms

• Processes:
  1. Glycogenolysis: Breakdown of liver glycogen.
  2. Gluconeogenesis: Conversion of non-glucose precursors to glucose.

3.6.4 - Regulation of Liver Glucose Production

• Complex regulation influenced by:
  • Plasma levels of insulin/glucagon.
  • Feedback from blood glucose levels.

3.7.1 - Effect of Carbohydrate Ingestion on Metabolism

• Ingesting carbohydrates enhances performance during exercise:
  • Helps maintain plasma glucose levels, supports high rates of muscle glucose uptake.

3.8.1 - Carbohydrate Metabolism Response to Training

• Endurance training reduces carbohydrate reliance during prolonged exercise
  • Results in decreased muscle glycogen use and blood glucose production.

3.8.2 - Muscle Glycogen Utilisation after Training

• Pre-exercise glycogen concentrations are higher in trained individuals.
  • Rate of glycogen use during exercise decreases post-training.

3.8.3 - Glucose Uptake Post Training

• Reduced muscle glucose uptake due to decreased GLUT4 translocation post-training,
  • Despite maintained total GLUT4 content.

3.8.4 - Muscle Carbohydrate Oxidation Post Training

• Training enhances skeletal muscle oxidative capacity, boosting fat oxidation.
  • Results in lower respiratory exchange ratio (RER).

3.8.5 - Liver Glucose Production Post Training

• Decrease in liver glucose production primarily due to less glycogenolysis following training, potentially driven by hormonal changes.

3.9.1 - Effect of Heat on Carbohydrate Metabolism

• Increased carbohydrate utilisation during submaximal exercise in heat:
  • Anaerobic/aerobic pathways both elevated due to increased muscle temperature and adrenaline levels.