Aerobic Metabolism Overview

Aerobic Metabolism Overview

• Aerobic metabolism is crucial for exercise lasting longer than a few minutes
• Initially, ATP-PC system provides energy for the first 30 seconds
• Anaerobic glycolysis is dominant for the next 2-3 minutes
• Beyond this, aerobic metabolism becomes increasingly important
• Aerobic metabolism utilizes carbohydrates, fats, and proteins as fuel sources
• Byproducts include CO2 and water

Energy Substrate Breakdown

• Carbohydrates: Glucose and glycogen are broken down through glycolysis
  • Glycolysis occurs in the sarcoplasm
  • Anaerobic glycolysis produces little ATP, while aerobic glycolysis produces more
• Fats: Fatty acids and triglycerides are broken down through beta-oxidation in the mitochondria
  • Beta-oxidation forms acetyl CoA
  • Fat metabolism produces more acetyl CoA than glucose
• Proteins: Amino acids are deaminated and transaminated, entering the Krebs cycle at various points
  • Proteins play a smaller role compared to carbohydrates and fats

Metabolic Pathways

• Glycolysis: The breakdown of glucose/glycogen to produce ATP
  • Involves 10/11 steps, producing pyruvate
• Krebs Cycle: Occurs in the mitochondrial matrix, producing a small amount of ATP (1 ATP per acetyl CoA)
  • Starts and ends with oxaloacetate
  • Removes H+, carried to the ETC by NADH and FADH2; CO2 is removed and exhaled
  • Produces 2 CO2, 3 NADH, 1 FADH2, 1 GTP/ATP per acetyl CoA
• Electron Transport Chain (ETC): Occurs in the inner mitochondrial membrane
  • Produces the majority of ATP during aerobic metabolism (oxidative phosphorylation)
  • Regenerates NAD+ for glycolysis and the Krebs cycle
  • Oxygen is the final electron acceptor; its absence halts ATP generation
  • Electrons move between acceptors, pumping H+ creating a gradient; ATP synthase produces ATP

Glucose Metabolism

• Glucose moves from the blood into muscle cells via facilitated diffusion
  • Insulin stimulates glucose uptake at rest
  • Muscle contraction stimulates glucose uptake during exercise (insulin suppressed)
• Muscle glycogen is broken down by glycogen phosphorylase to release glucose for glycolysis

Lactate Role

• Lactate is produced during high-intensity exercise
• The Cori cycle converts lactate to glucose
• Lactate can be used by other tissues for glycogen synthesis or pyruvate conversion

Substrate Utilization

• The body utilizes substrates based on availability, intensity, duration, and diet
• Anaerobic metabolism primarily uses glucose
• Aerobic metabolism utilizes a mix (carbohydrates and fats), prioritizing carbs at higher intensities

Exercise Intensity and Duration

• Higher intensity = greater reliance on carbohydrates
• Longer duration = shift toward fat metabolism as glycogen stores deplete

Dietary Recommendations

• Healthy carbohydrates are crucial for maximizing glycogen stores
• High carbohydrate diets improve athletic performance for endurance events

Lactate Threshold

• Lactate threshold is the exercise intensity where blood lactate increases exponentially
  • Untrained individuals reach this at 50-60% VO2 max
  • Trained individuals reach it at 65-80% VO2 max
  • Exceeding this threshold impairs performance due to H+ accumulation

OBLA (Onset of Blood Lactate Accumulation)

• OBLA is the intensity at which blood lactate reaches 4 mmol/L

Metabolic Recovery

• Post-exercise, the body recovers through resynthesis of PC stores, reducing intramuscular and blood acidity
• Elevated heart rate, respiration, and metabolic rate support recovery
• Oxygen deficit is the difference between consumed and needed oxygen
• Steady-state O2 consumption occurs when aerobic processes meet energy demands
• EPOC (Excess Post-exercise Oxygen Consumption) reflects oxygen taken in above resting levels after exercise.

Active Recovery

• Active recovery (light exercise below lactate threshold) reduces blood lactate
• Examples: cycling at 30-45% VO2 max or running at 55-60% VO2 max
• Active recovery maintains blood flow, facilitating lactate removal and glycogen synthesis

Aerobic Metabolic Adaptations

• Training increases mitochondrial density and enzymatic activity, increasing ATP production
• Capacity to utilize fats at higher workloads is improved, sparing glycogen stores
• Lactate threshold (LT) increases, allowing for higher intensity exercise for longer durations

Energy System Usage During Exercise

• Different energy systems dominate based on duration and intensity of exercise
• Sprints rely primarily on ATP-PC and anaerobic glycolysis
• Endurance events rely on aerobic metabolism

Measuring Aerobic Metabolism

• VO2: Oxygen consumption
  • Absolute VO2 (L/min): Not weight-adjusted
  • Relative VO2 (mL/kg/min): Weight-adjusted, proportional to aerobic ATP production
• RER (Respiratory Exchange Ratio): Ratio of VCO2 to VO2 indicating metabolic substrate mix
  • Lower RER = greater fat metabolism
  • Higher RER = greater carbohydrate metabolism

Endurance Events

• Endurance events utilize a combination of energy systems
• Short duration high intensity events may have high anaerobic component
• Long duration events rely heavily on aerobic metabolism