8.2 Exercise and metabolism

Exercise profoundly impacts metabolism, triggering immediate energy mobilization and long-term adaptations. The body shifts fuel utilization, enhances glucose uptake, and improves insulin sensitivity in response to physical activity.

Understanding these metabolic changes is crucial for grasping how exercise benefits health. From boosting mitochondrial function to improving lipid profiles, regular physical activity plays a key role in preventing metabolic disorders.

Metabolic Adaptations During Exercise

Energy Mobilization and Nervous System Activation

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• Acute exercise triggers immediate increase in energy demand mobilizing glucose and fatty acids for fuel
• Sympathetic nervous system activates during exercise releasing catecholamines (epinephrine, norepinephrine) stimulating lipolysis and glycogenolysis
• Blood flow to skeletal muscles increases dramatically enhancing oxygen and nutrient delivery while facilitating waste removal
  • Can increase up to 20-fold in active muscles
  • Achieved through vasodilation and increased cardiac output

Fuel Utilization and Metabolic Shifts

• Muscle glycogen stores preferentially utilized for energy production in initial stages of exercise
• Gluconeogenesis in liver upregulated to maintain blood glucose levels during prolonged exercise
  • Uses substrates like lactate, amino acids, and glycerol
• Respiratory exchange ratio (RER) shifts towards greater fat oxidation as exercise duration increases
  • RER closer to 1.0 indicates primarily carbohydrate use
  • RER closer to 0.7 indicates primarily fat use
• Lactate production increases during high-intensity exercise serving as both fuel source and signaling molecule
  • Can be oxidized by heart and less active muscles
  • Stimulates release of growth hormone and testosterone

Exercise Effects on Insulin Sensitivity

Glucose Transport and Utilization

• Regular exercise enhances insulin sensitivity in skeletal muscle, liver, and adipose tissue improving glucose uptake and utilization
• Exercise training increases expression and translocation of GLUT4 glucose transporters in muscle cells facilitating glucose disposal
  • GLUT4 translocation can increase up to 5-fold during exercise
• Habitual exercise enhances hepatic insulin sensitivity reducing glucose output and improving glycemic control
  • Can decrease hepatic glucose production by up to 30% in individuals with type 2 diabetes

Metabolic Adaptations and Health Benefits

• Chronic exercise leads to mitochondrial biogenesis enhancing cellular capacity for fatty acid oxidation and glucose metabolism
  • Can increase mitochondrial content by 50-100% with consistent training
• Regular physical activity improves lipid profiles reducing triglycerides and increasing HDL cholesterol levels
  • Can decrease triglycerides by 20-30% and increase HDL by 5-10%
• Exercise-induced adaptations in adipose tissue include increased lipolysis and reduced inflammation contributing to improved metabolic health
• Cumulative effects of regular exercise on metabolic health include reduced risk of type 2 diabetes, cardiovascular disease, and metabolic syndrome
  • Can decrease risk of type 2 diabetes by 30-50% in high-risk individuals

Skeletal Muscle Role in Glucose Use

Glucose Uptake Mechanisms

• Skeletal muscle primary site of glucose disposal during exercise accounting for up to 80% of glucose uptake
• Contraction-mediated glucose uptake in skeletal muscle occurs independently of insulin involving AMPK activation and calcium signaling
• Exercise stimulates translocation of GLUT4 glucose transporters to muscle cell membrane facilitating glucose entry
  • GLUT4 translocation can occur within minutes of exercise onset
• Rate of glucose uptake by skeletal muscle influenced by exercise intensity with higher intensities leading to greater glucose utilization
  • High-intensity exercise can increase glucose uptake up to 50-fold compared to resting state

Glycogen Utilization and Metabolic Flexibility

• Intramuscular glycogen serves as crucial energy source during exercise with depletion rate depending on exercise intensity and duration
  • Glycogen stores can be depleted by 50-70% during prolonged moderate-intensity exercise
• Skeletal muscle fibers exhibit metabolic flexibility switching between glucose and fatty acid oxidation based on exercise demands and substrate availability
• Post-exercise, skeletal muscle displays enhanced insulin sensitivity and glucose uptake for glycogen replenishment known as "glucose uptake window"
  • Can last up to 24-48 hours post-exercise, with greatest effect in first 30-60 minutes

Aerobic vs Anaerobic Exercise Metabolism

Energy Systems and Fuel Sources

• Aerobic exercise primarily relies on oxidative phosphorylation for ATP production while anaerobic exercise depends on glycolysis and phosphagen system
• Predominant fuel source in aerobic exercise mix of carbohydrates and fats whereas anaerobic exercise mainly utilizes muscle glycogen and phosphocreatine
• Oxygen consumption (VO2) significantly higher during aerobic exercise compared to anaerobic exercise
  • Aerobic exercise can sustain 60-80% of VO2max for extended periods
  • Anaerobic exercise can briefly reach 100% VO2max but not sustainable

Metabolic Byproducts and Recovery

• Anaerobic exercise leads to greater lactate accumulation and more pronounced decrease in muscle pH compared to aerobic exercise
  • Blood lactate can increase from 1-2 mmol/L at rest to over 20 mmol/L during intense anaerobic exercise
• Energy systems' contribution varies with exercise duration: anaerobic dominates in short, intense bursts while aerobic prevails in longer-duration activities
  • Anaerobic system predominant in activities lasting 10-90 seconds
  • Aerobic system takes over for activities lasting beyond 2-3 minutes
• Recovery metabolism differs: aerobic exercise has lower oxygen debt and faster recovery while anaerobic exercise requires longer for full metabolic recovery
  • Excess post-exercise oxygen consumption (EPOC) higher after anaerobic exercise
• Adaptations to training differ: aerobic exercise enhances mitochondrial density and capillarization while anaerobic exercise increases glycolytic enzyme activity and muscle buffering capacity
  • Aerobic training can increase mitochondrial density by 50-100%
  • Anaerobic training can increase phosphofructokinase activity by 20-30%