7.3 Role of hormones in substrate mobilization and utilization

Hormones play a crucial role in regulating energy during exercise. Insulin and glucagon work together to maintain blood glucose levels, while catecholamines like epinephrine boost fat breakdown for fuel. These hormonal shifts ensure your body has the energy it needs to keep moving.

As you exercise, growth hormone levels rise, promoting muscle growth and repair. This hormone, along with others, helps your body adapt to the demands of physical activity. Understanding these hormonal changes can help you optimize your workouts and recovery.

Insulin and Glucagon in Blood Glucose Regulation

Antagonistic Hormonal Actions

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• Insulin and glucagon work antagonistically to maintain blood glucose homeostasis during exercise
• Insulin promotes glucose uptake by cells, particularly in skeletal muscle and adipose tissue
  • Facilitates translocation of GLUT4 transporters to the cell membrane
• Glucagon stimulates glycogenolysis in the liver
  • Increases glucose release into the bloodstream to maintain blood glucose levels during exercise

Exercise-Induced Hormonal Shifts

• During prolonged exercise, insulin levels decrease while glucagon levels increase
  • Promotes glucose production and mobilization to meet energy demands
• Insulin-to-glucagon ratio shifts in favor of glucagon as exercise intensity and duration increase
  • Facilitates greater glucose availability for working muscles (glycogenolysis, gluconeogenesis)
• Insulin sensitivity in skeletal muscle increases during and after exercise
  • Enhances glucose uptake and glycogen replenishment post-exercise
  • Improves overall glucose metabolism (increased GLUT4 expression)

Catecholamine Effects on Lipolysis

Hormonal Activation of Lipolysis

• Catecholamines (epinephrine and norepinephrine) release from adrenal medulla and sympathetic nerve endings during exercise
• Stimulate β-adrenergic receptors on adipocytes, activating lipolytic enzymes
  • Hormone-sensitive lipase (HSL) activation
  • Adipose triglyceride lipase (ATGL) activation
• Activation of HSL and ATGL leads to increased lipolysis
  • Breaks down stored triglycerides into free fatty acids and glycerol
  • Provides alternative energy source for prolonged exercise

Fatty Acid Mobilization and Utilization

• Catecholamines enhance blood flow to adipose tissue
  • Facilitates transport of mobilized fatty acids to working muscles
• Rate of lipolysis and fatty acid mobilization increases with exercise intensity and duration
  • Crucial energy source for prolonged aerobic activities (marathon running, long-distance cycling)
• Promote fatty acid oxidation in skeletal muscle
  • Increase activity of carnitine palmitoyltransferase I (CPT-I)
  • CPT-I serves as rate-limiting enzyme for fatty acid entry into mitochondria

Growth Hormone and Muscle Hypertrophy

Growth Hormone Secretion and Action

• Growth hormone (GH) secretion from anterior pituitary gland increases during exercise
  • Particularly elevated during high-intensity and resistance training
• Stimulates production of insulin-like growth factor-1 (IGF-1)
  • IGF-1 production occurs in liver and locally in muscle tissue
  • Amplifies anabolic effects of GH (increased protein synthesis, cell proliferation)
• GH-IGF-1 axis promotes protein synthesis
  • Enhances amino acid uptake into muscle cells
  • Activates intracellular signaling pathways (mTOR pathway)

Anabolic Effects on Muscle Tissue

• Increases muscle protein synthesis and decreases protein breakdown
  • Leads to positive protein balance and muscle hypertrophy over time
• Promotes lipolysis and fatty acid oxidation
  • Spares glucose and protein for energy during exercise and recovery
• Anabolic effects most pronounced when combined with resistance exercise and adequate protein intake
  • Synergistic effect with mechanical stress and nutritional support
  • Enhances muscle repair and growth (increased muscle fiber size, strength gains)

Hormonal Interplay in Substrate Utilization

Exercise Duration and Intensity Effects

• Hormonal response to exercise depends on intensity, duration, and type
  • Each hormone plays specific role in substrate mobilization and utilization
• Short, high-intensity exercise predominates catecholamines and glucagon
  • Promotes glucose mobilization from liver glycogen
  • Supports rapid energy production (anaerobic glycolysis, phosphocreatine system)
• Prolonged exercise shifts hormonal milieu to favor fat oxidation
  • Increased growth hormone and cortisol levels support lipolysis
  • Protein sparing effect to preserve muscle tissue

Hormonal Balance and Energy Substrate Regulation

• Insulin levels decrease during prolonged exercise
  • Reduces glucose uptake by non-exercising tissues
  • Allows sustained glucose availability for working muscles
• Insulin and glucagon interplay crucial for blood glucose maintenance
  • Glucagon promotes hepatic glucose output as insulin levels decline
  • Helps prevent exercise-induced hypoglycemia
• Growth hormone and cortisol work synergistically
  • Mobilize fatty acids and amino acids
  • Provide alternative fuel sources as glycogen stores deplete in prolonged exercise (ultramarathons, triathlons)
• Post-exercise shift in anabolic (insulin, GH) and catabolic (cortisol, glucagon) hormone balance
  • Supports recovery, protein synthesis, and glycogen replenishment
  • Facilitates adaptations to exercise (increased muscle mass, improved endurance capacity)