3.5 Integration of lipid metabolism

Lipid metabolism is a complex dance of energy storage and use. It's tightly linked with carbs and proteins, sharing building blocks and control systems. Your body constantly juggles these fuel sources, adapting to what's available and what you need.

Hormones like insulin and glucagon are the choreographers of this metabolic ballet. They tell your body when to store fat, break it down, or burn it for energy. Understanding this helps explain how your body maintains energy balance and why things can go wrong in diseases like diabetes.

Interplay of Lipid Metabolism

Interconnections with Other Metabolic Pathways

Top images from around the web for Interconnections with Other Metabolic Pathways

• 

  Frontiers | Glucagon Receptor Signaling and Lipid Metabolism View original

  Is this image relevant?

• 

  Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways | OpenStax Biology 2e View original

  Is this image relevant?

• 

  Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways | Boundless Biology View original

  Is this image relevant?

• 

  Frontiers | Glucagon Receptor Signaling and Lipid Metabolism View original

  Is this image relevant?

• 

  Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways | OpenStax Biology 2e View original

  Is this image relevant?

1 of 3

Top images from around the web for Interconnections with Other Metabolic Pathways

• 

  Frontiers | Glucagon Receptor Signaling and Lipid Metabolism View original

  Is this image relevant?

• 

  Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways | OpenStax Biology 2e View original

  Is this image relevant?

• 

  Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways | Boundless Biology View original

  Is this image relevant?

• 

  Frontiers | Glucagon Receptor Signaling and Lipid Metabolism View original

  Is this image relevant?

• 

  Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways | OpenStax Biology 2e View original

  Is this image relevant?

1 of 3

• Lipid metabolism intertwines with carbohydrate and protein metabolism through shared intermediates and regulatory mechanisms
• Acetyl-CoA from fatty acid β-oxidation enters the citric acid cycle links lipid catabolism to energy production
• Excess carbohydrates convert to fatty acids via lipogenesis demonstrates bidirectional relationship between carbohydrate and lipid metabolism
• Glycerol from triglyceride breakdown converts to glucose through gluconeogenesis connects lipid catabolism to glucose production
• Glucose and amino acid availability influences lipid metabolism regulation highlights integration of major metabolic pathways
• Amino acids can be converted to acetyl-CoA for fatty acid synthesis (leucine, isoleucine)
• Citrate from the citric acid cycle serves as a precursor for fatty acid synthesis in the cytosol

Metabolic Flexibility and Energy Balance

• Metabolic flexibility allows switching between lipid and carbohydrate oxidation based on availability and energy demands
• Malonyl-CoA acts as a metabolic switch regulating fatty acid oxidation and synthesis
• Peroxisome proliferator-activated receptors (PPARs) coordinate lipid metabolism with overall energy homeostasis
• Acetyl-CoA carboxylase (ACC) activity influences both fatty acid synthesis and oxidation (inhibition promotes oxidation)
• AMP-activated protein kinase (AMPK) senses cellular energy status and regulates lipid metabolism accordingly
• Excess amino acids can be converted to fatty acids for storage when protein intake exceeds requirements

Hormonal Regulation of Lipid Metabolism

Insulin's Role in Lipid Metabolism

• Insulin promotes lipogenesis by activating lipogenic enzymes (fatty acid synthase, acetyl-CoA carboxylase)
• Insulin inhibits lipolysis through suppression of hormone-sensitive lipase activity
• Insulin decreases carnitine palmitoyltransferase I (CPT-I) activity reduces fatty acid entry into mitochondria for β-oxidation
• Insulin stimulates glucose uptake in adipose tissue provides substrate for glycerol-3-phosphate in triglyceride synthesis
• Insulin enhances lipoprotein lipase activity in adipose tissue increases uptake of fatty acids from circulating lipoproteins

Glucagon and Other Hormones in Lipid Regulation

• Glucagon stimulates lipolysis in adipose tissue by activating hormone-sensitive lipase leads to triglyceride breakdown
• Glucagon enhances fatty acid oxidation by increasing CPT-I expression and other β-oxidation enzymes
• Insulin-to-glucagon ratio determines predominance of lipid synthesis or breakdown in the body
• Epinephrine promotes rapid lipolysis in adipose tissue during stress or exercise
• Cortisol increases lipolysis and fatty acid oxidation during prolonged fasting or stress
• Growth hormone enhances lipolysis and reduces lipogenesis in adipose tissue
• Thyroid hormones increase overall metabolic rate and promote lipolysis

Importance of Lipid Metabolism in Energy Homeostasis

Lipids as Energy Storage and Fuel

• Adipose tissue stores large amounts of energy as triglycerides serves as primary long-term energy reserve
• Mobilization of stored lipids during fasting or increased energy demand provides crucial energy source (muscles, liver)
• Lipid metabolism contributes to glucose homeostasis by providing gluconeogenesis substrates and reducing peripheral glucose utilization
• Fatty acids serve as preferred fuel for cardiac muscle and skeletal muscle during rest and low-intensity exercise
• Brown adipose tissue uses lipids for thermogenesis and energy expenditure

Liver's Role in Lipid Homeostasis

• Liver synthesizes, stores, and distributes lipids to other tissues as needed
• Hepatic lipogenesis converts excess carbohydrates to fatty acids for storage or export
• Liver produces ketone bodies from fatty acids during prolonged fasting provides alternative fuel for brain and other tissues
• Hepatic production of very-low-density lipoproteins (VLDL) distributes lipids to peripheral tissues
• Liver regulates cholesterol homeostasis through synthesis, uptake, and excretion

Lipid Metabolism in Health and Disease

• Dysregulation of lipid metabolism leads to various pathological conditions (obesity, diabetes, cardiovascular diseases)
• Insulin resistance in adipose tissue increases lipolysis contributes to elevated circulating free fatty acids
• Excessive lipid accumulation in non-adipose tissues (liver, muscle) can lead to lipotoxicity and metabolic dysfunction
• Imbalances in lipid metabolism contribute to atherosclerosis development through altered lipoprotein profiles
• Brown adipose tissue dysfunction may contribute to obesity and metabolic disorders

Metabolic Adaptations During Fasting vs Exercise

Fasting-Induced Metabolic Changes

• Hormone-sensitive lipase activity increases during fasting promotes triglyceride breakdown in adipose tissue
• Prolonged fasting increases ketone body production in liver provides alternative fuel source for brain and other tissues
• Liver enhances gluconeogenesis capacity uses glycerol from triglyceride breakdown and amino acids from protein catabolism
• Brain gradually shifts from glucose to ketone bodies as primary fuel source conserves glucose for obligate glucose users
• Decreased insulin and increased glucagon levels coordinate fasting metabolic adaptations
• Protein catabolism increases to provide amino acids for gluconeogenesis and energy production
• Leptin levels decrease during fasting reduces energy expenditure and stimulates appetite

Exercise-Induced Metabolic Adaptations

• Prolonged exercise shifts towards increased fatty acid oxidation and decreased glucose utilization (glucose-fatty acid cycle or Randle cycle)
• Muscle tissue adapts to utilize fatty acids more efficiently increases β-oxidation and electron transport chain enzyme expression
• Intramuscular triglyceride stores serve as important fuel source during prolonged exercise
• Increased epinephrine levels during exercise promote rapid lipolysis in adipose tissue
• Enhanced mitochondrial biogenesis in muscle tissue improves capacity for fatty acid oxidation
• Upregulation of fatty acid transporters (CD36, FABP) in muscle enhances fatty acid uptake and utilization
• Glycogen depletion during prolonged exercise triggers increased reliance on fat oxidation