24.5 Metabolic States of the Body

The body's metabolism adapts to different nutritional states, shifting between absorptive, postabsorptive, and starvation modes. These changes involve complex hormonal and enzymatic responses that regulate energy storage, utilization, and conservation.

Understanding metabolic states is crucial for grasping how the body maintains energy balance. From insulin-driven glucose uptake after meals to ketone body production during fasting, these adaptations showcase the body's remarkable ability to manage energy resources.

Metabolic States and Adaptations

Metabolic states: absorptive, postabsorptive, starvation

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• Absorptive state occurs after a meal when nutrients are being absorbed and processed by the digestive system (glucose, amino acids, fatty acids)
• Postabsorptive state begins 3-5 hours after a meal when nutrient absorption is complete and the body shifts towards using stored energy (glycogen, triglycerides)
• Starvation state occurs during prolonged fasting, typically after 24 hours or more without food, leading to significant metabolic adaptations to conserve glucose and utilize alternative energy sources (ketone bodies, fatty acids)

Key processes in absorptive state

• Insulin secretion increases in response to elevated blood glucose levels, promoting glucose uptake by cells (skeletal muscle, adipose tissue)
• Glucose uptake by cells is enhanced, facilitated by insulin-dependent glucose transporters (GLUT4) that translocate to the cell membrane
• Glycogenesis is stimulated in the liver and skeletal muscles, storing excess glucose as glycogen for later use during periods of fasting
• Lipogenesis is promoted, converting excess glucose into triglycerides for storage in adipose tissue as an energy reserve
• Protein synthesis is upregulated, utilizing amino acids from digested proteins to build new proteins and repair tissues (muscle, enzymes, hormones)
• Anabolism is the predominant metabolic process, focusing on building complex molecules from simpler ones

Metabolic changes in postabsorptive state

• Blood glucose levels gradually decrease as the body utilizes stored glycogen from the liver and skeletal muscles
• Glucagon secretion increases, promoting glycogenolysis in the liver to maintain blood glucose levels and provide energy for the brain
• Lipolysis is initiated in adipose tissue, releasing fatty acids into the bloodstream for energy production via beta-oxidation
• Ketogenesis begins in the liver, producing ketone bodies (acetoacetate, beta-hydroxybutyrate) from fatty acids as an alternative fuel source for the brain and heart
• Protein catabolism increases, breaking down muscle proteins to provide amino acids for gluconeogenesis and energy production
• Glycolysis becomes an important pathway for glucose breakdown to produce energy (ATP)

Glucose metabolism during prolonged fasting

• Glycogen stores become depleted, and the body relies more heavily on gluconeogenesis to maintain blood glucose levels for the brain
• Gluconeogenesis increases, utilizing amino acids (alanine), lactate (Cori cycle), and glycerol (from triglyceride breakdown) as substrates to produce glucose in the liver
• Ketone body production ramps up, providing an alternative fuel source for the brain and other tissues to spare glucose
• Protein catabolism is reduced to conserve muscle mass, with the body prioritizing the use of fatty acids and ketone bodies for energy production
• Metabolic rate decreases to conserve energy, resulting in a decrease in body temperature (adaptive thermogenesis) and heart rate (bradycardia)

Metabolic Regulation and Energy Production

• Metabolism encompasses all chemical reactions in the body, including both anabolism and catabolism
• Homeostasis is maintained through complex regulatory mechanisms that balance energy intake and expenditure
• ATP is the primary energy currency of cells, produced through various metabolic pathways
• The citric acid cycle plays a central role in energy metabolism, oxidizing acetyl-CoA derived from carbohydrates, fats, and proteins