Chapter 24: Metabolism and Nutrition

Metabolism Overview

Definition: Metabolism is the sum of all energy-requiring and energy-consuming processes of the body, crucial for maintaining life functions.Factors Affecting Metabolism:

• Lean muscle mass: More muscle increases basal metabolic rate (BMR).

• Amount and quality of food consumed: Nutrient-dense foods can enhance metabolic processes.

• Physical demands on the body: Higher activity levels increase energy expenditure.

Chapter Objectives

• Describe processes involved in anabolic and catabolic reactions.

• List steps necessary for carbohydrate, lipid, and protein metabolism.

• Explain processes regulating glucose levels during absorptive and postabsorptive states.

• Discuss metabolism's role in thermoregulation.

• Summarize the importance of vitamins and minerals in the diet.

Introduction to Metabolism

Eating Importance:

Eating provides essential nutrients for cellular energy and physiological functions, including hormone production, immune responses, and tissue repair.

Balanced Diet:

Emphasizes the importance of fruits and vegetables for health and metabolism, as they provide vital vitamins, minerals, and antioxidants.

Types of Metabolic Reactions

24.1 Overview of Metabolic Reactions

Metabolic Processes: Constantly occurring chemical reactions in the body, including:

• Catabolism: Breakdown of food to obtain energy and produce metabolic intermediates.

• Anabolism: Synthesis of larger molecules from smaller ones (e.g., proteins from amino acids), crucial for growth, repair, and regulation.

Catabolic Reactions

Definition: Break down large organic molecules into smaller molecules, releasing energy, which is critical for ATP production.

• Energy Yield: Approximately 40% of energy is stored as ATP; 60% is released as heat, supporting thermoregulation.

• ATP Structure: Composed of adenine, ribose, and three phosphate groups; high-energy bonds between the phosphate groups are crucial for energy release and storage.

• Function of ATP: Powers cellular functions, facilitates tissue repair, muscle contraction, and can be stored for future energy needs.

Anabolic Reactions

Definition: Involve joining smaller molecules into larger molecules (e.g., building proteins from amino acids), essential for maintaining cellular integrity and function.

• Energy Requirement: Require ATP generated from catabolic reactions to drive the synthesis process.

Hormonal Regulation of Metabolism

• Catabolic Hormones: Hormones that stimulate breakdown and energy production, including cortisol (stress response), glucagon (in response to low glucose), and adrenaline (fight or flight response).

• Anabolic Hormones: Regulate synthesis of molecules and include insulin (promotes glucose uptake), growth hormone (stimulates growth), testosterone, and estrogen (influence muscle and fat distribution).

Disorders Related to Metabolism

Cushing Syndrome

Definition: Caused by excess cortisol; symptoms include rapid weight gain, especially around the abdomen and face, depression, and fatigue.

• Treatments: Reducing cortisol levels through medication, lifestyle changes, or removing tumors causing excess ACTH.

Addison's Disease

Definition: Insufficient cortisol production; symptoms include low blood pressure, fatigue, and darkening of the skin.

• Treatments: Cortisol replacement therapy to restore normal hormone levels.

Oxidation-Reduction Reactions

Definition: Transfer of electrons between molecules; essential in metabolic processes for energy release.

• Key Coenzymes: NAD (Nicotinamide Adenine Dinucleotide) and FAD (Flavin Adenine Dinucleotide) are crucial for facilitating oxidation-reduction reactions in metabolic pathways.

24.2 Carbohydrate Metabolism

Glycolysis:

Process: Breakdown of glucose into pyruvate, generating ATP through two phases:

• Energy-consuming phase: Creates two ATP molecules.

• Energy-yielding phase: Produces four ATPs, netting two ATPs and two NADH molecules.

Anaerobic vs. Aerobic Respiration

• Anaerobic Respiration: Converts pyruvate into lactic acid in the absence of oxygen, producing less energy.

• Aerobic Respiration: Occurs in the presence of oxygen, utilizing the Krebs cycle and electron transport chain to produce substantially more ATP.

Krebs Cycle (Citric Acid Cycle)

Process: Pyruvate is converted into acetyl CoA and further processed in a series of enzymatic reactions that produce ATP, NADH, and FADH2.

• Significance: Plays a key role in cellular respiration and energy production by oxidizing acetyl CoA.

Electron Transport Chain (ETC)

Function: Uses NADH and FADH2 to create ATP through oxidative phosphorylation; oxygen acts as the final electron acceptor, forming water.

Gluconeogenesis

Definition: Synthesis of glucose from non-carbohydrate sources, crucial during fasting or intense exercise.Process Steps: Pyruvate is converted into glucose-6-phosphate through multiple steps involving key enzymes and substrates, ensuring glucose availability.

24.3 Lipid Metabolism

Lipid Metabolism Overview:

Includes the oxidation of fatty acids and the synthesis of new lipids from glucose, which is essential for energy storage and hormone production.

Lipolysis

Definition: Breakdown of triglycerides into fatty acids and glycerol; essential for energy production, particularly when glucose levels are low.

Ketogenesis

Definition: Conversion of excess acetyl CoA into ketone bodies occurs when carbohydrate availability is low, providing an alternative energy source for the brain and other tissues during periods of starvation or prolonged fasting.

• Importance: Helps maintain energy supply when glucose is scarce.

Lipogenesis

Definition: Formation of fatty acids and lipids from excess glucose; primarily occurs in adipocytes and liver cells.

• Significance: Helps in long-term energy storage and management of excess caloric intake.

Aging and Metabolism

• Metabolic Decline: Metabolic rate decreases approximately 2% per decade after age 30, impacting overall energy levels and body composition.

• Maintaining Metabolism: Suggestions include regular exercise, a balanced diet rich in nutrients, staying hydrated, and effective stress management techniques to support metabolic health throughout aging.