Chapter 22: Metabolism and Energy Balance – Study Guide

Chapter 22: Metabolism and Energy Balance

22.1 Appetite and Satiety

This section explores the physiological and biochemical mechanisms that regulate hunger and fullness, focusing on hypothalamic centers and various influencing factors.

• Feeding Center vs. Satiety Center: The feeding center stimulates hunger, while the satiety center suppresses appetite. Both are located in the hypothalamus.

• Control Factors: Neural, hormonal, and psychological factors influence these centers. Examples include blood glucose levels, hormones like leptin and ghrelin, and emotional state.

• External and Internal Influences: Chemical signals (e.g., glucose, insulin), neural inputs (e.g., vagus nerve), psychological cues (e.g., stress), and environmental factors (e.g., food availability) all play roles in regulating food intake.

22.2 Energy Balance

Energy balance is the relationship between energy intake (food) and energy output (work and heat). Maintaining this balance is crucial for body weight regulation.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

• Energy Intake: Consists of carbohydrates, proteins, and fats consumed in the diet.

• Energy Output: The body expends energy as biological work (mechanical, chemical, transport) and heat.

• Types of Biological Work:

  • Mechanical work (e.g., muscle contraction)

  • Chemical work (e.g., synthesis of molecules)

  • Transport work (e.g., moving ions across membranes)

• Direct vs. Indirect Calorimetry: Direct calorimetry measures heat output; indirect calorimetry estimates energy expenditure from oxygen consumption and carbon dioxide production.

• Metabolic Rate: The rate at which the body uses energy, often measured in kilocalories (kcal).

• Energy Content of Nutrients:

  • Carbohydrate: 4 kcal/g

  • Protein: 4 kcal/g

  • Fat: 9 kcal/g

• Basal Metabolic Rate (BMR): The energy expenditure of a person at rest, in a thermoneutral environment, after a 12-hour fast.

• Resting Metabolic Rate (RMR): Similar to BMR but less strict conditions.

• Factors Influencing Metabolic Rate: Age, sex, genetics, hormones (e.g., thyroid hormone), body composition, and activity level.

• Diet-Induced Thermogenesis: The increase in metabolic rate after eating.

• Energy Storage: Excess energy is stored as glycogen (in liver and muscle) and fat (in adipose tissue).

• Glycogen vs. Fat Storage: Glycogen is more readily accessible but stores less energy; fat stores more energy but is less accessible.

22.3 Metabolism

Metabolism encompasses all chemical reactions in the body, divided into anabolic (building up) and catabolic (breaking down) pathways.

• Definition: Metabolism is the sum of all chemical reactions that occur in the body.

• Anabolic Pathways: Synthesize complex molecules from simpler ones (e.g., protein synthesis).

• Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy (e.g., glycolysis, beta-oxidation).

22.4 Fed-State Metabolism

During the fed state (absorptive state), the body processes nutrients absorbed from the digestive tract for immediate use or storage.

• Carbohydrate Metabolism: Absorbed glucose is used for ATP production, stored as glycogen, or converted to fat.

• Glucose Storage: Excess glucose is stored as glycogen in liver and muscle.

• Amino Acid Metabolism: Absorbed amino acids are used for protein synthesis or converted to energy or fat if in excess.

• Lipid Metabolism: Dietary fats are absorbed as chylomicrons, broken down into fatty acids and glycerol, and stored in adipose tissue.

• Lipoproteins: Transport lipids in the blood. Types include chylomicrons, VLDL, LDL, and HDL.

22.5 Fasted-State Metabolism

In the fasted state (postabsorptive state), the body mobilizes stored energy to maintain blood glucose and supply energy to tissues.

• Glycogenolysis: Breakdown of glycogen to glucose, primarily in the liver.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources (e.g., amino acids, lactate).

• Lipolysis: Breakdown of triglycerides into fatty acids and glycerol.

• Beta-Oxidation: Fatty acids are converted to acetyl-CoA for entry into the citric acid cycle.

• Protein Catabolism: Amino acids can be used for energy if necessary, with ammonia as a byproduct excreted as urea.

22.6 Homeostatic Control of Metabolism

Metabolic homeostasis is regulated by hormones, primarily insulin and glucagon, secreted by the pancreas.

• Insulin: Dominant in the fed state; promotes glucose uptake, glycogen synthesis, fat storage, and protein synthesis.

• Glucagon: Dominant in the fasted state; stimulates glycogenolysis, gluconeogenesis, and lipolysis.

• Insulin/Glucagon Ratio: Determines whether the body is in an anabolic (building) or catabolic (breaking down) state.

• Pancreatic Islet Cells:

  • Alpha (α) cells: secrete glucagon

  • Beta (β) cells: secrete insulin

• Regulation of Secretion: Blood glucose concentration is the primary regulator; amino acids and gastrointestinal hormones also play roles.

Diabetes Mellitus

Diabetes mellitus is a group of metabolic diseases characterized by chronic hyperglycemia due to defects in insulin secretion, insulin action, or both.

• Type 1 Diabetes: Autoimmune destruction of β cells leads to insulin deficiency. Prone to ketoacidosis.

• Type 2 Diabetes: Characterized by insulin resistance and relative insulin deficiency. Less prone to ketoacidosis.

• Symptoms: Hyperglycemia, polyuria, polydipsia, polyphagia, weight loss, and in severe cases, metabolic acidosis.

• Diagnosis: Glucose tolerance test, fasting plasma glucose, HbA1c levels.

• Treatment: Type 1: insulin therapy; Type 2: lifestyle modification, oral hypoglycemics, sometimes insulin.

Metabolic Syndrome

Metabolic syndrome is a cluster of conditions that increase the risk of cardiovascular disease and diabetes.

• Criteria: Central obesity, high blood pressure, elevated fasting glucose, high triglycerides, low HDL cholesterol.

• PPARs: Peroxisome proliferator-activated receptors are nuclear receptors that regulate gene expression involved in metabolism.

Key Terms and Definitions

• Glycogenesis: Formation of glycogen from glucose.

• Glycogenolysis: Breakdown of glycogen to glucose.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources.

• Lipogenesis: Synthesis of fatty acids from glucose or amino acids.

• Lipolysis: Breakdown of triglycerides into fatty acids and glycerol.

Important Equations

• Energy Balance Equation:

$\text{Energy Input} = \text{Energy Output} + \text{Energy Stored}$

• Metabolic Rate (Indirect Calorimetry):

$\text{Metabolic Rate} = \text{L O}_2 \text{ consumed/day} \times \text{kcal/L O}_2$

• Respiratory Quotient (RQ):

$RQ = \frac{\text{CO}_2 \text{ produced}}{\text{O}_2 \text{ consumed}}$

Sample Table: Comparison of Fed and Fasted States

Parameter	Fed State	Fasted State
Dominant Hormone	Insulin	Glucagon
Major Metabolic Pathways	Glycogenesis, Lipogenesis, Protein Synthesis	Glycogenolysis, Gluconeogenesis, Lipolysis
Energy Storage	Increased	Decreased (mobilization)
Blood Glucose	Rises then stabilizes	Maintained by hepatic output

Additional info: This study guide expands on the reading questions by providing definitions, explanations, and context for each major topic in Chapter 22, suitable for exam preparation in Anatomy & Physiology.