Metabolism and Energy Balance

Appetite and Satiety

The regulation of food intake is a complex process involving hormonal, neural, and psychological factors. The body uses various signals to maintain energy homeostasis.

• Leptin: A hormone secreted by adipocytes (fat cells) when fat stores increase. Leptin acts to decrease food intake, signaling satiety to the brain.

• Ghrelin: A hormone secreted by the stomach during fasting. Ghrelin increases hunger and stimulates appetite.

• Neural and psychological factors: The hypothalamus integrates hormonal signals and psychological cues to regulate appetite and satiety.

Example: After a large meal, leptin levels rise, reducing the desire to eat further, while during fasting, ghrelin levels increase, promoting hunger.

Energy Balance

Energy balance is achieved when energy input equals energy output, maintaining a stable body weight. Imbalances lead to weight gain or loss.

• Energy Input: The calories consumed from food and beverages.

• Energy Output: The sum of energy used for basal metabolism, physical activity, and thermogenesis.

• Types of Work:

  • Transport work: Movement of molecules across cell membranes.

  • Mechanical work: Movement of the body and within cells (e.g., muscle contraction).

  • Chemical work: Synthesis and storage of molecules for growth, maintenance, and energy storage.

• Voluntary energy output: Can be increased through physical activity; energy intake can be controlled by dietary choices.

Example: A person who increases daily exercise will increase energy output, potentially leading to weight loss if energy input remains constant.

Metabolic Rate and Oxygen Consumption

Metabolic rate is the rate at which the body expends energy. Oxygen consumption is a common method for estimating metabolic rate, as it reflects the rate of nutrient metabolism.

• Basal Metabolic Rate (BMR): The lowest metabolic rate, measured under strict conditions (rest, fasting, thermoneutral environment).

• Resting Metabolic Rate (RMR): Similar to BMR but measured while awake and at rest after a 12-hour fast.

Equation (for metabolic rate estimation):

Example: Measuring a person's oxygen consumption at rest provides an estimate of their RMR.

Factors Influencing Metabolic Rate

Several factors affect an individual's metabolic rate:

• Age and Sex: Men generally have higher RMR than women; metabolic rate declines with age.

• Lean Muscle Mass: Muscle tissue consumes more oxygen than fat tissue, increasing metabolic rate.

• Activity Level: Physical activity and muscle contractions raise metabolic rate.

• Diet: Proteins increase metabolic rate more than fats or carbohydrates after ingestion (diet-induced thermogenesis).

• Hormones: Thyroid hormones, epinephrine, and norepinephrine increase BMR.

• Genetics: Inherited traits influence metabolic efficiency and energy storage.

Example: After a protein-rich meal, metabolic rate increases more than after a fat-rich meal.

Energy Storage: Fat and Glycogen

The body stores energy primarily as fat and glycogen. The efficiency and capacity of these storage forms differ.

• Carbohydrates: Stored as glycogen in the liver and skeletal muscles; provides energy for 10-15 hours.

• Fats: Main energy storage form; 1 gram of fat provides 9 kilocalories, while 1 gram of protein or carbohydrate provides 4 kilocalories.

• Metabolic accessibility: Glycogen is more readily mobilized than fat, but fat stores are larger and more energy-dense.

Example: During prolonged fasting, the body shifts from using glycogen to fat stores for energy.

Metabolism: Anabolic and Catabolic Pathways

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

• Anabolic pathways: Synthesize larger molecules from smaller ones (e.g., protein synthesis).

• Catabolic pathways: Break down larger molecules into smaller ones (e.g., glycolysis).

• Metabolic States:

  • Fed (Absorptive) State: Follows a meal; nutrients are absorbed, used, and stored (anabolic).

  • Fasted (Postabsorptive) State: After nutrients are absorbed; body uses stored nutrients (catabolic).

• Fates of Ingested Energy:

  1. Immediate energy production (ATP synthesis)

  2. Synthesis of cellular components

  3. Storage as glycogen or fat

• Nutrient pools: Readily available nutrients in plasma for immediate use.

Key Metabolic Processes:

• Glycogenesis: Synthesis of glycogen from glucose.

• Lipogenesis: Synthesis of fat from glucose.

• Glycogenolysis: Breakdown of glycogen to glucose.

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

Fed-State and Fasted-State Metabolism

The body alternates between anabolic and catabolic states depending on nutrient availability.

• Fed-State (Anabolic):

  • Amino acids are used to synthesize proteins.

  • Fats are stored for future energy needs.

• Fasted-State (Catabolic):

  • Glycogen is converted to glucose for energy.

  • Proteins can be broken down for ATP production, starting with plasma amino acids, then muscle proteins.

  • Lipids undergo lipolysis, breaking down into fatty acids and glycerol. Fatty acids are converted to acyl groups (2 carbons) via beta-oxidation and enter the citric acid cycle. Glycerol can be used for gluconeogenesis or ketone body production (ketosis).

Example: During overnight fasting, the liver breaks down glycogen and initiates gluconeogenesis to maintain blood glucose levels.

Homeostatic Control of Metabolism: The Pancreas

The pancreas plays a central role in metabolic regulation through its endocrine and exocrine functions.

• Islets of Langerhans: Clusters of endocrine cells in the pancreas (<2% of pancreatic mass).

• Beta cells: Secrete insulin (dominant in fed state; promotes anabolism).

• Alpha cells: Secrete glucagon (dominant in fasted state; promotes catabolism).

• Insulin:Glucagon ratio: Regulates the switch between anabolic and catabolic metabolism.

Example: After a meal, insulin secretion increases, promoting glucose uptake and storage; during fasting, glucagon secretion increases, promoting glucose release.

Diabetes Mellitus and Metabolic Syndrome

Diabetes mellitus is a group of disorders characterized by chronic high blood glucose due to insulin issues.

• Type 1 Diabetes: Autoimmune destruction of beta cells; no insulin produced. Requires insulin therapy.

• Type 2 Diabetes: Insulin resistance; cells do not respond to insulin. May progress to insulin deficiency.

• Gestational Diabetes: Develops during pregnancy; increases risk of type 2 diabetes for mother and child.

• Metabolic Syndrome: Diagnosed when type 2 diabetes, atherosclerosis, and high blood pressure are present together.

Example: A patient with type 2 diabetes may initially have normal insulin levels but poor glucose control due to insulin resistance.

Regulation of Body Temperature

Body temperature is tightly regulated to maintain optimal enzyme function and metabolic processes.

• Heat Production: Generated by metabolism and muscle activity.

• Unregulated Heat: From voluntary muscle contractions and normal metabolism.

• Regulated Heat: For maintaining core body temperature.

• Thermoregulation: The hypothalamus acts as the body's thermostat, adjusting heat production and loss.

Body Temperature Adjustments:

• Hot Flashes: Temporary decreases in the hypothalamic set point, often due to low estrogen.

• Fever: Elevated set point as part of immune response; mild to moderate fevers are generally safe, but high fevers can cause brain damage.

• Hyperthermia: Abnormally high body temperature. Includes:

  • Heat Exhaustion: Severe dehydration, core temp 37.5–39°C (99.5–102.2°F), symptoms include cramps, nausea, headache, pale and sweaty skin.

  • Heat Stroke: More severe; flushed, dry skin, high mortality rate (~50%). Requires rapid cooling.

• Hypothermia: Abnormally low body temperature; slows enzymatic reactions and oxygen consumption, can lead to unconsciousness.

Example: Athletes exercising in hot, humid conditions are at risk for heat exhaustion and heat stroke if not properly hydrated and cooled.

Summary Table: Key Hormones and Their Roles

Additional info: The above notes expand on the original content by providing definitions, examples, and a summary table for clarity and completeness.