Energy Metabolism

Key Terms and Definitions

• Metabolism: The sum of all chemical reactions that occur within a living organism to maintain life, including both anabolic (building up) and catabolic (breaking down) processes.

• Aerobic Metabolism: Metabolic processes that require oxygen, primarily occurring in the mitochondria of cells. Example: the TCA cycle and electron transport chain.

• Anaerobic Metabolism: Metabolic processes that do not require oxygen, occurring in the cytoplasm. Example: glycolysis and the Cori cycle.

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy. Example: protein synthesis.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. Example: glycolysis.

• ATP (Adenosine Triphosphate): The primary energy carrier in cells. Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate: .

• Glycolysis: The metabolic pathway that converts glucose (6 carbons) into pyruvate (3 carbons each), producing ATP and NADH. Occurs in the cytoplasm.

• TCA Cycle (Krebs Cycle): A series of reactions in the mitochondria that oxidize acetyl CoA to CO2, generating NADH, FADH2, and ATP.

• Electron Transport Chain: A sequence of proteins in the mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, producing ATP.

• Beta Oxidation: The process of breaking down fatty acids into acetyl CoA units in the mitochondria.

• Lipolysis: The breakdown of triglycerides into glycerol and free fatty acids.

• Glycogenolysis: The breakdown of glycogen into glucose.

• Glycogenesis: The synthesis of glycogen from glucose.

• Deamination: The removal of an amino group from an amino acid, preparing it for energy production or excretion.

• Transamination: The transfer of an amino group from one amino acid to a keto acid, forming new amino acids.

• Cori Cycle: The process by which lactate produced in muscles during anaerobic glycolysis is transported to the liver and converted back to glucose.

• Pyruvate: A 3-carbon compound produced from glucose during glycolysis.

• Lactate: A 3-carbon compound formed from pyruvate under anaerobic conditions.

• Acetyl CoA: A 2-carbon compound formed from pyruvate, fatty acids, or amino acids, entering the TCA cycle.

Metabolic Pathways and Energy Sources

• Most Concentrated Source of Energy: Lipids (fats) provide the most energy per gram (9 kcal/g).

• Protein Synthesis: Occurs in ribosomes within the cytoplasm of cells.

• Anaerobic Metabolism Location: Cytoplasm; pathways include glycolysis and the Cori cycle.

• Aerobic Metabolism Location: Mitochondria; pathways include TCA cycle, electron transport chain, and beta oxidation.

• Anabolic Pathways: Glycogenesis, protein synthesis, lipogenesis.

• Compound Used Directly for Energy: ATP; energy is released by hydrolysis of the terminal phosphate bond.

• Cells Relying on Anaerobic Metabolism: Red blood cells (lack mitochondria) and fast-twitch muscle fibers during intense exercise.

• Ketogenic Nutrients: Fatty acids and some amino acids (leucine, lysine) that cannot be converted to glucose.

• Glucogenic Nutrients: Most amino acids and glycerol, which can be converted to glucose.

• Anabolic Hormones: Insulin, growth hormone, testosterone.

• Catabolic Hormones: Glucagon, cortisol, epinephrine.

• Most Metabolically Active Tissue: Liver.

• Tissues Performing Gluconeogenesis: Liver and kidneys.

• Glycolysis Substrate and Products: Begins with glucose; ends with pyruvate (aerobic) or lactate (anaerobic).

• Low Oxygen/Anaerobic Pyruvate Fate: Pyruvate is converted to lactate via the Cori cycle.

Alcohol Metabolism

Key Terms and Definitions

• Ethanol: The type of alcohol found in alcoholic beverages; metabolized primarily in the liver.

• Glycerol: A component of triglycerides; not an alcohol in the context of alcoholic beverages.

• Moderate Alcohol Consumption: Up to one drink per day for women and two for men.

• Alcohol Abuse: Excessive or harmful consumption of alcohol.

• Alcoholism: Chronic disease characterized by uncontrolled drinking and preoccupation with alcohol.

• Binge Drinking: Consuming large amounts of alcohol in a short period (typically 5+ drinks for men, 4+ for women).

• Alcohol Dehydrogenase: The primary enzyme that metabolizes ethanol in the stomach and liver.

• Fatty Liver: Accumulation of fat in liver cells due to excessive alcohol intake.

Alcohol as a Nutrient

• Is Alcohol a Nutrient? Alcohol provides energy (7 kcal/g) but is not essential for life; thus, it is not considered a nutrient.

• Alcohol Metabolizing Enzymes: Alcohol dehydrogenase (stomach, liver), microsomal ethanol oxidizing system (MEOS, liver).

• Health Risks of Alcohol Consumption: Liver disease, cardiovascular disease, cancer, addiction, impaired judgment.

• Brain Effects of Alcohol:

  1. Impaired judgment

  2. Loss of motor coordination

  3. Memory impairment

  4. Depression of respiratory centers

• Food Impact on Alcohol Absorption: Consuming alcohol with food slows absorption and reduces peak blood alcohol levels.

• Signs of Alcoholism: Craving, loss of control, physical dependence, tolerance.

Energy Balance, Body Weight, and Eating Disorders

Key Terms and Definitions

• Appetite: The psychological desire to eat.

• Hunger: The physiological need for food.

• Satiation: The feeling of fullness during a meal that leads to the cessation of eating.

• Satiety: The feeling of fullness after a meal that suppresses hunger until the next meal.

• Hypothalamus: Brain region regulating hunger and satiety.

• Thermogenesis: The production of heat in the body, contributing to energy expenditure.

• Basal Metabolism/BMR: The energy expended at rest to maintain vital body functions. (approximate).

• Physical Activity: Energy expended during movement and exercise.

• Thermic Effect of Food: Energy required for digestion, absorption, and metabolism of food.

• Adaptive Thermogenesis: Changes in energy expenditure due to environmental or physiological factors.

• Body Composition: The proportion of fat, muscle, bone, and other tissues in the body.

• BMI (Body Mass Index): A measure of body weight relative to height.

• Underweight: BMI < 18.5

• Overweight: BMI 25–29.9

• Visceral Fat: Fat stored within the abdominal cavity, associated with increased health risks.

• Central Obesity: Excess fat around the trunk and abdomen.

• Insulin Resistance: Reduced sensitivity to insulin, often associated with obesity and type 2 diabetes.

• Inflammation: Chronic low-grade inflammation is linked to obesity and metabolic diseases.

• Anorexia Nervosa: Eating disorder characterized by self-starvation and excessive weight loss.

• Bulimia Nervosa: Eating disorder involving binge eating followed by purging.

• Binge Eating Disorder: Recurrent episodes of eating large quantities of food without purging.

• RED-S (Relative Energy Deficiency in Sport): Condition affecting athletes due to insufficient energy intake.

Energy Balance and Its Consequences

• Energy Balance: The relationship between energy intake and energy expenditure. Positive balance leads to weight gain; negative balance leads to weight loss.

• Consequences of Imbalance: Underweight, overweight, obesity, and associated health risks.

Influences on Food Intake

• Physical Influences: Hunger, satiety, physical activity.

• Emotional Influences: Stress, mood, psychological factors.

• Environmental Influences: Availability of food, social settings, cultural norms.

Components of Energy Expenditure

• Basal Metabolic Rate (BMR): Largest component; influenced by age, sex, genetics, body composition.

• Physical Activity: Most variable component; influenced by exercise habits.

• Thermic Effect of Food: Typically 5–10% of total energy expenditure.

• Adaptive Thermogenesis: Minor component; influenced by temperature, stress, illness.

Body Weight vs. Body Composition

• Body Weight: Total mass of the body.

• Body Composition: Proportion of fat, muscle, bone, and other tissues; assessed by skinfold measurements, bioelectrical impedance, DEXA scans.

Body Weight and Chronic Diseases

• Obesity: Increases risk for heart disease, diabetes, cancer, and other chronic conditions.

• Underweight: Associated with malnutrition, osteoporosis, and immune dysfunction.

Eating Disorders: Identification and Treatment

• Major Eating Disorders: Anorexia nervosa, bulimia nervosa, binge eating disorder, RED-S.

• Criteria: Diagnostic criteria include behavioral patterns, physical symptoms, and psychological factors.

• Treatment: Multidisciplinary approach including medical, nutritional, and psychological support.