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Chapter 24: Nutrition, Metabolism, and Energy Balance – Study Notes

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Nutrition, Metabolism, and Energy Balance

Overview

This chapter explores how nutrients are metabolized for building blocks or energy, and how energy intake and output are regulated in the human body. The content is divided into three main parts: Nutrients, Metabolism, and Energy Balance.

Chapter 24 flowchart: Nutrients, Metabolism, and Energy Balance

Part 1: Nutrients

Definition and Categories of Nutrients

Nutrients are substances used by the body to promote normal growth, development, and maintenance. Essential nutrients cannot be synthesized in adequate amounts by the body and must be obtained from the diet. Calories represent food energy, with 1 calorie (C) equivalent to 1 kilocalorie, the energy required to raise the temperature of water by 1°C.

  • Major Nutrients: Carbohydrates, lipids, proteins

  • Micronutrients: Vitamins and minerals

  • Caloric Values: Carbohydrates and proteins yield 4 calories/gram; lipids yield 9 calories/gram

Example: According to USDA's MyPlate, a healthy diet includes roughly half fruits and vegetables, half grains and proteins, and dairy as a glass of milk.

Carbohydrates

  • Simple Carbohydrates: Monosaccharides and disaccharides from fruits, sugarcane, honey, milk

  • Complex Carbohydrates: Polysaccharides from grains, fruits, vegetables

  • Glucose: Primary fuel for ATP synthesis; essential for neurons and red blood cells

  • Dietary Fiber: Provided by insoluble cellulose and soluble polysaccharides

Lipids

  • Triglycerides: Most abundant dietary lipids; can be saturated (animal sources, coconut oil, trans fats) or unsaturated (plant sources)

  • Essential Fatty Acids: Linoleic and linolenic acids must be consumed

  • Cholesterol: Found in animal products; most synthesized by the liver

  • Functions: Absorption of fat-soluble vitamins, cellular fuel, myelin sheath and cell membrane formation, adipose tissue, regulatory molecules

Proteins

  • Complete Proteins: Contain all essential amino acids (eggs, milk, fish, meats)

  • Incomplete Proteins: Lacking one or more essential amino acids (legumes, nuts, cereals)

  • Functions: Structural and functional molecules, synthesis of new molecules, energy source

  • Nitrogen Balance: Positive (protein synthesis exceeds breakdown), Negative (breakdown exceeds synthesis)

Vitamins and Minerals

  • Vitamins: Mostly act as coenzymes; fat-soluble (A, D, E, K) absorbed with lipids; water-soluble (B-complex, C) absorbed with water

  • Minerals: Work with other molecules, incorporated into tissues, ionized in fluids, or bound to organic compounds

Part 2: Metabolism

Metabolic Processes

Metabolism is the sum of all biochemical reactions in the body. It includes:

  • Anabolism: Synthesis of larger molecules from smaller ones

  • Catabolism: Breakdown of large molecules into simpler ones

Cellular respiration is a catabolic process where food molecules are broken down to release energy for ATP synthesis.

Oxidation-Reduction Reactions

  • Oxidation: Loss of electrons

  • Reduction: Gain of electrons

  • Importance: Transfer of energy between molecules; enzyme-catalyzed reactions require coenzymes (e.g., NAD+, FAD)

ATP Synthesis Mechanisms

  • Substrate-Level Phosphorylation: Direct transfer of phosphate groups to ADP

  • Oxidative Phosphorylation: Energy from food fuels creates a proton gradient used to attach phosphates to ADP

Carbohydrate Metabolism

  • Glycolysis: Anaerobic process in cytosol; converts glucose to pyruvic acid, NADH, and ATP

  • Citric Acid Cycle: Aerobic process in mitochondria; produces CO2, NADH, FADH2, ATP

  • Electron Transport Chain: Oxygen-requiring; produces water and ATP

  • Net Energy Gain: 30 ATP per glucose molecule

Glucose Handling

  • Glycogenesis: Formation of glycogen from glucose

  • Glycogenolysis: Breakdown of glycogen to glucose

  • Gluconeogenesis: Formation of glucose from non-glucose molecules

Lipid Metabolism

  • Oxidation: Glycerol enters glycolysis; fatty acids undergo beta oxidation to acetyl CoA

  • Lipogenesis: Formation of triglycerides for storage

  • Lipolysis: Breakdown of triglycerides for energy

  • Ketogenesis: Formation of ketone bodies from acetyl CoA

  • Structural Materials: Phospholipids for membranes, cholesterol for hormones

Protein Metabolism

  • Transamination: Transfer of amine group to α-ketoglutaric acid

  • Deamination: Removal of amine group as ammonia, converted to urea

  • Protein Synthesis: Amino acids used for structural and functional proteins

Metabolic States of the Body

  • Absorptive State: Anabolism exceeds catabolism; insulin directs nutrient storage

  • Postabsorptive State: Catabolism exceeds anabolism; glucagon and sympathetic nervous system mobilize energy reserves

Metabolic Role of the Liver

  • Functions: Metabolism of carbohydrates, fats, proteins; storage of vitamins and minerals; detoxification

  • Lipoproteins: VLDL (triglyceride transport), LDL (cholesterol transport), HDL (cholesterol removal)

  • Cholesterol Regulation: Negative feedback, dietary influence, saturated/unsaturated/trans fats

Part 3: Energy Balance

Body Energy Balance

  • Energy Intake: Energy produced during food oxidation

  • Energy Output: Energy lost as heat, used for work, or stored

  • Stable Weight: When intake equals output

Regulation of Food Intake

  • Obesity: BMI > 30; increases risk for several diseases

  • BMI Formula:

  • Hypothalamic Control: Peptides regulate hunger and satiety

  • Short-Term Regulation: Neural signals, nutrient levels, GI hormones

  • Long-Term Regulation: Leptin from adipose cells suppresses hunger

Factors Affecting Metabolic Rate

  • Basal Metabolic Rate (BMR): Energy for essential activities; influenced by age, sex, temperature, thyroxine

  • Total Metabolic Rate (TMR): Energy for all activities; increases with muscle activity and food intake

Body Temperature Regulation

  • Core vs. Shell Temperature: Core (internal organs) is highest; shell (skin) is lowest

  • Heat Exchange Mechanisms: Radiation, conduction, convection, evaporation

  • Hypothalamic Regulation: Heat-loss and heat-promoting centers

  • Heat-Promoting Mechanisms: Vasoconstriction, shivering, increased metabolism, thyroxine release

  • Heat-Loss Mechanisms: Vasodilation, sweating, behavioral changes

  • Fever: Pyrogens reset hypothalamic temperature

Developmental Aspects of Nutrition and Metabolism

  • Early Nutrition: Critical for brain, muscle, and bone development

  • Genetic Disorders: Affect metabolism (e.g., cystic fibrosis, phenylketonuria)

  • Type II Diabetes: Significant in middle age

  • Metabolic Syndrome: Visceral fat increases risk for heart disease and stroke

  • Metabolic Rate Decline: Affects digestion and absorption with age

Additional info:

  • Cross-references to other chapters: Chemical bonding, membrane transport, hypothalamus, hormones, blood flow, chylomicrons, bile, ketone bodies

  • Recent discoveries: Body weight sensing, metabolism without enzymes, lipolysis in white fat for temperature regulation

  • Library research topics: Nutritional guidelines, popular diets, metabolic disorders, vitamin/mineral supplements, effects of inability to sweat

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