Nutrition, Metabolism, and Energy Balance

Introduction

This chapter explores the essential nutrients required for human health, the metabolic pathways that convert these nutrients into energy, and the mechanisms regulating energy balance and body temperature. Understanding these processes is fundamental for Anatomy & Physiology students, as they underpin cellular function, tissue maintenance, and overall homeostasis.

Part 1—Nutrients

Categories of Nutrients

Nutrients are substances in food necessary for growth, maintenance, and repair. They are classified into five categories:

• Macronutrients: Carbohydrates, lipids, and proteins (required in large amounts)

• Micronutrients: Vitamins and minerals (required in small amounts)

• Water: Essential for life, makes up a significant portion of food volume

Essential nutrients must be obtained from the diet because the body cannot synthesize them in sufficient quantities.

Energy Value of Nutrients

• Measured in kilocalories (kcal): 1 kcal raises the temperature of 1 kg of water by 1°C.

• Carbohydrates and proteins: 4 kcal/g

• Lipids: 9 kcal/g

Dietary guidelines recommend balanced intake from fruits, vegetables, grains, protein, and dairy.

Carbohydrates

Carbohydrates are primarily derived from plants and serve as the main energy source for cells.

• Sugars: Monosaccharides and disaccharides (fruits, honey, milk)

• Starch: Polysaccharides (grains, vegetables)

• Fiber: Insoluble (cellulose) aids digestion; soluble (pectin) lowers cholesterol

• Uses: Glucose is used for ATP production; excess is stored as glycogen or fat

• Dietary Requirement: 45–65% of total calories, mainly from complex carbohydrates

Lipids

Lipids are a concentrated energy source and are essential for cell structure and function.

• Types: Saturated, unsaturated, and trans fats; cholesterol

• Essential Fatty Acids: Linoleic (omega-6) and linolenic (omega-3)

• Uses: Energy storage, insulation, cell membrane structure, hormone synthesis

• Dietary Requirement: 20–35% of total caloric intake; limit saturated fats

Proteins

Proteins are vital for structural and functional roles in the body.

• Complete Proteins: Animal products and soybeans (contain all essential amino acids)

• Incomplete Proteins: Legumes, nuts, grains (lack one or more essential amino acids)

• Uses: Structural (keratin, collagen), functional (enzymes, hormones)

• Nitrogen Balance: Positive (growth, repair), negative (stress, malnutrition)

• Dietary Requirement: 0.8 g/kg body weight per day

Vitamins and Minerals

Vitamins act as coenzymes, while minerals have structural and regulatory roles.

• Water-soluble vitamins: B complex, C (not stored, excess excreted)

• Fat-soluble vitamins: A, D, E, K (stored, excess can be toxic)

• Antioxidants: Vitamins A, C, E, and selenium neutralize free radicals

• Minerals: Calcium, phosphorus, potassium, sulfur, sodium, chlorine, magnesium

• Roles: Bone strength, nerve function, oxygen transport, hormone synthesis

Part 2—Metabolism

Metabolic Pathways

Metabolism encompasses all biochemical reactions in the body, divided into:

• Anabolism: Building larger molecules from smaller ones (e.g., protein synthesis)

• Catabolism: Breaking down complex molecules into simpler ones (e.g., glycolysis)

Three stages of nutrient processing:

1. Digestion and absorption

2. Anabolic and catabolic reactions in cells

3. Complete breakdown in mitochondria (producing ATP)

Oxidation-Reduction Reactions

Cellular metabolism relies on redox reactions, where electrons are transferred between molecules.

• Oxidation: Loss of electrons or hydrogen

• Reduction: Gain of electrons or hydrogen

• Coenzymes: NAD+ and FAD accept electrons during these reactions

ATP Synthesis

ATP is produced by two mechanisms:

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP

• Oxidative phosphorylation: Electron transport chain and chemiosmosis in mitochondria

Carbohydrate Metabolism

Glycolysis

Glycolysis is the anaerobic breakdown of glucose in the cytosol, yielding pyruvate, ATP, and NADH.

• Phase 1: Sugar activation – Glucose is phosphorylated twice

• Phase 2: Sugar cleavage – Fructose-1,6-bisphosphate splits into two 3-carbon fragments

• Phase 3: Sugar oxidation and ATP formation – Fragments are oxidized, ATP is produced

Citric Acid Cycle (Krebs Cycle)

Occurs in the mitochondrial matrix, further oxidizing pyruvate to produce CO2, NADH, FADH2, and ATP.

• Acetyl CoA enters the cycle, combining with oxaloacetate to form citrate

• Cycle regenerates oxaloacetate and produces high-energy electron carriers

Oxidative Phosphorylation

Electron transport chain uses oxygen to generate a proton gradient, driving ATP synthesis via ATP synthase.

• Electrons from NADH and FADH2 are passed through protein complexes

• Proton gradient powers ATP synthase

Summary of ATP Production

Complete oxidation of one glucose molecule yields approximately 30–32 ATP.

Glycogenesis, Glycogenolysis, and Gluconeogenesis

• Glycogenesis: Formation of glycogen from glucose

• Glycogenolysis: Breakdown of glycogen to release glucose

• Gluconeogenesis: Formation of glucose from noncarbohydrate sources

Lipid Metabolism

Oxidation of Glycerol and Fatty Acids

Triglycerides are broken down into glycerol and fatty acids, which are oxidized for energy.

• Glycerol: Converted to glycolysis intermediate

• Fatty acids: Undergo beta oxidation to form acetyl CoA

Lipogenesis and Lipolysis

• Lipogenesis: Synthesis of triglycerides from excess nutrients

• Lipolysis: Breakdown of triglycerides into fatty acids and glycerol

• Ketogenesis: Formation of ketone bodies from excess acetyl CoA

Amino Acid Metabolism

Degradation of Amino Acids

Amino acids are deaminated and converted to intermediates for energy production or gluconeogenesis.

• Transamination: Transfer of amine group to keto acid

• Oxidative deamination: Removal of amine group as ammonium

• Keto acid modification: Conversion to citric acid cycle intermediates

Protein Synthesis

Amino acids are used to build structural and functional proteins, regulated by hormones and requiring all essential amino acids.

Energy Balance and Metabolic States

Catabolic-Anabolic Balance

The body maintains a balance between breaking down and building up molecules, using nutrient pools for energy and biosynthesis.

Fed State (Absorptive State)

Occurs during and shortly after eating; anabolism exceeds catabolism, and nutrients are stored.

• Glucose: Used for ATP, stored as glycogen or fat

• Triglycerides: Stored in adipose tissue

• Amino acids: Used for protein synthesis or converted to fat

• Insulin: Main hormone directing these processes

Fasting State (Postabsorptive State)

Occurs when the GI tract is empty; catabolism of stored nutrients maintains blood glucose.

• Glycogenolysis: Releases glucose from liver and muscle

• Lipolysis: Releases fatty acids and glycerol

• Gluconeogenesis: Produces glucose from amino acids and glycerol

• Glucagon: Main hormone stimulating these processes

Liver Functions and Cholesterol Metabolism

Liver Metabolic Functions

The liver metabolizes, stores, and detoxifies nutrients, regulates blood cholesterol, and synthesizes lipoproteins.

Cholesterol Transport and Regulation

• Lipoproteins: Transport cholesterol and triglycerides in blood

• Types: VLDL, LDL, HDL, chylomicrons

• Blood cholesterol: Levels and ratios are important for cardiovascular health

• Diet and lifestyle: Affect cholesterol synthesis and excretion

Energy Balance and Body Temperature Regulation

Energy Balance

Energy intake must equal energy output to maintain stable body weight. Output includes heat, work, and storage.

Regulation of Food Intake

Neural and hormonal signals regulate hunger and satiety, involving hypothalamic centers and hormones like leptin, insulin, ghrelin, and CCK.

Basal Metabolic Rate (BMR) and Total Metabolic Rate (TMR)

• BMR: Energy needed for essential activities at rest

• TMR: Total energy expenditure, increases with activity and food intake

Body Temperature Regulation

Body temperature is maintained by balancing heat production and loss, regulated by the hypothalamus.

• Heat production: Basal metabolism, muscular activity, hormones

• Heat loss: Radiation, conduction, convection, evaporation

Clinical Aspects and Developmental Considerations

Metabolic Disorders

• Diabetes mellitus: Insulin deficiency or resistance

• Phenylketonuria (PKU): Inability to metabolize phenylalanine

• Metabolic syndrome: Cluster of risk factors for cardiovascular disease and diabetes

Developmental Aspects

Nutrition and metabolism change throughout life, affecting growth, maintenance, and aging. Adequate nutrition is critical during fetal development and early childhood, while metabolic rate declines with age.

Summary Table: Macronutrients

Key Equations

• ATP yield from glucose:

• BMI calculation:

Additional info: Academic context was added to clarify metabolic pathways, clinical relevance, and regulatory mechanisms. Tables and images were included only when directly relevant to the explanation.