CHAPTER 24

Nutrition, Metabolism, and Energy Balance

Introduction

  • Understanding the conversion of nutrients into energy is crucial for advising patients on optimal dietary choices, which can enhance overall body performance.

Part 1: Nutrients

Definition of Nutrients

  • Nutrients are substances obtained from food necessary for growth, maintenance, and repair of the body.

Classification of Nutrients
Major Categories:

  • Macronutrients: Comprise three primary nutrient types that constitute the majority of consumed food:

    • Carbohydrates

    • Lipids (fats)

    • Proteins

  • Micronutrients: Required in smaller amounts, including:

    • Vitamins

    • Minerals

  • Water: Also classified as a nutrient due to its essential role in the body.

Essential and Nonessential Nutrients

  • Essential Nutrients: Must be consumed through diet as the body cannot synthesize them; approximately 45–50 nutrients are deemed essential.

  • Nonessential Nutrients: Can be synthesized by the body from other nutrients, albeit they still play vital roles in life.

Energy Measurement

  • Energy content in food is measured in kilocalories (kcal).

    • 1 calorie is defined as the heat required to raise the temperature of 1 kg of water by 1°C.

    • Note: 1 dietary Calorie = 1 kcal.

Dietary Guidelines

  • The USDA's MyPlate guidelines outline healthy eating principles:

    • Emphasize portion control.

    • Incorporate abundant fruits and vegetables.

    • Prioritize whole grains.

    • Limit junk food consumption.

Part 2: Role of Carbohydrates, Lipids, and Proteins

Carbohydrates
Dietary Sources

  • Primarily sourced from plants:

    • Starch in grains and vegetables (complex carbohydrates).

    • Sugars (mono- and disaccharides) from fruits, sugarcane, sugar beets, honey, and milk.

    • Fiber Types:

    • Insoluble Fiber: Cellulose from vegetables; provides roughage.

    • Soluble Fiber: Pectin from apples and citrus; lowers blood cholesterol levels.

Functions in the Body

  • Glucose: Most utilized fuel to produce ATP (adenosine triphosphate).

    • Fat is an alternative energy source for some cells.

    • Neurons and red blood cells (RBCs) exclusively depend on glucose; lack of glucose can lead to quick neuronal death.

  • Excess glucose is converted into glycogen or fat for storage.

  • Fructose and galactose are converted to glucose in the liver before entry into the bloodstream.

Dietary Requirements

  • Recommended daily carbohydrate intake:

    • 45–65% of total caloric intake, primarily from complex carbohydrates.

    • Limit simple sugars to avoid obesity and nutritional deficiencies.

    • The average American diet constitutes roughly 46% carbohydrates, often higher in low-income populations due to cost-effectiveness of staples like rice and bread.

Lipids
Dietary Sources

  • Triglycerides: Main lipid form, found in:

    • Saturated fats from meat, dairy, tropical oils, hydrogenated oils (trans fats).

    • Unsaturated fats from seeds, nuts, olive oil, and vegetable oils.

  • Cholesterol: Found in egg yolk, meats, organ meats, shellfish, and dairy.

    • Approximately 85% is produced by the liver.

Essential Fatty Acids

  • The liver synthesizes various fatty acids; however, two are essential and must be obtained through diet:

    • Linoleic Acid: An omega-6 fatty acid, part of lecithin.

    • Linolenic Acid: An omega-3 fatty acid, both found in most vegetable oils.

Functions in the Body

  • Adipose tissue provides:

    • Protection

    • Insulation

    • Fuel storage

  • Phospholipids: Vital for myelin sheaths and cell membranes.

  • Cholesterol: Stabilizes membranes and serves as a precursor for bile salts and steroid hormones.

  • Prostaglandins: Play roles in smooth muscle contraction, blood pressure control, and inflammation.

  • Fats enhance absorption of fat-soluble vitamins.

Dietary Recommendations

  • Fats should make up 20-35% of total calories.

  • Saturated fats should comprise less than 10% of total fat intake.

  • Cholesterol intake should be minimized, especially among those with high blood cholesterol, as it is linked to cardiovascular diseases.

Proteins
Dietary Sources

  • Complete Proteins: Found in animal products (eggs, milk, fish, meats) and soybeans; contain all essential amino acids.

  • Incomplete Proteins: Present in legumes, nuts, and cereals; lack one or more essential amino acids.

  • Combining legumes and grains can provide a full amino acid profile.

Functions in the Body

  • Structural Materials: Such as keratin (skin), collagen, elastin (connective tissues), and muscle fibers.

  • Functional Molecules: Enzymes and some hormones.

  • Three factors influencing amino acid use for protein synthesis versus energy burning:

    • All-or-None Rule: All required amino acids must be present for protein synthesis.

    • Adequacy of Caloric Intake: Insufficient caloric intake results in proteins being used for energy.

    • Hormonal Controls: Anabolic hormones enhance protein synthesis; cortisol promotes breakdown.

Nitrogen Balance

  • Defined as the ratio of protein synthesis versus breakdown:

    • Positive Nitrogen Balance: Synthesis exceeds breakdown (normal in childhood, pregnancy, recovery).

    • Negative Nitrogen Balance: Breakdown exceeds synthesis (seen in stress, infections, poor dietary intake, starvation).

Dietary Recommendations

  • Protein needs depend on age, size, metabolic rate, and nitrogen balance; generally 0.8 g of protein per kg of body weight is recommended.

  • Typical American diets provide excess protein compared to needed amounts.

Part 3: Vitamins and Minerals

Vitamins
Importance

  • Organic compounds essential for the body's utilization of nutrients, most functioning as coenzymes.

  • Certain vitamins are synthesized in the body:

    • Vitamin D (synthesized in the skin)

    • Some B vitamins and K are synthesized by intestinal bacteria.

    • Beta-carotene from carrots is converted to vitamin A.

Types of Vitamins

  • Water-Soluble Vitamins: Include B complex and vitamin C; absorbed with water and not stored in the body, leading to excretion within one hour if unused.

  • Fat-Soluble Vitamins: Include vitamins A, D, E, and K; absorbed with lipid products and stored in the body (except K).

    • Potential Risks: Overconsumption can lead to health issues.

Antioxidants

  • Vitamins C, E, A, and the mineral selenium serve as antioxidants that neutralize free radicals generated during metabolism, with sources including broccoli and other vegetables.

Minerals
Overview

  • Seven essential minerals needed in moderate amounts: calcium, phosphorus, potassium, sulfur, sodium, chlorine, magnesium.

  • Others required in trace amounts, aiding in nutrient function and balancing uptake and excretion for detoxification.

Functions

  • Calcium, Phosphorus, Magnesium: Form the hard structure of bones.

  • Iron: Critical for oxygen binding in hemoglobin.

  • Iodine: Necessary for synthesizing thyroid hormones.

  • Sodium, Chloride: Primary electrolytes in blood, crucial for maintaining fluid balance and cellular function.

Part 4: Metabolism

Definition

  • Metabolism encompasses all biochemical reactions in a cell involving nutrients.

Anabolism and Catabolism
Anabolism

  • The process of building larger molecules from smaller ones, e.g., synthesizing proteins from amino acids.

Catabolism

  • The breakdown of complex structures into simpler ones, e.g., protein degradation to amino acids.

Cellular Respiration

  • A key catabolic pathway capturing energy to form ATP. The goal is to trap chemical energy, also storing it as glycogen or fats for later use.

Process Stages of Metabolism

  1. Digestion: Nutrient breakdown and absorption, followed by transportation to tissues.

  2. Cellular Processing: Totaling synthesis of lipids, proteins, or glycogen, and catabolism (glycolysis) into pyruvic acid and acetyl CoA within the cytoplasm.

  3. Oxidative Breakdown: Occurs in mitochondria, leading to CO₂, water, and ATP production; includes glycolysis and the Krebs cycle.

Carbohydrate Metabolism

  • Entry of glucose into cells involves phosphorylation to form glucose-6-phosphate, preventing its escape from cells and facilitating continuous glucose uptake.

  • Complete glucose metabolism requires three main pathways: glycolysis, Krebs cycle, and electron transport chain (oxidative phosphorylation).

Lipid Metabolism

  • Lipids yield higher energy than carbohydrates or proteins: 9 kcal/gram vs. 4 kcal/gram.

  • Fat digestion products are transported as chylomicrons and hydrolyzed into fatty acids and glycerol.

  • Lipogenesis: Synthesis of triglycerides when ATP and glucose levels are high; stored primarily in adipose tissue and other fat deposits.

  • Lipolysis: Breakdown of stored fats into fatty acids and glycerol, favored in energy-deficient situations.

Protein Metabolism

  • Proteins require continual breakdown and replacement; amino acids are recycled into new proteins or other compounds.

  • In excess, amino acids can be oxidized for energy or converted to fat for storage.

  • Hormonal control regulates protein synthesis, requiring all essential amino acids for functionality.