Chp. 24

Introduction to Nutrition, Metabolism, and Building Blocks

Nutrient Definitions:
- Nutrient: A substance in food the body uses to promote normal growth, maintenance, and repair.
- Essential Nutrient: A nutrient that cannot be made by the body (or cannot be made in sufficient amounts) and must be provided by the diet.
- Calorie: A unit used to measure the energy value of food. In nutrition, this often refers to a kilocalorie (kcal), which is the amount of heat energy needed to raise the temperature of $1\,kg$ of water by $1\,^{\circ}C$ .
Major Nutrient Categories:
- Macronutrients: Categories of nutrients that make up the bulk of what we eat. These include Carbohydrates, Lipids, and Proteins.
- Micronutrients: Essential nutrients required in trace or small amounts, specifically Vitamins and Minerals.
Carbohydrate Classifications and Sources:
- Complex Carbohydrates: These include starches found in plant-based foods. Notable sources include potatoes, rice, and whole grain breads.
- Simple Carbohydrates: Sugars found naturally in fruits and processed into sugary drinks.
Lipid Classifications and Sources:
- Saturated Fats: Primarily found in meat and certain plant products such as coconut.
- Unsaturated Fats: Found in plant-based oils like olive oil, as well as seeds and nuts.
Proteins and Amino Acid Balance:
- Complete Proteins: Proteins that contain all of the body's essential amino acids in the proper portions for human needs. Key sources include eggs, fish, and milk.
- Incomplete Proteins: Proteins that are low in one or more essential amino acids (e.g., corn, peas).
- Nitrogen Balance: The state in which the rate of protein synthesis equals the rate of protein breakdown and loss.
  - Positive Nitrogen Balance (P): Occurs when the amount of protein incorporated into tissues exceeds the amount broken down for energy. Examples include children during growth, pregnancy, and tissue repair following illness or injury.
  - Negative Nitrogen Balance (N): Occurs when protein breakdown for energy exceeds the amount being incorporated into tissues. Examples include physical or emotional stress (infection, injury, burns), starvation, or when dietary protein quality/quantity is poor.

Major Cellular Uses of Macronutrients

Carbohydrates:
- Serve as the major energy fuel (specifically glucose) for forming ATP.
- Form the glycocalyx on the exterior surface of the plasma membrane.
Lipids:
- Provide insulation and serve as a reserve body fuel.
- Form the basic structure of the plasma membrane (phospholipids and cholesterol).
Proteins:
- Act as important structural materials of the body, such as keratin (hair/nails) and collagen (connective tissue).
- Regulate body functions as enzymes and some hormones.

Vitamins and Minerals

General Function of Vitamins: Most vitamins function as coenzymes, which act with an enzyme to accomplish a particular chemical task.
Vitamin Classifications:
- Fat-Soluble Vitamins: These are stored in the body's fat tissues and liver. They include Vitamin A (retinol), Vitamin D, Vitamin E (tocopherol), and Vitamin K (phylloquinone).
- Water-Soluble Vitamins: These are not stored in significant amounts; excesses are generally excreted. They include Vitamin C (ascorbic acid) and B vitamins such as Vitamin $B_{2}$ (riboflavin), Vitamin $B_{3}$ (niacin), Vitamin $B_{9}$ (folic acid).
- Toxicity Risk: Overconsumption of fat-soluble vitamins (hypervitaminosis) is more hazardous than water-soluble vitamins because the body stores them in fatty tissues rather than excreting them in urine.
Essential Minerals:
- Major Minerals: Required in amounts greater than $200\,mg$ per day. These include Sodium ( $Na$ ), Potassium ( $K$ ), and Calcium ( $Ca$ ).
- Functions of Specific Minerals:
  - Calcium ( $Ca$ ): Critical for bone and tooth formation, blood clotting, and nerve/muscle function.
  - Iron ( $Fe$ ): A component of hemoglobin (for oxygen transport) and electron carriers in energy metabolism.
  - Zinc ( $Zn$ ): Component of several enzymes; not primarily for thyroid hormones (iodine serves that role).
  - Phosphorus ( $P$ ): Essential for bone formation and ATP structure.
  - Sodium ( $Na$ ): Important for water balance, blood pressure, and nerve function.
- Dietary Sources: Minerals are abundant in vegetables, legumes, milk, and some meats.

Principles of Metabolism and Redox Reactions

Foundational Definitions:
- Metabolism: The sum of all biochemical reactions occurring in the body.
- Catabolism: Degradative reactions; the breakdown of complex structures into simpler ones (typically releasing energy).
- Anabolism: Synthetic reactions; the building of larger molecules from smaller ones (typically requiring energy).
Oxidation-Reduction (Redox) Reactions:
- Oxidation: The loss of electrons from a substance; in biological systems, this often involves the loss of hydrogen atoms.
- Reduction: The gain of electrons by a substance; the addition of hydrogen atoms.
- Energy Transfer: "Oxidized" substances lose energy, while "reduced" substances gain energy. Energy-rich electrons move between substances.
- Glucose Oxidation Equation: $C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6H_{2}O + 6CO_{2} + 32\,ATP + \text{heat}$
  - Glucose is oxidized to carbon dioxide (loses hydrogen/electrons).
  - Oxygen is reduced to water (gains hydrogen/electrons).
Role of Coenzymes: Coenzymes (such as $NAD^{+}$ and $FAD$ ) act as hydrogen or electron acceptors when a substrate is oxidized, carrying them to the electron transport chain.
Phosphorylation Mechanisms:
- Substrate-level Phosphorylation: Involves the direct transfer of a phosphate group from a substrate to ADP. This occurs in both the cytosol (during glycolysis) and the mitochondrial matrix (during the citric acid cycle).
- Oxidative Phosphorylation: A more complex process requiring ATP synthase and the creation of a proton ( $H^{+}$ ) gradient. This occurs exclusively in the mitochondrial matrix/inner membrane.

Carbohydrate and Lipid Metabolism

Glucose and Glycogen Management:
- Glycogen: A polysaccharide that serves as the storage form of glucose in the liver and skeletal muscle.
- Glycogenesis: The formation of glycogen from glucose subunits when glucose levels exceed the demand for ATP.
- Glycogenolysis: The breakdown of glycogen into glucose subunits; stimulated by low blood glucose levels.
- Gluconeogenesis: The formation of "new" glucose from noncarbohydrate molecules (e.g., glycerol or amino acids); stimulated by low blood glucose levels.
Lipid Metabolism and Ketones:
- Ketone Bodies: Organic compounds (such as acetoacetic acid and acetone) produced during the incomplete breakdown of fat.
- Stimulus for Formation: Ketone bodies are produced when carbohydrates are unavailable or cannot be used, forcing the body to rely heavily on fat for fuel (e.g., starvation, uncontrolled diabetes mellitus).
- Consequences of Excess: Excessive levels lead to ketosis, which can cause metabolic acidosis (ketoacidosis), potentially leading to coma or death.

Amino Acid Metabolism and Protein Synthesis

Amino Acid Degradation:
1. Transamination: An amine group is transferred from an amino acid to a citric acid cycle keto acid, typically generating glutamate.
2. Oxidative Deamination: The amine group is removed from glutamate as ammonium ( $NH_{4}^{+}$ ) and combined with $CO_{2}$ in the liver to form urea, which is excreted.
3. Keto Acid Modification: The remaining keto acids are altered to produce metabolites that can enter the citric acid cycle.
Synthesis Requirements:
- The "all-or-none" rule applies: all essential amino acids must be present at the same time and in sufficient quantities to produce a protein.
- Adults require eight essential amino acids from the diet.
- Nonessential amino acids are synthesized in the liver by transferring groups from other acids to citric acid cycle intermediates.
- If essential amino acids are lacking, body proteins are broken down, resulting in a negative nitrogen balance.

Energy Balance: Fed and Fasting States

Nutrient Pools:
- Amino Acid Pool: The body's total supply of free amino acids. They cannot be stored as proteins; excess is converted to fat, glycogen, or used in the citric acid cycle as keto acids.
- Carbohydrate Pool: Total supply of glucose and glycogen available for immediate use.
- Fat Pool: Total supply of lipids in the body.
The Fed (Absorptive) State:
- Occurs during and for about four hours after eating.
- Anabolism exceeds catabolism.
- Glucose is the primary energy fuel.
- Nutrients move from the GI tract to the blood.
- Insulin: The primary hormone directing the fed state; it stimulates glucose and amino acid uptake by cells and inhibits gluconeogenesis.
The Fasting (Postabsorptive) State:
- The period when the GI tract is empty and body reserves are broken down for energy.
- Catabolism exceeds anabolism.
- Glucose Sparing: The use of non-carbohydrate fuels (fats) to save glucose for the brain.
- Glucagon: The primary hormone directing the fasting state; it stimulates gluconeogenesis and glycogenolysis.
- Other Regulators: Increased sympathetic nervous system activity and epinephrine trigger glycogenolysis and lipolysis.
Sources of Blood Glucose During Fasting:
1. Liver Glycogenolysis: Breakdown of liver glycogen.
2. Skeletal Muscle Glycogenolysis: Breakdown of muscle glycogen (indirectly via glycolysis/lactic acid).
3. Lipolysis: Breakdown of adipose tissue (glycerol used for gluconeogenesis).
4. Protein Catabolism: Use of cellular proteins (amino acids used for gluconeogenesis).

Liver Functions and Cholesterol Transport

Metabolic Role: The liver is the body's main metabolic organ, responsible for processing nearly every nutrient, storing vitamins and minerals, and detoxifying substances like drugs, alcohol, and metabolic waste.
Cholesterol and Lipoproteins:
- Cholesterol Sources: Approximately $85\%$ of blood cholesterol is produced endogenously by the liver and intestinal cells; only $15\%$ comes from the diet.
- Lipoproteins: Lipid-protein complexes used to transport hydrophobic lipids (cholesterol/triglycerides) in the blood.
- Classification:
  - Chylomicron: Highest lipid proportion; carries absorbed lipids from the intestine to the liver.
  - Very Low-Density Lipoprotein (VLDL): Carries triglycerides from the liver to adipose tissue.
  - Low-Density Lipoprotein (LDL): Carries cholesterol from the liver to peripheral tissues. High levels are "bad" as they contribute to arterial plaque.
  - High-Density Lipoprotein (HDL): Highest protein proportion; carries cholesterol from tissues back to the liver for excretion. High levels are "good."
- Desirable Levels: Total blood cholesterol should generally be below $200\,mg/dL$ .

Metabolism and Thermoregulation

Metabolic Rates:
- Basal Metabolic Rate (BMR): The energy the body needs at rest to perform only essential activities (breathing, heartbeat).
  - Measurement Conditions: Measured in the postabsorptive (fasting) state, in a reclining/resting position, and in a thermally neutral environment.
  - Influencing Factors: Surface area, age, gender, stress, and hormones.
  - Thyroxine: The major hormone controlling BMR; it increases oxygen consumption and heat production by accelerating metabolic rate.
- Total Metabolic Rate (TMR): The total rate of kilocalorie consumption needed to fuel all ongoing activities (including exercise and food digestion).
Body Temperature Regulation:
- Core: Includes organs within the skull and the thoracic/abdominal cavities. Temperatures are precisely regulated and highest ( $37\,^{\circ}C$ or $98.6\,^{\circ}F$ ).
- Shell: Includes the skin. Temperatures fluctuate substantially and are generally lower than the core.
- Heat Exchange Mechanisms:
  - Radiation: Loss of heat via infrared waves.
  - Conduction: Transfer of heat via direct contact.
  - Convection: Transfer of heat through air currents (warm air rises, cool air falls).
  - Evaporation: Heat loss as water vaporizes from body surfaces.
Hypothalamic Control:
- The hypothalamus is the body's main integrating center/thermostat.
- Responses to Low Temperature: Cutaneous blood vessel constriction, shivering (skeletal muscle activity producing heat), and release of epinephrine/norepinephrine (non-shivering thermogenesis).
- Responses to High Temperature: Cutaneous blood vessel dilation (bringing heat to the surface) and increased sweating (evaporative cooling).