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

Nutrition, Metabolism, and Energy Balance

Major Nutrient Categories and Terminology

Nutrition is the study of nutrients and how the body uses them for growth, maintenance, and repair. Nutrients are substances in food that promote normal physiological functions. The energy value of foods is measured in kilocalories (kcal), where 1 kcal is the amount of heat needed to raise 1 kg of water by 1°C.

• Major nutrients: Carbohydrates, proteins, lipids, and water

• Other nutrients: Vitamins and minerals (required in minute amounts)

• Essential nutrients: Nutrients that cannot be synthesized by the body in sufficient quantities and must be obtained from the diet (about 45-50 molecules)

• Food groups: Grains, fruits, vegetables, meats and fish, milk products

Once absorbed, nutrients participate in metabolism, which includes:

• Anabolism: Building larger molecules from smaller ones (requires energy)

• Catabolism: Breaking down complex molecules into simpler ones (releases energy)

Carbohydrate Uses and Metabolism

Carbohydrates are the body's preferred energy source. Dietary recommendations suggest 45-65% of caloric intake should come from carbohydrates, with an emphasis on complex carbohydrates.

• Polysaccharides: Starch (plant storage), glycogen (animal storage), cellulose (insoluble fiber), and soluble fiber

• Disaccharides: Sucrose, maltose, lactose (broken down into monosaccharides)

• Monosaccharides: Glucose (main fuel), others converted to glucose

Glucose Metabolism: Glucose is metabolized through three main processes:

1. Glycolysis: Occurs in the cytosol; 1 glucose → 2 pyruvic acid + 2 ATP (net) + 2 reduced coenzymes

2. Krebs Cycle: Occurs in mitochondria; requires O2; each pyruvic acid → 3 CO2 + 5 reduced coenzymes + 1 ATP (per glucose: 6 CO2, 10 reduced coenzymes, 2 ATP)

3. Electron Transport Chain & Oxidative Phosphorylation: Occurs in mitochondria; uses reduced coenzymes and O2 to generate ATP and water. 12 reduced coenzymes from one glucose yield 28 ATP (net).

Total ATP yield from one glucose:

• Direct ATP: 4

• From reduced coenzymes: 28

• Total: 32 ATP

Key Processes:

• Glycogenesis: Formation of glycogen from excess glucose (mainly in liver and skeletal muscle)

• Glycogenolysis: Breakdown of glycogen to release glucose when blood glucose is low

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources (amino acids, glycerol) when glucose and glycogen are depleted

Example: After a carbohydrate-rich meal, excess glucose is stored as glycogen in the liver. During fasting, glycogenolysis and gluconeogenesis maintain blood glucose levels.

Protein Uses and Metabolism

Proteins are essential for tissue structure, enzymes, transport, immunity, and some hormones. The recommended daily intake is 0.8 g per kg of body weight.

• Complete proteins: Contain all essential amino acids (e.g., eggs, milk, fish, meats)

• Incomplete proteins: Lacking one or more essential amino acids (e.g., legumes, nuts, cereals)

Protein Metabolism:

• Amino acids are used to build body proteins or, if in excess, are broken down for energy.

• Deamination: Removal of the amine group (NH2) from amino acids before they can be used for energy. The amine group is converted to ammonia, then to urea in the liver, and excreted in urine.

Example: During starvation, body proteins are broken down, and amino acids are deaminated for energy production.

Lipid Uses and Metabolism

Lipids are a concentrated energy source and are important for cell membranes, hormone synthesis, and vitamin absorption. Dietary lipids should be ≤30% of total caloric intake, with saturated fats ≤10% of total fat.

• Triglycerides: Glycerol backbone + 3 fatty acids (saturated or unsaturated)

• Essential fatty acids: Linoleic acid (must be obtained from diet)

• Cholesterol: Obtained from diet and synthesized by the liver; used for bile salts, steroid hormones, and cell membranes

Lipid Metabolism:

• Glycerol can enter glycolysis; fatty acids are broken down in mitochondria via beta-oxidation.

• Fats yield 9 kcal/g (vs. 4 kcal/g for carbohydrates and proteins).

• Excess lipids are stored as triglycerides in adipose tissue.

Example: During prolonged exercise, fatty acids become the primary fuel for muscle cells.

Role of Vitamins

Vitamins are organic compounds required in small amounts for normal metabolism, often functioning as coenzymes.

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

• Fat-soluble vitamins: A, D, E, K (stored in liver, except K; can accumulate to toxic levels)

• Some vitamins can be synthesized by the body (e.g., vitamin D in skin, vitamin K and some B vitamins by intestinal bacteria, vitamin A from beta-carotene)

• Vitamin B12 absorption requires intrinsic factor

Example: Deficiency of vitamin C leads to scurvy; excess vitamin A can cause toxicity.

Role of Minerals

Minerals are inorganic elements required for various physiological functions. Seven are needed in moderate amounts; others are trace minerals.

• Major minerals: Calcium, phosphorus, potassium, sulfur, sodium, chlorine, magnesium

• Functions: Strengthen tissues (e.g., bone), maintain fluid balance, form parts of proteins, hormones, and phospholipids

• Most minerals are ionized in body fluids or bound to organic compounds

Example: Calcium and phosphorus are major components of bone and teeth.

Absorptive and Post-Absorptive States

The body alternates between the absorptive (fed) and post-absorptive (fasting) states to regulate nutrient use and storage.

Absorptive State

• Occurs during and shortly after eating

• Anabolism exceeds catabolism; energy storage predominates

• Glucose is the main energy source; excess stored as glycogen and fat

• Amino acids used for protein synthesis; excess converted to fat

• Insulin is the main regulatory hormone

Post-Absorptive State

• Occurs between meals; catabolism exceeds anabolism

• Goal: Maintain blood glucose (70-110 mg/100 ml)

• Processes: Glycogenolysis, lipolysis, gluconeogenesis, protein catabolism (if needed)

• Glucagon is the main regulatory hormone

Example: Overnight fasting triggers the post-absorptive state, with the liver releasing glucose to maintain blood levels.

Metabolic Rate and Role of the Liver

Metabolic rate is the body's rate of energy output, measured as total heat produced. Basal metabolic rate (BMR) is measured at rest in the post-absorptive state and reflects the energy needed for essential functions. Thyroxine is the most important hormone influencing BMR.

• Metabolic rate equation:

$\text{BMR} = \text{Kcal used per hour per square meter of body surface area}$

• Average person at rest uses about 100 kcal/hour

Role of the Liver in Metabolism:

• Carbohydrate metabolism: Glycogen formation and breakdown, gluconeogenesis, conversion of glucose to fat

• Protein metabolism: Deamination of amino acids, urea formation, plasma protein synthesis

• Lipid metabolism: Lipid storage, fatty acid breakdown, lipoprotein and cholesterol synthesis

• Storage of vitamins and minerals

• Detoxification of alcohol and drugs

• Processing of bilirubin from red blood cell breakdown

• Metabolism of bloodborne hormones

Example: The liver converts excess amino acids to urea for excretion and regulates blood glucose by storing or releasing glucose as needed.

Nutrient	Main Functions	Dietary Sources	Storage/Excess
Carbohydrates	Energy source, structural molecules	Grains, fruits, vegetables	Stored as glycogen; excess converted to fat
Proteins	Tissue structure, enzymes, hormones	Meats, dairy, legumes	Excess deaminated; nitrogen excreted as urea
Lipids	Energy storage, membranes, hormones	Oils, meats, dairy	Stored as triglycerides in adipose tissue
Vitamins	Coenzymes, antioxidant functions	Fruits, vegetables, animal products	Fat-soluble stored in liver; water-soluble excreted
Minerals	Structural, electrolyte balance, cofactors	Varied (dairy, meats, vegetables)	Some stored in bone, others in body fluids