Chapter 24: Nutrition, Metabolism, and Body Temperature Regulation: Study Notes

Nutrition, Metabolism, and Body Temperature Regulation

Nutrition

Nutrition is the process by which the body takes in and utilizes food substances for growth, maintenance, and repair. The energy content of food is measured in kilocalories (kcal), commonly referred to as "Calories" on food labels.

• Nutrient: A substance in food used to promote normal growth, maintenance, and repair.

• Major Nutrients: Carbohydrates, lipids, proteins, and water.

• Vitamins: Organic compounds required in small amounts for growth and health, often functioning as coenzymes.

• Minerals: Inorganic elements needed for various physiological functions.

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

Carbohydrates

• Primarily derived from plants; minor sources include milk and meat.

• Complex carbohydrates (polysaccharides) are digested into monosaccharides.

• Glucose is the main energy source for ATP production; other monosaccharides are converted to glucose by the liver.

• Some carbohydrates are incorporated into cell membranes.

• Recommended intake: 125-175 g/day; most Americans consume 200-300 g/day.

• Excess carbohydrates are stored as glycogen (in liver and muscle) or as fat in adipose tissue.

Lipids

• Consumed as saturated fats (animal products) and unsaturated fats (seeds, nuts, vegetable oils).

• Cholesterol is found in egg yolk, meats, and dairy products.

• Essential fatty acids: Linoleic and linolenic acids, found in vegetable oils.

• Functions: Absorption of fat-soluble vitamins, energy source for liver and muscle, structural components (phospholipids, myelin), insulation, and hormone synthesis.

• Dietary recommendations (American Heart Association):

  • Fats: ≤30% of total caloric intake

  • Saturated fats: ≤10% of total fat intake

  • Cholesterol: ≤250 mg/day

Proteins

• Complete proteins: Contain all 8 essential amino acids (mainly animal products).

• Incomplete proteins: Most plant proteins; proper combinations (e.g., grains + legumes) provide all essential amino acids.

• Functions: Structural materials (muscle, connective tissue, cell membranes), enzymes, hormones.

• Proteins are used for energy if carbohydrate intake is insufficient.

• Nitrogen balance:

  • Positive: Protein synthesis > breakdown (growth, pregnancy, recovery)

  • Negative: Protein breakdown > synthesis (starvation, illness, trauma)

• Recommended intake: 0.8 g/kg body weight per day.

Vitamins

• Mostly function as coenzymes in metabolic reactions.

• Most must be obtained from the diet, except:

  • Vitamin D (synthesized in skin)

  • Vitamin K and some B vitamins (produced by intestinal bacteria)

• Water-soluble: Vitamin C and B vitamins (except B12); excess excreted in urine.

• Fat-soluble: Vitamins A, D, E, K; stored in body fat, risk of toxicity with excess intake.

Minerals

• Required in moderate amounts: Ca2+, P, K+, S, Na+, Cl-, Mg2+; and trace amounts of others.

• Functions: Bone mineralization, structural components, electrolytes, coenzymes.

• Main sources: Animal products, vegetables, legumes.

Metabolism

Metabolism encompasses all chemical reactions in the body, divided into anabolic (building) and catabolic (breaking down) processes. The main goal is to produce ATP for cellular work.

• Metabolism: All chemical reactions in a cell.

• Anabolism: Synthesis of larger molecules from smaller ones.

• Catabolism: Breakdown of large molecules into smaller ones.

• Cellular respiration: Catabolic process that breaks down food to produce ATP.

• Oxidation: Loss of electrons (often as hydrogen), releasing energy.

• Reduction: Gain of electrons (often as hydrogen).

Mechanisms of ATP Synthesis

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from an intermediate substrate.

• Oxidative phosphorylation (chemiosmosis): ATP generated via the electron transport chain and redox reactions.

Carbohydrate Metabolism

Glucose is the primary fuel for ATP production, undergoing a series of redox reactions in the cell.

• Complete oxidation of glucose yields 40-44 ATP molecules.

• Three main phases:

  1. Glycolysis: Glucose (6C) → 2 pyruvate (3C each); yields 2 ATP.

  2. Krebs Cycle: Pyruvate → CO2; yields 2 ATP.

  3. Electron Transport System (ETS): Produces 36-40 ATP; requires oxygen (aerobic respiration).

• Anaerobic respiration: Occurs without oxygen; only 2 ATP produced, with lactic acid as a byproduct (must be detoxified by the liver).

• Excess glucose is stored as glycogen (glycogenesis) or fat (lipogenesis).

• If glucose is low, the liver can synthesize glucose from proteins or fats (gluconeogenesis).

Lipid Metabolism

Lipids are a secondary energy source, mainly used when carbohydrates are insufficient. Only neutral fats are used for ATP production.

• Lipids are transported as chylomicrons in the lymph and blood.

• Unused glycerol and fatty acids are stored as triglycerides (lipogenesis).

• Stored fats are broken down (lipolysis) when needed for energy.

• Excessive fat breakdown leads to ketone body formation (ketosis), which can cause metabolic acidosis if unchecked.

• Fats are also important for cell membranes, myelin, and hormone synthesis.

Protein Metabolism

Proteins are primarily used for structural and functional roles, but can be used for energy if necessary.

• Amino acids are deaminated (removal of amine group), converted to glucose, pyruvate, or Krebs cycle intermediates.

• Amine group is converted to urea in the liver and excreted in urine.

• Nonessential amino acids are synthesized in the liver.

• Protein turnover is high: 500-1000 lbs. synthesized over a lifetime.

The Liver: Central Metabolic Organ

• Processes and stores nutrients from all major groups.

• Maintains blood energy sources, metabolizes hormones, detoxifies substances.

• Synthesizes cholesterol, bile salts, and lipoproteins.

Cholesterol and Lipoprotein Transport

Cholesterol is transported in the blood by different types of lipoproteins, each with distinct roles and health implications.

Lipoprotein	Main Function	Health Impact
VLDL (Very Low Density Lipoprotein)	Transports triglycerides from liver to adipose tissue	Precursor to LDL
LDL (Low Density Lipoprotein)	Transports cholesterol to peripheral tissues	"Bad" cholesterol; associated with atherosclerosis
HDL (High Density Lipoprotein)	Transports excess cholesterol from tissues to liver for excretion	"Good" cholesterol; protective effect

Key Equations

• ATP Yield from Glucose Oxidation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 40-44\ \text{ATP}$

• Protein Intake Recommendation: $\text{Daily Protein (g)} = 0.8 \times \text{Body Weight (kg)}$

Example: Nitrogen Balance

• Positive nitrogen balance: Occurs during growth, pregnancy, or recovery from illness (protein synthesis > breakdown).

• Negative nitrogen balance: Occurs during starvation, illness, or trauma (protein breakdown > synthesis).

Additional info:

• Body energy balance and temperature regulation are closely linked to metabolic processes, but detailed mechanisms are not included in this summary.