Nutrition Lecture 2

Digestion, Absorption, Metabolism, and Energy Balance

• Digestion is the process of converting ingested nutrients into absorbable forms. It involves mechanical (e.g., chewing) and chemical (enzymatic) processes.
• Stages of feeding:
  • Ingestion: Consumption of food.
  • Digestion: Mechanical (chewing, churning) and chemical (enzymatic breakdown).
    • Carbohydrates: Broken down by amylase.
    • Proteins: Broken down by proteases (e.g., pepsin).
    • Fats: Broken down by lipases into fatty acids and glycerol.
  • Absorption: Nutrients enter the bloodstream/lymphatic system.
  • Assimilation: Nutrients used for energy, growth, and repair.
  • Respiration: Metabolic pathways like glycolysis, Krebs cycle.
  • Excretion/Egestion: Removal of waste and undigested matter.
• Digestion of Nutrients:
  • Carbohydrates: Digestion begins in the mouth with salivary amylase; further breakdown in the small intestine by pancreatic amylase.
  • Proteins: Begins in the stomach with pepsin and HCl; further breakdown in the small intestine with enzymes like trypsin and chymotrypsin.
  • Lipids (Fats): Begins in the small intestine; bile emulsifies fats, lipase breaks them down.
• Food Processing:
  • Methods to transform raw ingredients into consumable products.
  • Reasons: Food safety, enhanced quality, increased convenience.

Categories of Food Processing

• Mechanical: Grinding, chopping, mixing.
• Thermal: Baking, frying, boiling, steaming.
• Chemical: Preservatives, colorants, flavor enhancers.
• Biological: Fermentation, enzymatic reactions.

Impact of Processing on Nutrient Content

• Nutrient retention depends on chemical stability, extent of processing, environmental conditions, and food form (fresh, frozen, canned).

Heat Processing Techniques

• Baking (Dry Heat): Loss of heat-sensitive vitamins (thiamin, vitamin C).
• Microwaving: Retains some vitamins and minerals.
• Frying: May lose water-soluble vitamins but retains fat-soluble ones.
• Grilling: Moderate loss of water-soluble vitamins.
• Boiling: Significant loss of water-soluble vitamins.
• Steaming: Preserves most nutrients.
• Blanching: Reduces nutrient loss by limiting cooking time.
• Pasteurization: Kills pathogens while preserving nutritional quality.

Mechanical Food Processing

• Grinding: Reduces particle size, increases surface area, may cause some nutrient loss (depending on refinement).
• Chopping: Breaks food into smaller pieces; minimal nutrient loss unless oxidation occurs.
• Blending: Liquefies/purees food; may degrade heat-sensitive vitamins; increases some nutrient bioavailability (like carotenoids).

Chemical Food Processing Techniques

• Uses additives to preserve, enhance, and modify food.
  • Preservatives: Prevent spoilage (e.g., sodium benzoate).
  • Antioxidants: Prevent oxidation (e.g., vitamin C).
  • Flavor Enhancers: Enhance taste.
  • Color Additives: Restore or enhance color.
  • Sweeteners: Provide sweetness.
  • Emulsifiers: Combine incompatible ingredients.
  • Thickeners/Stabilizers: Improve texture.
  • pH Control Agents: Maintain desired acidity/alkalinity.
  • Anti-Caking Agents: Prevent clumping.
  • Leavening Agents: Cause gases for light texture.

Common Chemical Processing Methods

• Curing/Salting: Draws out moisture; preserves protein and minerals; increased sodium content.
• Pickling: Preserves food by acidity; may reduce water-soluble vitamins.
• Emulsification: Mixes ingredients with emulsifiers.
• Hydrogenation: Converts unsaturated fats to solid fats (unhealthy trans fats).
• Acidification: Lowers pH to inhibit bacterial growth.

Biological Food Processing Techniques

• Fermentation: Uses microorganisms to convert sugars; enhances B-vitamins and probiotics.
• Enzymatic Browning: A reaction causing fruits/vegetables to brown due to oxidation; prevent by controlling pH, temperature, or using antioxidants.

Energy Balance

• Basal Metabolic Rate (BMR): Minimum energy for basic functions (60-75% daily expenditure).
• Factors affecting BMR: Age, gender, body composition, genetics, hormones, temperature.
• BMR Calculation: Harris-Benedict Equation (men/women); simplified estimate (24 kcal/kg/day).
• Glycemic Index (GI): Ranking of carbohydrates based on blood glucose rise.
  • Low GI: Gradual rise (e.g., oats).
  • Medium GI: Moderate rise (e.g., brown rice).
  • High GI: Rapid rise (e.g., white bread).

Glycemic Index Calculation

• 50g of available carbohydrates are given to participants.
• Blood glucose measured over 2 hours.
• Compared to a reference food (glucose/white bread).
• Average blood glucose response determines GI score.

Energy Balance Concepts

• Energy Balance: Calories consumed = calories burned; crucial for weight maintenance.
• Positive Energy Balance: More calories in than out; weight gain.
• Negative Energy Balance: More calories out than in; weight loss.
• Total Daily Energy Expenditure (TDEE): Components: BMR, TEF (Thermic Effect of Food), PAL (Physical Activity Level), NEAT (Non-Exercise Activity Thermogenesis).

Factors influencing Energy Balance

• Diet composition: High protein increases TEF.
• Physical activity: Boosts calorie expenditure and muscle mass.
• Metabolic Rate: Varies with factors mentioned earlier.
• Genetics: Influences metabolism.

Regulation of Energy Balance

• Hormonal Regulation: (e.g., insulin, glucagon, leptin, ghrelin)
• Neurological Signals: Hypothalamus controls hunger and satiety.

Nutritional and Metabolic Disorders

• Obesity: Prolonged positive energy balance; associated risks.
• Underweight/Malnutrition: Chronic negative energy balance; associated risks.
• Metabolic Syndrome: Cluster of conditions (insulin resistance, high blood pressure).
• Eating Disorders: Anorexia nervosa, bulimia, binge-eating disorder.

Key Metabolic Processes

• Glycolysis: Breakdown of glucose to pyruvate; ATP and NADH produced.
• Pyruvate Oxidation: Pyruvate to Acetyl-CoA; CO2, NADH, and Acetyl-CoA produced.
• Krebs Cycle: Cyclic reactions producing ATP, NADH, and FADH₂ from Acetyl-CoA.
• Electron Transport Chain (ETC): Transferring electrons for ATP generation; oxygen as final electron acceptor.

Energy Balance and Metabolic Disorders

• Hypermetabolism: Increased metabolic rate leading to weight loss.
• Hypometabolism: Decreased metabolic rate causing weight gain.
• Insulin Resistance: Cells not responding to insulin leading to high blood sugar.

Key Definitions and Medical Terms

• Metabolism: All chemical reactions in the body.
• Anabolism: Building complex molecules.
• Catabolism: Breaking down complex molecules.
• Homeostasis: Maintaining stable internal conditions.
• Enzyme: Protein speeding up reactions.
• ATP: Primary energy carrier.
• Gluconeogenesis: Creating glucose from non-carbohydrates.
• Lipolysis: Breakdown of fats.
• Ketosis: Using fat-derived ketones for energy.