Clinical Nutrition: Ingestion, Digestion, Absorption, and Metabolism

VN120: Clinical Nutrition

Lecture # 2.0

Overview

  • Topic: Ingestion, Digestion, Absorption, and Metabolism

  • Key Organs Involved:

    • Salivary glands

    • Gallbladder

    • Liver

    • Esophagus

    • Pharynx

    • Mouth

    • Stomach

    • Pancreas

    • Small intestine

    • Large intestine

    • Anus

    • Rectum

Digestive Function Overview

Digestive Process

  • The digestive tract breaks down food into smaller substances through:

    • Mechanical digestion

    • Chemical digestion

  • Distinct organ structures perform specific functions as part of the gastrointestinal (GI) system.

Basic Digestive Flow

  • Ingestion: The process of engulfing food into the mouth for chewing and swallowing into the stomach.

  • Digestion: The breakdown of food in the GI tract to release nutrients that are absorbable by the body.

  • Absorption: The process of taking nutrients into the cells lining the GI tract.

  • Transport: Movement of nutrients through the circulatory system from one area of the body to another.

  • Metabolism: Chemical changes in cells that produce energy, tissue building materials, and metabolic control processes.

    • Anabolism: The building up of larger substances from smaller units.

    • Catabolism: The breaking down of larger substances into smaller units.

The Digestive Process

Mechanical and Chemical Digestion

Mechanical Digestion

  • Involves GI motility, supported by:

    • Muscles

    • Nerves

Chemical Digestion

  • Involves GI secretions:

    • Hydrochloric acid and buffer ions

    • Enzymes

    • Mucus

    • Water and electrolytes

    • Bile

Digestion: Mouth & Esophagus

  • Mechanical Digestion:

    • Mastication initiates breakdown of food.

    • Chewed food is swallowed and moves down the esophagus.

    • Gastroesophageal sphincter relaxes to allow food entry into the stomach and constricts to retain food.

  • Chemical Digestion:

    • Salivary glands secrete enzymes and mucous material binding food particles.

    • Lingual lipase from glands at the back of the tongue aids fat digestion.

    • Mucous glands in the esophagus help move food towards the stomach.

Digestion in the Stomach

  • Mechanical Digestion:

    • Food enters the fundus, where stomach muscles knead, mix, and propel food forward.

    • By the time food reaches the antrum, it becomes a semiliquid mix known as chyme.

    • The pyloric valve controls the release of chyme into the duodenum.

  • Chemical Digestion:

    • Hydrochloric acid is secreted to aid digestion.

    • Mucus protects the stomach lining and binds food.

    • Chief cells secrete zymogen pepsinogen, activated by HCl to pepsin, which aids protein breakdown.

Digestion in the Small Intestine

Duodenum

  • First part of the small intestine (8-10 inches).

  • Acidic chyme triggers the release of proteases and enzymes like enterokinase.

    • Activation of Zymogens:

    • Trypsinogen to trypsin

    • Chymotrypsinogen to chymotrypsin

    • Procarboxypeptidase to carboxypeptidase

    • Function: Breakdown polypeptides into simple amino acids for absorption.

Mechanical Digestion in the Small Intestine

  • Process:

    • Peristaltic waves push food forward.

    • Pendular movements stir chyme.

    • Segmentation rings chop food into lumps.

    • Longitudinal rotation rolls food in a spiral motion.

    • Surface villi motions stir and mix chyme.

Chemical Digestion

  • Secretions from the pancreas and small intestines are critical:

    • Pancreatic Enzymes for macronutrient digestion:

    • Amylase breaks down carbohydrates to glucose.

    • Lipase breaks down lipids to free fatty acids.

    • Proteases breakdown proteins to amino acids.

      • Includes trypsin, chymotrypsin, and carboxypeptidase.

Gallbladder

  • Stores and concentrates bile produced by the liver.

  • Functions of Bile:

    • Breaks down fats during digestion.

    • Carries away waste products.

Completion of Protein Digestion in the Small Intestine

  • Two protein-splitting enzymes complete breakdown:

    • Aminopeptidase: Attacks nitrogen-containing amino acids, releasing them one at a time.

    • Dipeptidase: Final enzyme breaking down remaining dipeptides into individual amino acids for absorption.

Absorption and Metabolism

Absorption Definition

  • Involves the distribution, use, and storage of nutrients throughout the body.

Nutrient Transformation

  • At this stage of digestion:

    • Carbohydrates: Reduced to monosaccharides (glucose, fructose, galactose).

    • Fats: Reduced to fatty acids and monoglycerides.

    • Proteins: Reduced to individual amino acids.

    • Vitamins and Minerals: Liberated from food.

  • Factors Influencing Nutrient Absorption:

    • Bioavailability

    • Quantity of nutrient in the GI tract

    • Competition at absorption sites

    • Forms of nutrients present

Absorption Mechanisms in the Small Intestine

Structures Involved

  • Mucosal folds, Villi, Microvilli: Enhance absorptive surface area.

Absorption Processes

  • Mechanisms:

    • Simple diffusion

    • Facilitated diffusion

    • Active transport

    • Pinocytosis

Absorption in the Large Intestine

  • Main Function: Water absorption; some remains for feces.

  • Dietary Fiber: Adds bulk to feces.

  • Nutrients: Absorbed through blood or lymph.

  • Transport Mechanisms:

    • Vascular System: Carries water-soluble nutrients and oxygen, and transports waste via portal circulation.

    • Lymphatic System: Provides an alternative route for fat-soluble nutrient absorption.

Metabolism Overview

  • At this point, macronutrients are:

    • Broken down into building blocks

    • Absorbed into bloodstream or lymphatic system

    • Ready for conversion into energy or storage

  • Micronutrients are also available for absorption.

Metabolic Processes

  • Metabolism Location: Takes place in the mitochondria of cells.

Catabolism vs. Anabolism

  • Catabolism: Breaking down large substances into smaller units.

  • Anabolism: Building larger substances from smaller units.

  • Krebs Cycle: Mitochondria convert energy into usable forms.

Energy Yield from Macronutrients

  • Conversion Table:

    • Carbohydrate: $4 ext{ kcal/g}$

    • Fat: $9 ext{ kcal/g}$

    • Protein: $4 ext{ kcal/g}$

    • Alcohol: $7 ext{ kcal/g}$

Energy Density and Storage

  • Energy Density: Metabolism of glucose yields less energy than fat.

  • Energy Source Preference: The body prefers glucose as the primary energy source.

  • Storage Mechanisms:

    • Excess glucose converts to glycogen.

    • Added protein is converted into glucose or fat when in excess.

    • When glycogen stores are full, surplus energy from all macronutrients is stored as fat.

Errors in Digestion and Metabolism

Genetic Defects

  • Missing enzymes can disrupt normal nutrient metabolism. Examples include:

    • Phenylketonuria: Abnormal accumulation of phenylalanine.

    • Galactosemia: Impaired breakdown of sugar galactose.

    • Glycogen Storage Diseases: Issues in glycogen breakdown.

  • Other Conditions:

    • Intolerances (e.g., lactose intolerance).

    • Allergies.

Energy Balance

Fundamentals of Energy Balance

  • Calories In vs. Calories Out: Maintains life and health.

  • Imbalance can lead to underweight or overweight conditions.

Human Energy System

Energy Needs

  • Involuntary body processes (greatest energy consumption).

  • Voluntary work and exercise demand energy as well.

  • Measurement of Energy:

    • Kilocalorie (kcal): Heat required to raise 1 kg of water by 1° C.

    • Joule (J): International energy measure unit.

  • Conversion from kilocalories to kilojoules: Multiply by $4.184$.

Food as Energy Fuel

  • ATP: The metabolic end product derived from energy-yielding foods.

  • Fuel Factors:

    • Carbohydrate: 4 kcal/g

    • Fat: 9 kcal/g

    • Protein: 4 kcal/g

    • Alcohol: 7 kcal/g

Energy and Nutrient Density

  • Energy Density: Foods high in energy concentration.

  • Nutrient Density: Foods rich in vitamins and minerals opposed to empty calorie foods.

Balancing Intake and Output

Effects of Imbalance

  • Balancing energy intake and output is crucial for maintaining health.

Recommendations for Dietary Energy Intake

General Guidelines

  • Life Cycle Specificity:

    • Growth periods require extra energy for tissue development.

    • Energy needs decrease with age, but this decline varies between individuals.

  • Dietary Reference Intakes: Set at two standard deviations above average to meet the needs of 97.5% of the population.

  • Dietary Guidelines:

    • Emphasize a healthy eating pattern at an appropriate calorie level.

    • Align with Physical Activity Guidelines for Americans.

    • Recommendations aligned with MyPlate principles.