Nutrition: An Applied Approach - The Gastrointestinal System

Nutrition: An Applied Approach

Chapter 3: The Gastrointestinal System

Overview

  • Main Focus: Understanding the structure and function of the gastrointestinal (GI) system from food intake to nutrient absorption and disorders related to specific foods.

Organization of the Body

Atoms and Molecules

  • Atoms: Smallest units of matter.

  • Molecules: Composed of groups of atoms bonded in specific configurations.

    • Examples:

    • Water (H₂O)

    • Carbon dioxide (CO₂)

Biological Molecules

  • Carbohydrates, proteins, fats, and vitamins are large molecules typically involved in biological processes.

  • Goals of Digestion:

    • Breakdown large molecules into smaller molecules.

    • Absorb smaller molecules into body cells.

Cellular Organization

  • Cells: Smallest unit of life; formed from molecules processed through digestion.

  • Cell Composition:

    • Cell Membrane:

    • Composed of phospholipid bilayers.

    • Phospholipid tails face inward; heads face outward.

    • Embedded with cholesterol and proteins.

    • Cytoplasm: Liquid within the cell.

    • Organelles: Special structures within cells (e.g., nucleus, mitochondria).

Tissues and Organs

  • Tissues: Groups of cells functioning together.

    • Examples:

    • Muscle tissue

    • Nervous tissue

  • Organs: Combinations of multiple tissue types performing specific functions.

    • Examples:

    • Stomach

    • Heart

    • Brain

  • Organ Systems: Groups of organs collectively performing functions.

    • Example: Gastrointestinal system.

The Drives to Eat

Definitions

  • Appetite: The desire to eat influenced by sensory stimuli (sight, smell, thought of food).

  • Hunger: A physiological drive indicating a need for food.

  • Anorexia: A condition in which an individual lacks appetite despite needing food, often linked to illnesses or medications.

Mechanisms of Appetite Regulation

  • Hypothalamus: Contains the feeding center and satiety center to regulate hunger.

  • Nerve Cells:

    • Located in the stomach and small intestine; send signals to the hypothalamus.

  • Hormones:

    • Insulin and glucagon influenced by blood glucose levels.

    • Ghrelin (stimulates hunger), Cholecystokinin (CCK), Leptin (promote satiety).

Effects of Food on Satiety

  • Satiety Values:

    • Proteins: Highest satiety value.

    • Carbohydrates: Lower satiety value than fats.

    • Bulky foods: More filling.

    • Solid foods: More filling than liquids or semisolids.

Digestive Process

Definitions

  • Digestion: Breaking down food into component molecules (mechanically or chemically).

  • Absorption: Transport of digested molecules into circulation through the small intestine.

  • Elimination: Removal of waste from the body.

Gastrointestinal (GI) Tract

  • A long tube consisting of organs for processing food through digestion, absorption, and elimination.

  • Includes: Stomach, intestines, and sphincters controlling passage.

  • Accessory Digestive Organs:

    • Salivary glands, liver, gallbladder, pancreas assist in digestion but are not part of the GI tract.

GI Tract Organs and Functions

  1. Mouth:

    • Ingestion: Entry point for food.

    • Mechanical Digestion: Chewing (mastication) prepares food.

    • Chemical Digestion: Begins with salivary amylase (carbohydrates) and lingual lipase (lipids).

  2. Esophagus:

    • Propels food to stomach through peristalsis.

  3. Stomach:

    • Mechanical & Chemical Digestion:

      • Produces chyme through mixing with gastric juices.

      • Pepsin (proteins) and gastric lipase (fats) are activated by hydrochloric acid (HCl).

  4. Small Intestine:

    • Continues chemical digestion with enzymes and bile.

    • Absorbs nutrients into bloodstream and lymphatic system.

  5. Large Intestine:

    • Storage of undigested material, absorption of water/nutrients.

  6. Rectum:

    • Temporary storage for feces prior to elimination.

Accessory Organs of Digestion

  • Liver: Produces bile for fat emulsification.

  • Gallbladder: Stores bile, releasing it into the small intestine.

  • Pancreas: Produces digestive enzymes and bicarbonate to neutralize chyme.

Absorption Mechanisms

  • Mainly occurs in the small intestine (duodenum, jejunum, ileum).

  • Nutrient Absorption:

    • Water-soluble nutrients enter portal vein (to liver).

    • Fat-soluble nutrients enter lymphatic vessels.

  • Methods of Nutrient Transfer:

    • Passive diffusion, facilitated diffusion, active transport, endocytosis.

The Gut Microbiome

  • Composition of beneficial bacteria and microbes in the GI tract.

  • Functions:

    • Aid digestion and synthesize vitamins.

    • Regulate appetite and energy metabolism.

    • Stimulate the immune system.

Neuromuscular System's Role

  • Components:

    • Muscles mix and move food (both voluntary and involuntary).

    • Nerves regulate contractions and secretions (enteric nervous system, autonomic nervous system, central nervous system).

GI Tract Disorders

  1. Heartburn:

    • Caused by HCl in the esophagus.

    • Chronic form is Gastroesophageal reflux disease (GERD).

  2. Peptic Ulcers:

    • Erosion caused by HCl and pepsin; associated with Helicobacter pylori.

  3. Vomiting:

    • Often linked to infections (e.g., norovirus); cyclic vomiting syndrome (CVS) is severe and chronic.

  4. Diarrhea: Range of causes including intolerances and infections, leading to dehydration risks.

  5. Constipation:

    • Defined as no stool for 2 or more days.

  6. Leaky Gut:

    • Associated with various health issues; may arise due to antibiotics, poor diet, high alcohol consumption.

  7. Irritable Bowel Syndrome (IBS):

    • Involves abnormal colon function, symptoms include cramps and alternating diarrhea/constipation.

  8. Cancers:

    • Possible in any GI region, common types include oral, pancreatic, and colorectal cancer.

Food-Related Disorders

Food Intolerance

  • Symptoms include gas, pain, diarrhea; not an immune response.

Food Allergy

  • An immune hypersensitivity to food components (usually proteins).

Celiac Disease

  • Autoimmune disorder leading to gluten intolerance, affecting nutrient absorption.

Non-Celiac Gluten Sensitivity

  • Negative reactions to gluten without celiac disease; symptoms include bloating and headache, improved by gluten-free diet.

Final Notes

  • Consultation Recommended: For any suspected food sensitivity or gastrointestinal disorder, consultation with a healthcare provider is advised.

While both Cholecystokinin (CCK) and Leptin are hormones that promote satiety (the feeling of fullness) by signaling the hypothalamus, they differ in their origin and the time frame of their regulation:

  1. CCK (Cholecystokinin):

    • Origin: It is primarily produced and secreted by the small intestine.

    • Function: It acts as a short-term satiety signal. It is released during digestion in response to the presence of food (particularly proteins and fats) to signal the brain to terminate a meal.

  2. Leptin:

    • Origin: It is produced by adipose (fat) tissue.

    • Function: It acts as a long-term regulator of energy balance. Its levels in the blood are generally proportional to the amount of body fat, informing the hypothalamus about the body's long-term energy stores to help regulate appetite and metabolism over time.

In short, CCK helps manage how much you eat in a single sitting, whereas Leptin helps manage your body weight and energy stores over the long term.