Chapter 3: Digestion – The Human Body & Digestion

The Human Body & Digestion

Introduction to Digestion

The process of digestion involves the breakdown of food into absorbable components, allowing the body to extract and utilize nutrients. The gastrointestinal (GI) tract and accessory organs work together to accomplish digestion, absorption, and elimination.

Why We Eat

Hunger vs. Appetite

• Hunger: The physiological drive for food, nonspecific, and can be satisfied by a variety of foods.

• Appetite: The psychological desire to consume specific foods, often triggered by environmental cues and satisfied by particular foods.

Neural and Hormonal Regulation of Eating

• Hypothalamus: Brain region that triggers feelings of hunger or satiety by receiving signals from the stomach and small intestine.

• Enteric Nervous System: Network of neurons in the GI tract that communicates with the brain to indicate fullness or emptiness.

Hormones Involved in Hunger and Satiety

• Insulin & Glucagon: Regulate blood glucose levels.

• Ghrelin: Stimulates hunger by acting on the hypothalamus.

• CCK (Cholecystokinin): Stimulates satiety center, suppressing hunger.

• Leptin: Suppresses hunger by acting on the hypothalamus.

Factors Affecting Satiety

• Protein-rich foods provide the highest satiety, followed by fats, then carbohydrates.

• High-fat diets are more satiating than high-carbohydrate diets.

• Meals high in fiber and water increase stomach distension and satiety.

• Solid foods are more filling than liquids or semisolids.

Organs Involved in Digestion

The digestive system consists of the GI tract (mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum) and accessory organs (salivary glands, liver, gallbladder, pancreas).

Stomach

Gastric Secretions and Functions

• Gastrin: Hormone that stimulates the secretion of gastric juice.

• Gastric Juice: Contains hydrochloric acid (HCl), pepsin, gastric lipase, and intrinsic factor.

• HCl: Denatures proteins and activates pepsin.

• Pepsin: Begins protein digestion.

• Gastric Lipase: Digests fats.

• Intrinsic Factor: Essential for vitamin B12 absorption.

• Food is mixed with gastric juice to form chyme.

Motility in the GI Tract

• Peristalsis: Wave-like muscle contractions that move food through the GI tract.

• Segmentation: Rhythmic contractions that mix food and increase contact with digestive juices.

Stomach pH and Protection

• The stomach is highly acidic (pH 1-3) due to HCl.

• A mucus layer and bicarbonate secretion protect the stomach lining from acid damage.

Stomach Ulcer (Peptic Ulcer)

• Caused by Helicobacter pylori infection or excessive acid production.

• Symptoms: Abdominal pain, nausea, vomiting.

• Treatment: Acid-reducing medications and antibiotics.

Gastroesophageal Reflux Disease (GERD)

• Occurs when stomach acid flows back into the esophagus, causing heartburn and discomfort.

• Recommendations: Avoid trigger foods (mint, chocolate, alcohol, caffeine, acidic and spicy foods), eat smaller meals, limit fatty foods, stay upright after eating, and avoid smoking.

• Foods that may help: High-fiber, alkaline, and watery foods.

Small Intestine

Primary Site of Digestion and Absorption

• 90% of nutrient digestion and absorption occurs here.

• Carbohydrates, fats, proteins, vitamins, minerals, and water are absorbed.

Hormones and Enzymes in the Small Intestine

• Secretin: Stimulates pancreatic bicarbonate release to neutralize acid.

• Gastric Inhibitory Peptide (GIP): Stimulates insulin secretion, reduces stomach acid, slows gastric emptying.

• Cholecystokinin (CCK): Stimulates digestive enzyme secretion and suppresses hunger.

• Enzymes: Pancreatic amylase (carbohydrates), pancreatic lipase (fats), proteases (proteins), sucrase/maltase/lactase (carbohydrates).

Villi and Microvilli

• Finger-like projections (villi) and microscopic extensions (microvilli) increase the surface area of the small intestine by 600x, enhancing nutrient absorption.

Celiac Disease

• Autoimmune disorder triggered by gluten (gliadin) in wheat, barley, and rye.

• Damages the small intestine lining, reducing nutrient absorption and causing GI distress.

• Treatment: Strict gluten-free diet.

Gluten-Free Diet

• Contains Gluten: Wheat, rye, barley, triticale, bulgur, malt, etc.

• May Contain Gluten: Oats, processed foods, sauces, gravies, etc.

• Gluten-Free: Potato, corn, rice, buckwheat, quinoa, legumes, nuts, etc.

Large Intestine (Colon)

Functions of the Large Intestine

• Stores undigested food material.

• Absorbs water, short-chain fatty acids, and electrolytes.

• Beneficial bacteria assist with final digestion and produce vitamins.

• Forms and eliminates feces.

Ulcerative Colitis

• Chronic inflammation and ulceration of the colon mucosa.

• Treatment: Medications, surgery, immunosuppressants, dietary management, and bowel rest as needed.

Microbiome

• Gut bacteria finish digesting nutrients, ferment fiber, produce vitamins, inhibit harmful bacteria, and reduce constipation/diarrhea risk.

Probiotic Foods

• Contain live beneficial microorganisms (e.g., Lactobacillus, Bifidobacterium).

• Examples: Yogurt, kefir, kombucha, sauerkraut, kimchi.

• Supplements should list CFU at the end of shelf life.

Prebiotic Foods

• Non-digestible fibers that feed beneficial gut bacteria (e.g., inulin, FOS, GOS, resistant starch).

• Sources: Chicory root, garlic, onions, leeks, asparagus, green bananas, oats, barley, beans, lentils, soybeans.

Enteric Nervous System (ENS)

• Known as the "second brain," the ENS is the largest and most complex part of the peripheral nervous system, with ~600 million neurons.

• Controls secretions, blood flow, hormone release, and motility in the GI tract.

• Gut and brain are in constant communication, influencing mood, behavior, and GI function.

Accessory Organs in Digestion

Liver

• Produces bile for fat emulsification, stores vitamins, and detoxifies substances.

Pancreas

• Secretes digestive enzymes (amylase, lipase, protease) and hormones (insulin, glucagon).

• Releases bicarbonate to neutralize acidic chyme.

Gallbladder

• Stores and releases bile in response to CCK.

• Bile emulsifies lipids, making them more accessible to digestive enzymes.

Absorption and Nutrient Transport

Absorption

• Process of moving molecules across cell membranes into the body.

• Most absorption occurs in the small intestine; a small amount occurs in the stomach.

Transport of Nutrients

• Water-Soluble Nutrients: Absorbed into the blood via capillaries (e.g., amino acids, monosaccharides, water-soluble vitamins, minerals, short-chain fatty acids).

• Fat-Soluble Nutrients: Absorbed into the lymphatic system due to their size (e.g., long-chain fatty acids, cholesterol, fat-soluble vitamins).

Summary Table: Key Digestive Organs and Functions

Organ	Main Function(s)	Key Secretions
Mouth	Mechanical and chemical digestion begins	Saliva (amylase)
Stomach	Mixes food, protein digestion, forms chyme	HCl, pepsin, gastric lipase, intrinsic factor
Small Intestine	Major site of digestion and absorption	Enzymes, bile, bicarbonate
Large Intestine	Absorbs water/electrolytes, forms feces	Beneficial bacteria
Liver	Produces bile, detoxifies, stores nutrients	Bile
Gallbladder	Stores/releases bile	Bile (released on CCK signal)
Pancreas	Secretes digestive enzymes, hormones	Amylase, lipase, protease, bicarbonate, insulin, glucagon