Chapter 3: The Human Body – Why We Really Eat What We Eat? (Digestion, Absorption, and Food-Related Disorders)

Chapter 3: The Human Body – Why We Really Eat What We Eat?

Overview

This chapter explores the physiological and psychological reasons behind eating, the organization of the human body as it relates to nutrition, the processes of digestion and absorption, and disorders related to specific foods. Understanding these concepts is fundamental for students of nutrition, as they form the basis for how nutrients are processed and utilized by the body.

Organization of the Body

Levels of Organization

The human body is organized into several hierarchical levels, each playing a role in nutrition and digestion:

• Atoms: The basic units of matter (e.g., carbon, hydrogen, oxygen).

• Molecules: Combinations of atoms (e.g., water, proteins, carbohydrates, fats, vitamins).

• Cells: The smallest functional units of life, composed of molecules.

• Tissues: Groups of similar cells performing a specific function.

• Organs: Structures composed of different tissues working together (e.g., stomach, intestines).

• Organ Systems: Groups of organs that perform related functions (e.g., digestive system).

Example: The digestive system is an organ system that includes the mouth, esophagus, stomach, intestines, and accessory organs.

Macromolecules and Digestion

• Macromolecules: Carbohydrates, proteins, fats, and vitamins are large molecules essential for body function.

• Goals of Digestion:

  • Break down large molecules into smaller, absorbable units.

  • Absorb these smaller molecules into the cells of the body for use in metabolism and energy production.

Definition: Digestion is the process by which food is broken down into smaller components that can be absorbed by the body.

Why Do We Want to Eat?

Physiological and Psychological Drives

• Appetite: The psychological desire to eat, often triggered by sensory inputs such as sight, smell, or thought of food.

• Hunger: The physiological drive to eat, initiated when the body senses a need for food.

Example: Smelling freshly baked bread can stimulate appetite even if you are not physiologically hungry.

The Role of the Hypothalamus

• The hypothalamus is a region of the brain that regulates hunger and satiety through two centers:

  • Feeding center: Stimulates the desire to eat.

  • Satiety center: Signals fullness and suppresses eating.

• Nerve cells in the stomach and small intestine send signals to the hypothalamus about the presence or absence of food.

• Hormones such as insulin and glucagon (released in response to blood glucose levels) also communicate with the hypothalamus.

Hormonal Regulation of Hunger and Satiety

• Hormones: Chemical messengers produced by specialized glands, traveling through the bloodstream to target organs.

  • Ghrelin: Produced in the stomach; stimulates hunger.

  • Cholecystokinin (CCK): Produced in the small intestine; promotes satiety.

  • Leptin: Produced in adipose (fat) tissue; signals satiety and reduces appetite.

Food Properties Affecting Satiety

• Proteins have the highest satiety value, making you feel full longer.

• Carbohydrates have a lower satiety value than fats.

• Bulky foods (high in fiber and water) increase satiety.

• Solid foods are more filling than semisolid or liquid foods.

Digestion: The Gastrointestinal (GI) Tract

Structure and Function

• The GI tract is a series of organs arranged as a long tube through which food passes.

• Main organs: mouth, esophagus, stomach, small intestine, large intestine.

• Sphincters: Muscles that control the passage of material from one organ to the next.

Digestion in the Mouth

• Mechanical digestion: Chewing breaks food into smaller pieces.

• Chemical digestion: Salivary amylase (an enzyme from salivary glands) begins the breakdown of carbohydrates.

Swallowing and the Esophagus

• The epiglottis covers the trachea during swallowing to prevent food from entering the airway.

• Food moves from the mouth to the stomach via the esophagus.

• Peristalsis: Rhythmic muscular contractions that move food through the GI tract.

• The gastroesophageal sphincter separates the esophagus from the stomach.

Digestion in the Stomach

• The stomach mixes, digests, and stores food.

• Mechanical digestion: Mixing food with gastric juice.

• Chemical digestion:

  • Hydrochloric acid (HCl): Denatures proteins and activates pepsin.

  • Intrinsic factor: Essential for vitamin B12 absorption.

  • Pepsin: Enzyme that digests proteins.

  • Gastric lipase: Enzyme that digests fats.

• Chyme: The semisolid product of mechanical and chemical digestion in the stomach.

Digestion in the Small Intestine

• Chyme is released from the stomach through the pyloric sphincter into the small intestine.

• Chemical digestion continues with the help of pancreatic enzymes and bile.

• The small intestine is divided into three sections:

  • Duodenum

  • Jejunum

  • Ileum

Digestion in the Large Intestine

• Undigested food passes through the ileocecal valve into the large intestine.

• Very little digestion occurs here; water and some nutrients are absorbed.

• Material is stored for 12–24 hours before elimination.

Accessory Organs

• Salivary glands: Produce saliva and enzymes for carbohydrate digestion.

• Liver: Produces bile, which emulsifies fats.

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

• Gallbladder: Stores and releases bile.

Absorption

Process and Sites of Absorption

• Absorption: The process of taking molecules across a cell membrane and into the body’s cells.

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

• The stomach absorbs only a small amount of certain substances (e.g., alcohol, some medications).

Structures Facilitating Absorption

• Villi: Finger-like folds in the lining of the small intestine that increase surface area and are in close contact with nutrient molecules.

• Brush border: Composed of microvilli, further increasing the surface area for absorption.

Transport of Nutrients

• Water-soluble nutrients (carbohydrates, proteins, minerals, some vitamins) enter the portal vein and are transported to the liver.

• Fat-soluble nutrients (lipids, some vitamins) enter the lymphatic vessels and are transported directly to the bloodstream.

Neuromuscular Regulation of the GI Tract

Muscular and Nervous Control

• The GI tract is regulated by both voluntary and involuntary muscles.

• Nerves control contractions and secretions:

  • Enteric nervous system (ENS): Local control within the GI tract.

  • Other branches of the autonomic nervous system.

  • The central nervous system (CNS).

Disorders of the GI Tract

Common Disorders

• Heartburn: Caused by hydrochloric acid entering the esophagus.

• Gastroesophageal reflux disease (GERD): Chronic, painful heartburn occurring more than twice per week.

• Peptic ulcers: Erosion of the GI tract lining by HCl and pepsin; often associated with Helicobacter pylori infection.

• Vomiting: Often due to GI infection; Cyclic vomiting syndrome (CVS) is a chronic condition with severe, recurrent vomiting.

• Diarrhea: Can result from food intolerances, infections, stress, or bowel disorders; may cause dehydration, especially dangerous for children and the elderly.

• Constipation: No stool passed for two or more days.

Signs and Symptoms of Dehydration

Symptoms in Adults	Symptoms in Children
Dizziness	No tears when crying
Dry mouth	No wet diaper for 3+ hours
Infrequent urination	High fever
Sunken eyes	Sunken abdomen, eyes, or cheeks
Lethargy	Irritability or listlessness

Irritable Bowel Syndrome (IBS)

• A disorder interfering with normal colon function.

• Symptoms: Abdominal cramps, bloating, diarrhea, or constipation.

• More common in women than men.

Disorders Related to Foods

Food Intolerance vs. Food Allergy

• Food intolerance: Adverse reaction to a food that does not involve the immune system (e.g., lactose intolerance); symptoms include gas, pain, and diarrhea.

• Food allergy: Immune system reaction to a food component (e.g., peanut allergy); can be life-threatening.

Celiac Disease

• An autoimmune and genetic disorder causing complete intolerance to gluten (a protein in wheat, rye, barley, triticale).

• Gluten ingestion damages the small intestine, leading to poor nutrient absorption.

• Requires a strict gluten-free diet.

Additional info: For further study, students should review the roles of digestive enzymes, the impact of dietary fiber on GI health, and the mechanisms of nutrient transport across cell membranes.