1. Jamie Pope, Steven Nizielski, and Alison McCook
NUTRITION
for a Changing World
FIRST EDITION
Chapter 3
Digestion
A Gut Feeling
© 2016 by W. H. Freeman and Company & Scientific American

2. Chapter 3
Objectives
Summarize the body’s organization from cells to organ systems
Explain the four main functions of the gastrointestinal (GI) tract
Identify and describe the function of the organs and accessory organs of the GI tract
Describe mechanisms by which food is propelled through the GI tract
Explain the importance of enzymes in digestion
Describe three ways nutrients can be transported across a cell membrane
Explain how carbohydrates, proteins, and fat are absorbed from the small intestine
Identify three nutrition-related GI disorders, and summarize potential nutritional (or dietary) implications and interventions

3. Is Gluten Really All That Bad?
Gastroenterologist Joseph Murray, MD
Increase in celiac disease in the United States
Celiac disease impairs digestion if untreated
Can lead to malnutrition
Learn about digestion to better understand what happens in celiac disease
Gastroenterologist Joseph Murray, MD, moved to the United States in 1988 where he began to see patients with stomach problems and weight loss, which upon testing showed signs of celiac disease. He has since seen more than a tenfold increase in celiac disease where he works at the Mayo Clinic in Rochester, Minnesota. Celiac disease is a disease where the body mounts an immune response against gluten, a protein found in wheat, rye, and barley. If people with celiac disease eat gluten, their immune system attacks the villi in the intestine, causing malabsorption of nutrients and malnutrition. They can suffer from stunted growth, anemia, and bone loss. This disease demonstrates the importance of proper digestion and absorption by the gastrointestinal (GI) tract. When the GI tract is damaged or impaired, as in untreated celiac disease, malabsorption causes malnutrition, no matter how much the person eats.

4. Organization of the Body
Cells are the smallest functional unit of living organisms
Tissue is a collection of cells with a similar origin, organized to form organs
Organ system is a group of organs that function together
Cells are the smallest functional unit of living organisms.
Tissue is a collection of cells with a similar origin, organized to form organs , such as the stomach, intestine, liver, kidney, heart, and lungs.
An organ system is a groups of organs that function together, such as the gastrointestinal system, cardiovascular system, and nervous system

5. Organization of the Body

6. Overview of the Digestive Process
Ingested food travels through the gastrointestinal tract from mouth to anus, where they are
Digested
Absorbed
Excreted
The gastrointestinal tract goes from the mouth to the anus. Food is ingested in the mouth and travels through the esophagus, stomach, small intestine, large intestine, and anus.
Food is digested and broken into smaller units in the GI tract. It is then absorbed. Nutrients pass into the bloodstream or lymphatic system to transport through the body. Anything not absorbed is excreted through the anus (the opening at the other end).

7. Overview of the Digestive Process
The gastrointestinal tract goes from the mouth to the anus. Food is ingested in the mouth and travels through the esophagus, stomach, small intestine, large intestine, and anus.
Food is digested and broken into smaller units in the GI tract. It is then absorbed. Nutrients pass into the bloodstream or lymphatic system to transport through the body. Anything not absorbed is excreted through the anus (the opening at the other end).

8. Digestive System Organs of the GI Tract Mouth Esophagus Stomach
Small intestine
Large intestine

9. Digestive System Accessory Organs Salivary Glands Liver Gallbladder
Pancreas

10. Digestion Mechanical Digestion Begins in the Mouth Teeth masticate
Food bolus
Esophagus
Peristalsis
Segmentation

11. Digestion

12. Digestion Mechanical Digestion Small Intestine Peristalsis
Segmentation

13. Digestion Mechanical Digestion Stomach Smooth muscle contractions
Storage
Chyme

14. Digestion Chemical Digestion Enzymes Hormones Mouth Salivary glands
Amylase
Lipsase

15. Digestion

16. Digestion Chemical Digestion Stomach Gastric juices Chyme
Acid
Proteases
Lipase
Chyme
Pyloric Sphincter

17. Digestion

18. Digestion
Chemical Digestion
Small Intestine
Duodenum
Jejunum
Ileum

19. Digestion Chemical Digestion Small Intestine Pancreas
Pancreatic juices
Bicarbonate
Amylase
Proteases
Lipase

20. Digestion
Cholecystokinin (CCK)
Gallbladder
Bile made in the liver

21. Digestion and Absorption
Small Intestine
Brush Border
Villi
Microvilli
Enzymes
Surface area for absorption

22. Absorption
Celiac disease causes an abnormal reaction to the protein gluten in wheat, which caused damage to the villi in the small intestine. This damage to the villi causes them to flatten out and interrupts digestion and absorption. This causes malabsorption and over time this damage increases the risk of cancer and mortality. This does not happen in people who do not have celiac disease.

23. Absorption

24. Absorption

25. Absorption Circulatory system Lymphatic system
Distribute nutrients to cells of the body
Celiac disease

26. Absorption
Celiac disease

27. Large Intestine Cecum Colon Rectum Bacterial flora
Undigested material enters the colon, where little digestion or absorption occurs, except with electrolytes, water, and some vitamins (K, biotin, and folate). There is a lot of bacteria in the colon that eats undigested material, such as fiber. Mucus is secreted to protect the lining and leftover material, mostly fiber, is excreted as feces.

28. Large Intestine
Undigested material enters the colon, where little digestion or absorption occurs, except with electrolytes, water, and some vitamins (K, biotin, and folate). There is a lot of bacteria in the colon that eats undigested material, such as fiber. Mucus is secreted to protect the lining and leftover material, mostly fiber, is excreted as feces.

29. Bacteria in the Gut Probiotics Beneficial bacteria Prebiotics
Food for the beneficial bacteria
Chicory, whole-grain rye, oats, wheat , barley, leeks, onions, and garlic

30. Digestive Disorders Vomiting Diarrhea Constipation Hemorrhoids
Diverticular disease

31. Digestive Disorders

32. Digestive Disorders Gastritis Ulcer Acid reflux
Gastroesophageal reflux disease (GERD)

33. Digestive Disorders

34. Digestive Disorders

35. Digestive Disorders Intestinal Irritable bowel syndrome (IBS)
Inflammatory bowel disease (IBD)
Gallstones

37. Celiac Disease Autoimmune disease with many symptoms
Occurs in about 1 percent of Americans
Completely treatable by avoiding gluten
Typical symptoms of celiac disease are diarrhea and weight loss. Other major symptoms are skin rash and neurological problems, making it hard to diagnose. Symptoms improve with a gluten-free diet.
Since it is a poorly understood condition, many people self-diagnose as having a non-celiac gluten sensitivity. A non-celiac gluten sensitivity appears in <10 percent of individuals who self-diagnose.

38. Summary
The process of digestion makes nutrients available for absorption and use in the body
The primary organs of the gastrointestinal tract include the mouth, stomach, small intestine, and large intestine
The digestive system includes accessory organs that secrete fluids that contain a variety of agents that aid in digestion; this includes the salivary glands, liver, gallbladder, and pancreas
Mechanical digestion, which begins in the mouth, involves the physical fragmentation of foods into small particles. Food is mixed with digestive fluids and propelled along the GI tract through two patterns of contractions of the smooth muscles called peristalsis and segmentation

39. Summary (Cont’d)
Chemical digestion involves specific enzymes and other secretions that break down chemical compounds in food
Enzymes are protein molecules that catalyze, or speed up, the rate at which a chemical reaction produces new compounds with altered chemical structures
Hormones are chemical messengers that participate in the control and regulation of body processes

40. Summary (Cont’d)
The stomach secretes gastric juice that contains hydrochloric acid and nutrient-specific enzymes that, together with peristaltic contractions, break down food into a semiliquid called chyme
The small intestine is the primary site where the chemical digestion of food and the absorption of nutrients are accomplished. The small intestine has structural features—villi and microvilli (often called the brush border)—that increase its absorptive area

41. Summary (Cont’d)
Nutrients enter mucosal cells along the GI tract through simple diffusion and facilitated diffusion, as well as through active transport
From the brush border, nutrients are distributed to tissues and organs through either the circulatory system or the lymphatic system
Probiotics are live, beneficial bacteria found in fermented foods that can restore or maintain a healthy balance of “friendly” bacteria in the GI tract
Prebiotics are nondigestible carbohydrates broken down by bacteria in the large intestine that foster the growth of the beneficial bacteria
Digestive disorders affect millions of people each year and diminish quality of life and overall health. Diet may play an important role in the prevention and treatment of these conditions