1. Williams' Basic Nutrition & Diet Therapy
Chapter 5
Digestion, Absorption, Transport and Metabolism
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved. 1

2. Digestion (p. 64) Basic principles Principle of change
The body cannot use food as it is eaten.
Food must be changed into simpler substances to be absorbed and then used by cells to sustain life.
Principle of wholeness
The parts of the digestive process comprise a continuous whole.
Food components travel through the gastrointestinal (GI) system until they are delivered to cells or excreted.
The digestive system is highly complex. The organs work both individually and as a whole to provide nutrients to the body.
What is digestion? (It is the process by which food is broken down in the gastrointestinal tract, releasing nutrients in forms that the body can use.)
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

3. Digestion (cont’d) (p. 65)
Point out different organs.
Describe how food flows through the digestive tract.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

4. Mechanical and Chemical Digestion (p. 65)
Mechanical and chemical actions make up the digestive process.
Food must undergo these changes to be delivered to cells.
Specific actions occurring during digestion of carbohydrates, proteins, and fats are discussed in other chapters.
Digestion of each individual nutrient occurs at the same time.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

5. Mechanical Digestion (p. 66)
Mechanical digestion: gastrointestinal motility
Muscles in GI wall produce:
Tonic contractions, which produce continuous movement
Periodic muscle contraction and relaxation, which mix food mass and move it forward
What is peristalsis? (Alternating contractions and relaxations that force food forward.)
Explain the importance of rhythm in mechanical digestion.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

6. Mechanical Digestion (p. 66)
Mechanical digestion: gastrointestinal motility (cont’d)
Nerves from esophagus to anus:
Control muscle tone in wall
Regulate rate and intensity of contractions
Coordinate various movements
Problems with the nerves in the digestive tract can cause problems with the digestive process.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

7. Chemical Digestion (p. 66)
Chemical digestion: gastrointestinal secretions
Hydrochloric acid and buffer ions: produce the correct pH necessary for enzyme activity
Enzymes: specific digestive proteins for breaking down nutrients
Mucus: lubricates and protects the GI tract tissues and helps mix the food mass
Water and electrolytes: carry and circulate the products of digestion through the tract and into the tissues
Bile: divides fat into smaller pieces to assist fat enzymes
GI secretions make chemical digestion possible.
pH can affect the ability of the enzymes to work; stomach environment is acidic, small intestine environment is alkaline.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

8. Digestion in the Mouth and Esophagus (p. 66)
Mechanical digestion
Mastication breaks down food
Food is swallowed and passes down esophagus
Muscles at tongue base facilitate process
Gastroesophageal sphincter at stomach entrance relaxes, allowing food to enter, then constricts to retain food
What is mastication? (Biting and chewing)
This is the start of the digestive process: breaking food into smaller pieces and moving it into the stomach.
If the gastroesophageal sphincter is not working properly, acid and mixed food seep back into esophagus, resulting in heartburn.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

9. Digestion in the Mouth and Esophagus (p. 66)
Chemical digestion
Salivary glands secrete material containing salivary amylase or ptyalin-What does amylase breakdown?
Ebner’s glands at the back of the tongue secrete a lingual lipase
Salivary glands also secrete a mucous material to lubricate and bind food particles, facilitating the swallowing of the food bolus
Secretions from the mucous glands in the esophagus help move food toward the stomach
What is amylase? (A starch-splitting enzyme)
What is lipase? (A fat-splitting enzyme)
What is a food bolus? (A lump of food material)
Not much chemical change occurs in the mouth because the food does not stay there long.
This is a preparatory stage to get the food ready to pass to the stomach and small intestine.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

10. Digestion in the Stomach (p. 67)
Problems with valves and sphincters can cause reflux and discomfort.
The fundus is the upper portion of the stomach.
The duodenum is the first part of the small intestine.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

11. Digestion in the Stomach (p. 67)
Mechanical digestion
Under sphincter control, the food enters upper portion of the stomach (fundus)
Stomach muscles knead, store, mix, and propel food mass forward
By the time food mass reaches the lower portion of the stomach (antrum), it is a semiliquid acid/food mix called chyme
Pyloric valve slowly releases chyme into the first section of the small intestine (duodenum)
High-calorie foods—especially fatty foods—empty out of the stomach at a slower rate; thus the feeling of fullness lasts longer.
Caloric density influences the rate in which the stomach empties.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

12. Digestion in the Stomach (p. 67)
Chemical digestion: three types of acid secretions
Hydrochloric acid: parietal cells in the stomach lining secrete acid to promote gastric enzyme activity; pH is lower in the stomach meaning more acidic than in the small intestine where it is more alkaline-pH higher
Mucus: secretions protect the stomach lining from the erosive effect of the acid and also bind and mix the food mass and help move it along
Enzymes: pepsinogen is secreted by stomach cells and is activated by acid to become pepsin, a protein-splitting enzyme
Various sensations and emotions stimulate the nerve impulses that trigger these secretions.
Anger and stress can increase secretions.
Sadness and fear can decrease secretions and diminish blood flow.
Ask students whether they feel more or less hungry when stressed or fearful. Do they follow these patterns?
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

13. Digestion in the Small Intestine (p. 67)
Mechanical digestion
Peristaltic waves push food forward
Pendular movements stir chyme
Segmentation rings chop food mass into lumps
Longitudinal rotation rolls food in spiral motion, mixing it
Surface villi motions stir and mix chyme
What do surface villi accomplish? (They stir and mix the chyme at the intestinal wall, exposing additional nutrients for absorption.)
The majority of food is digested in the small intestine.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

14. Digestion in the Small Intestine (p. 68)
Chemical digestion: pancreatic enzymes
Carbohydrate: pancreatic amylase converts starch to maltose and sucrose
Protein: trypsin and chymotrypsin split large protein molecules into small peptide fragments and eventually into single amino acids; carboxypeptidase removes end amino acids from peptide chains; remember peptidase breaks down proteins
Fat: pancreatic lipase converts fat to glycerides and fatty acids
What organs are involved in chemical digestion? (The small intestine, pancreas, liver, and gallbladder secrete digestive enzymes and other substances.)
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

15. Digestion in the Small Intestine (p. 68)
Chemical digestion: intestinal enzymes
Carbohydrate: Disaccharidases convert disaccharides into monosaccharides.
Protein: Enterokinase activates trypsinogen from the pancreas to become trypsin; amino peptidase removes end amino acids from polypeptides; dipeptidase splits dipeptides into amino acids.
Fat: Intestinal lipase splits fat into glycerides and fatty acids.
Ask students to explain where these enzymes originate.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

16. Digestion in the Small Intestine (p. 68)
Chemical digestion
Mucus: protects mucosal lining
Bile: emulsifying agent, aids fat digestion and absorption (remember produced by the liver)
Hormones: secretin, produced by mucosal glands in first part of small intestine for alkaline environment, will stimulate the pancreas to release enzymes and other substances; cholecystokinin triggers release of bile, which will break down fats in the duodenum
Mucus is important because the contents entering from the stomach are highly acidic.
Bile breaks fat into smaller pieces so it can be broken down easier.
What does secretin do? (Controls acidity and secretion of enzymes from the pancreas)
Secretin keeps the environment alkaline at a pH of 8 so pancreatic enzymes can work.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

17. Digestion in the Small Intestine (p. 69)
The pancreas, gallbladder, and liver are the accessory organs in digestion.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

18. Absorption and Transport (p. 69)
At this point in the process:
Carbohydrates: reduced to simple sugars (glucose, fructose, galactose)
Fats: changed into fatty acids and glycerides
Proteins: changed into single amino acids
Vitamins and minerals: liberated from food
When digestion is complete, food has been transformed into simple end products ready for use by cells.
This transformation has prepared the food for absorption with a water base for solution and transport.
Now the food is prepared for absorption as part of the GI circulation.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

19. Absorption and Transport (p. 69)
Amount of nutrients consumed depends on bioavailability:
Amount of nutrient present
Competition between nutrients for absorption sites
Form in which nutrient is present
If the nutrients are not necessary, they are excreted.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

20. Small Intestine (p. 71) Small intestine: three absorbing structures
Mucosal folds: surface of small intestine piles into many folds
Villi: small, fingerlike projections
Microvilli: cover each villus
Make inner surface 600 times greater than outer surface of intestinal wall
Each villus can receive protein and carbohydrate material as well as receive fat materials in its lymph vessels.
These lymph vessels are called lacteal.
If the entire surface of the small intestine were spread out, it would be half the size of a basketball court.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

21. Small Intestine (p. 72)
Intestinal wall. Diagram of villi of the human intestine showing the structure and blood and lymph vessels.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

22. Absorption Processes (p. 71)
Simple diffusion: particles move outward in all directions toward areas of lesser concentration
Facilitated diffusion: uses a protein channel for carrier-assisted movement of larger particles
Active transport: carrier partner (e.g., sodium) moves particles across a membrane
Pinocytosis: larger materials are engulfed by a cell
Small particles that do not need a protein channel use simple diffusion.
An example of active transport is glucose using sodium to carry it across membranes.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

23. Absorption Processes (p. 72)
Diagram illustrating transport pathways through the cell membrane.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

24. Absorption in the Large Intestine (p. 73)
Water: main absorptive task of large intestine is to absorb water; small amount remains for feces
Dietary fiber: contributes bulk to help form feces
Macronutrients and micronutrients: absorbed to lymph or blood
Vitamin K is synthesized in the large intestine
Gas is formed from bacterial action not being able to digestible food in the large intestine
Only about 100 mL of water is left in the large intestine after the digestion process.
Dietary fiber contributes to gas.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

25. Effects of Digestion Gas is a normal by-product of digestion
GI tract holds approx. 3 oz of gas moves along with food
Results from trapped air bubbles occurring when eat too fast or drinks through a straw, or takes extra air while eating
Avoid carbonated beverages, do not gulp, chew with mouth closed, do not drink from a can or through straws
Do not avoid while overly nervous
Copyright © 2013, 2009, 2005 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

26. Transport (p. 73) Vascular system: veins and arteries
Carry water-soluble nutrients, oxygen, other vital substances
Transport wastes for removal
Portal circulation first carries nutrients to liver for cell enzyme work
Lymphatic system
Carries non–water-soluble fatty materials
Lymph vessels in villi absorb materials such as fatty materials
Route to larger lymph vessels
Eventually to blood stream through thoracic duct
The vascular system is responsible for supplying the body with nutrients, oxygen, and other vital substances.
Water-soluble products of digestion can be absorbed into the vascular system.
Nutrients travel to the liver by the portal circulation, then to cells throughout the body.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

27. Metabolism (p. 73) At this point, individual macronutrients in food:
Have been broken down
Absorbed into bloodstream or lymphatic system
Next, nutrients can be converted into energy or stored
Body is highly efficient; stores excess for later use.
Explain that the body never rests, so when a person is not eating, stored energy is used.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

28. Energy for Fuel (p. 73)
Mitochondria of cells is where metabolism takes place
Two forms of metabolism
Catabolism: breaking down of larger substances into smaller units
Anabolism: building up of larger substances from smaller units
The Krebs cycle—or citric acid cycle or TCA cycle—is the hub of energy formation from food particles occurring in mitochondria.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

29. Energy for Fuel (p. 73)
Metabolic processes ensure that the body has energy in the form of adenosine triphosphate (ATP)
Metabolism is the sum total of all chemical changes in the body
Metabolism of glucose from carbohydrates yields less energy than metabolism of fat, but glucose is the body’s primary source of energy
Protein can be an energy source, but it is relatively inefficient
The rate of ATP production fluctuates depending on energy needs at a given time.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

30. Stored Energy (p. 74)
Energy beyond that needed at present is stored for future
Glucose is converted to glycogen via glycogenesis, stored in liver or muscles
When glycogen reserves are full, excess is stored as fat via lipogenesis
Excess protein/amino acids converted to glucose via gluconeogenesis
If the amount of food consumed yields more energy (kilocalories) than needed, the excess is stored in the body for later use.
Alcohol is not considered a nutrient but adds to overall energy intake and may be stored as fat.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

31. Stored Energy (p. 74)
Both glycogen and stored fat are available for use when energy demands require it.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

32. Errors in Digestion and Metabolism (p. 75)
Genetic defects: cell is missing enzyme controlling metabolism of a specific nutrient
Phenylketonuria (PKU)
Enzyme responsible for metabolizing essential amino acid phenylalanine is missing
Untreated, causes permanent mental retardation and CNS damage
With proper treatment, affected children may have normal and healthy lives
What foods can a person with phenylketonuria eat? Tell the students about special supplements made for this population.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

33. Errors in Digestion and Metabolism (p. 75)
Genetic defects (cont’d)
Galactosemia
Enzyme responsible for metabolizing galactose to glucose is missing
All sources of lactose must be eliminated from diet
Untreated, can cause brain and liver damage
Screening and treatment can enable normal life
What foods would be harmful to a person with galactosemia? (Any source of lactose, such as dairy products)
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

34. Errors in Digestion and Metabolism (p. 77)
Genetic defects (cont’d)
Glycogen storage diseases (GSD)
Group of rare genetic defects
Absence of enzymes required for synthesis or breakdown of glycogen
Form of disease depends on enzyme missing
Because the liver is the primary site of glycogen metabolism, hepatic forms of GSD affect the entire body.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

35. Other Intolerances or Allergies (p. 78)
Not genetic inborn errors of metabolism
Rather, caused by food intolerances or allergies
Lactose intolerance: deficiency of any of the disaccharidases in small intestine
Insufficient lactase to break down milk
Abdominal cramping and diarrhea
Products containing lactose must be avoided
The higher the amount of lactose in the food, the greater the discomfort. List foods that commonly cause discomfort and foods that do not.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

36. Other Intolerances or Allergies (p. 78)
Inappropriate immune responses
Not necessarily a problem with digestion or metabolism
Covered in Chapter 18
Allergies are an inappropriate response to an otherwise harmless substance.
Ask students to name some food allergies.
Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.