individuals who had a digestive disorders or difficulties such as ulcers or irritable bowel syndrome

75

3

Learning Outcomes After reading this chapter, you will be able to:

3.1 Describe the processes and organs involved in digestion.

3.2 Explain how food is propelled through the gas- trointestinal tract.

3.3 Identify the role of enzymes and other secre- tions in chemical digestion.

3.4 Describe how digested nutrients are absorbed.

3.5 Explain how hormones and the nervous sys- tem regulate digestion.

3.6 Explain how absorbed nutrients are trans- ported throughout the body.

3.7 Discuss the most common digestive disorders.

True or False? 1. Saliva can alter the taste of food. T/F 2. Without mucus, the stomach would digest itself. T/F 3. The major function of bile is to emulsify fats. T/F 4. Acid reflux is caused by gas in the stom-ach. T/F 5. The primary function of the large intes-tine is to absorb water. T/F 6. Feces contain a high amount of bacteria. T/F

7. The lymphatic system transports all nutrients through the body once they’ve been absorbed. T/F 8. Hormones play an important role in digestion. T/F 9. Diarrhea is always caused by bacterial infection. T/F

10. Irritable bowel syndrome is caused by an allergy to gluten. T/F See page 110 for the answers.

Digestion, Absorption, and Transport

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76 Chapter 3 | Digestion, Absorption, and Transport

The digestion of food begins even before you take that first bite. Just the sight and smell of homemade apple pie stimulates the release of saliva in the mouth. The secretion of saliva and other digestive juices starts a cascade of

events that prepares the body for digestion, the chemical and mechanical

processes by which the body breaks food down into individual nutrient

molecules ready for absorption. Food components that aren’t absorbed are

excreted as waste (feces) by elimination. Although these are complex

processes, they go largely unnoticed. You consciously chew and swallow the

pie, but you don’t feel the release of chemicals or the muscular contractions

that cause it to be digested or the absorption of nutrient molecules through

the intestinal lining cells. In fact, you may be unaware of the entire process

until about 48 hours after eating, when the body is ready to eliminate waste.

In this chapter, we explore the processes of digestion, absorption, and

elimination, the organs involved, and the other biological mechanisms that

regulate our bodies’ processing of food and nutrients. We also discuss the causes

and treatments of some common gastrointestinal conditions and disorders.

What Are the Processes and Organs Involved in Digestion? LO 3.1 Describe the processes and organs involved in digestion.

Digestion, absorption, and elimination occur in the gastrointestinal (GI) tract, a mus- cular tube approximately 20–24 feet long in an adult. Stretched vertically, the tube would be about as high as a two-story building. It provides a barrier between the food within the lumen (the hollow interior of the tract), which is technically external, and our body cells, which are internal.

Although the prefix gastro- means “stomach,” the GI tract actually extends from the mouth to the anus. Its six organs are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Food moves from one organ to the next by propulsion. Various sphincters along the way allow food to pass. These muscular rings act like one-way doors, allowing the mixture of food and digestive juices to flow into the next organ but not to flow back. Focus Figure 3.1 illustrates the organs and processes of the digestive system.

Digestion Begins in the Mouth The body digests food chemically, by the actions of digestive secretions, and mechanically, by the actions of the teeth and the powerful muscles of the GI tract. Both chemical digestion and mechanical digestion begin in the mouth. During mastication, the teeth, powered by strong jaw muscles, mechanically cut and grind food into smaller pieces as the tongue mixes it with saliva. Saliva dissolves small food particles, which allows them to react with the taste buds so we can savor food. About 99 percent water, saliva moistens and binds food to lubricate it for comfortable swallowing and traveling down the esophagus. Saliva also contains enzymes, compounds that help accelerate the rate of chemical reactions. Enzymes are discussed in detail later in this chapter. The primary enzyme in saliva is sali- vary amylase, which begins to break down carbohydrates. (You can taste this enzyme working when you eat a starch-containing food such as a cracker; as the enzyme breaks down starch into sugars, the flavor becomes sweeter.)

digestion Process that breaks down food into individual molecules small enough to be absorbed through the intestinal wall.

absorption Process of moving nutrients from the GI tract into the circulatory system.

elimination Excretion of undigested and unabsorbed food through the feces.

gastrointestinal (GI) tract Tubular organ system including the mouth, pharynx, esopha- gus, stomach, and small and large intestines, by means of which food is digested, nutrients absorbed, and wastes expelled.

lumen Channel or inside space of a vessel such as the intestine or artery.

propulsion Process that moves food along the GI tract during digestion.

sphincters Circular rings of muscle that open and close in response to nerve input.

chemical digestion Breaking down food through enzymatic reactions.

mechanical digestion Breaking down food by chewing, grinding, squeezing, and mov- ing it through the GI tract by peristalsis and segmentation.

mastication Chewing food.

saliva Secretion from the salivary glands that softens and lubricates food and begins the chemical breakdown of starch.

Whole foods must first be broken down into individual nutrients that can be used by the body’s cells.

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What Are the Processes and Organs Involved in Digestion? 77

Head to Mastering Nutrition and watch a narrated video tour of this figure by author Joan Salge Blake.

Figure 3.1 The Digestive SystemFOCUS

PANCREAS

GALLBLADDER

LIVER Produces bile to digest fats.

Stores bile before release into the small intestine through the bile duct.

Produce saliva, a mixture of water, mucus, enzymes, and other chemicals

SALIVARY GLANDS

MOUTH

PHARYNX AND ESOPHAGUS

STOMACH

SMALL INTESTINE

LARGE INTESTINE

RECTUM

The human digestive system consists of the organs of the gastrointestinal (GI) tract and associated accessory organs. The processing of food in the GI tract involves ingestion, mechanical digestion, chemical digestion, propulsion, absorption, and elimination.

Ingestion Food enters the GI tract via the mouth.

Mechanical digestion Mastication tears, shreds, and mixes food with saliva, forming a bolus.

Chemical digestion Salivary amylase begins carbohydrate breakdown.

Produces digestive enzymes and bicarbonate ions that are released into the small intestine via the pancreatic duct.

Mechanical digestion This process mixes and churns the bolus with acid, enzymes, and gastric fluid into a liquid called chyme.

Chemical digestion Pepsin begins digestion of proteins.

Absorption A few fat-soluble substances are absorbed through the stomach wall.

Mechanical digestion and Propulsion Segmentation mixes chyme with digestive juices; peristaltic waves move it along the tract.

Chemical digestion Digestive enzymes from the pancreas and brush border digest most classes of food.

Absorption Nutrients are absorbed into blood and lymph through enterocytes.

Chemical digestion Some remaining food residues are digested by bacteria.

Absorption Salts, water, and some vitamins are reabsorbed.

Propulsion Compacts waste into feces.

Elimination Feces are temporarily stored before voluntary release through the anus.

Propulsion Swallowing and peristalsis move the bolus from mouth to stomach.

ORGANS OF THE GI TRACT ACCESSORY ORGANS

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78 Chapter 3 | Digestion, Absorption, and Transport

Once food has been adequately chewed and moistened, the tongue rolls it into a bolus and thrusts it into the pharynx to be swallowed. The pharynx is the gateway to the esophagus, as well as to the trachea (or windpipe, the tube that connects to the lungs). Normally, a flap of cartilage called the epiglottis closes off the trachea during swallowing, so that food doesn’t accidentally “go down the wrong pipe” (see Figure 3.2). When the epiglottis doesn’t work properly, food can get lodged in the trachea and potentially cause choking.

The esophagus has only one function—to transport food and fluids from the mouth to the stomach. As food passes through the pharynx, the upper esophageal sphincter opens, allowing the bolus to enter the esophagus. After swallowing, rhythmic muscular contractions, with the help of gravity, move the bolus toward the stomach. The esophagus narrows at the bottom (just above the stomach) and ends at the lower esophageal sphincter (LES) (Figure 3.3). Under normal conditions, when the bolus reaches the stomach, the LES relaxes and allows food to pass into the stomach. The stomach also relaxes to comfortably receive the bolus. After food enters the stomach, the LES closes to prevent the stomach contents from regurgitating backward into the esophagus.

The Stomach Stores, Mixes, and Prepares Food for Digestion The primary function of the stomach is to mix food with various gastric juices to chemi- cally break it down into smaller and smaller pieces (Figure 3.4). The stomach lining includes four layers. The innermost layer contains goblet cells and gastric pits or ducts, which contain gastric glands that secrete a variety of critical digestive juices. Various other cells in the stomach lining, among them parietal (puh-RAHY-i-tl ) cells, chief cells, and mucous neck cells, secrete other gastric juices and mucus. The gastric juices are discussed later in this chapter.

Mechanical digestion in the stomach occurs as the longitudinal, circular, and diagonal muscles that surround the organ forcefully push, churn, and mix the contents of the stomach with the gastric juices. These powerful muscles can also stretch to accom- modate different volumes of food. An empty stomach can hold a little less than a cup, but the numerous folds of the stomach lining can stretch out after a large meal to hold up to 1 gallon (4 liters).1 For several hours, food is continuously churned and mixed in the stomach.

bolus Soft mass of chewed food.

pharynx Area of the GI tract between the mouth and the esophagus; also called the throat.

esophagus Tube that connects the mouth to the stomach.

epiglottis Cartilage at the back of the tongue that closes off the trachea during swallowing.

upper esophageal sphincter Muscular ring located at the top of the esophagus.

lower esophageal sphincter (LES) Mus- cular ring located between the base of the esophagus and the stomach.

stomach J-shaped muscular organ that mixes and churns food with digestive juices and acid to form chyme.

goblet cells Cells throughout the GI tract that secrete mucus.

gastric pits Indentations or small pits in the stomach lining where the gastric glands are located; gastric glands produce gastric juices.

parietal cells Specialized cells in the stomach that secrete the gastric juices hydrochloric acid and intrinsic factor.

chief cells Specialized cells in the stomach that secrete pepsinogen, an inactive form of the protein-digesting enzyme pepsin.

mucus Secretion produced throughout the GI tract that moistens and lubricates food and protects membranes.

▲ Figure 3.2 The Role of the Epiglottis The epiglottis prevents food from entering the trachea during swallowing.

Esophagus

Trachea (open)

Esophagus

Trachea (closed)

Bolus

Tongue

Epiglottis (up)

Bolus

Tongue

Epiglottis (down)

▲ Figure 3.3 Sphincters at Work Sphincters control the passage of food by contracting or relaxing.

The LES (lower esophageal sphincter) relaxes after swallowing to allow the bolus to enter the stomach.

1 The LES contracts to prevent stomach contents from returning to the esophagus.

2

Esophagus

Bolus LES relaxes

Stomach

Esophagus

LES contracts

Stomach

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What Are the Processes and Organs Involved in Digestion? 79

Once food has been adequately chewed and moistened, the tongue rolls it into a bolus and thrusts it into the pharynx to be swallowed. The pharynx is the gateway to the esophagus, as well as to the trachea (or windpipe, the tube that connects to the lungs). Normally, a flap of cartilage called the epiglottis closes off the trachea during swallowing, so that food doesn’t accidentally “go down the wrong pipe” (see Figure 3.2). When the epiglottis doesn’t work properly, food can get lodged in the trachea and potentially cause choking.

The esophagus has only one function—to transport food and fluids from the mouth to the stomach. As food passes through the pharynx, the upper esophageal sphincter opens, allowing the bolus to enter the esophagus. After swallowing, rhythmic muscular contractions, with the help of gravity, move the bolus toward the stomach. The esophagus narrows at the bottom (just above the stomach) and ends at the lower esophageal sphincter (LES) (Figure 3.3). Under normal conditions, when the bolus reaches the stomach, the LES relaxes and allows food to pass into the stomach. The stomach also relaxes to comfortably receive the bolus. After food enters the stomach, the LES closes to prevent the stomach contents from regurgitating backward into the esophagus.

The Stomach Stores, Mixes, and Prepares Food for Digestion The primary function of the stomach is to mix food with various gastric juices to chemi- cally break it down into smaller and smaller pieces (Figure 3.4). The stomach lining includes four layers. The innermost layer contains goblet cells and gastric pits or ducts, which contain gastric glands that secrete a variety of critical digestive juices. Various other cells in the stomach lining, among them parietal (puh-RAHY-i-tl ) cells, chief cells, and mucous neck cells, secrete other gastric juices and mucus. The gastric juices are discussed later in this chapter.

Mechanical digestion in the stomach occurs as the longitudinal, circular, and diagonal muscles that surround the organ forcefully push, churn, and mix the contents of the stomach with the gastric juices. These powerful muscles can also stretch to accom- modate different volumes of food. An empty stomach can hold a little less than a cup, but the numerous folds of the stomach lining can stretch out after a large meal to hold up to 1 gallon (4 liters).1 For several hours, food is continuously churned and mixed in the stomach.

bolus Soft mass of chewed food.

pharynx Area of the GI tract between the mouth and the esophagus; also called the throat.

esophagus Tube that connects the mouth to the stomach.

epiglottis Cartilage at the back of the tongue that closes off the trachea during swallowing.

upper esophageal sphincter Muscular ring located at the top of the esophagus.

lower esophageal sphincter (LES) Mus- cular ring located between the base of the esophagus and the stomach.

stomach J-shaped muscular organ that mixes and churns food with digestive juices and acid to form chyme.

goblet cells Cells throughout the GI tract that secrete mucus.

gastric pits Indentations or small pits in the stomach lining where the gastric glands are located; gastric glands produce gastric juices.

parietal cells Specialized cells in the stomach that secrete the gastric juices hydrochloric acid and intrinsic factor.

chief cells Specialized cells in the stomach that secrete pepsinogen, an inactive form of the protein-digesting enzyme pepsin.

mucus Secretion produced throughout the GI tract that moistens and lubricates food and protects membranes.

▲ Figure 3.4 Anatomy of the Stomach The cross section of the stomach illustrates the gastric cells that secrete digestive juices.

Longitudinal muscles

Stomach Cross section of inner stomach walls

Small intestine

Esophagus

Lower esophageal sphincter

Circular muscles

Diagonal muscles

Inner stomach walls

Chief cells

Gastric pits

Parietal cells

Submucosa

Gastric glands

Goblet cells

By the time the mixture reaches the lower portion of the stomach, it is a semiliquid mass called chyme, which contains digestive secretions plus the original food. As the chyme accumulates near the pyloric sphincter, between the stomach and the small intestine, the sphincter relaxes and the chyme gradually enters the small intestine. You eat much faster than you can digest and absorb food, so the stomach also acts as a holding tank for chyme until it can be released into the small intestine. Approximately 1–5 milliliters (1 teaspoon) of chyme is released into the small intestine every 30 seconds.2 The pyloric sphincter prevents chyme from exiting the stomach too soon and blocks the intestinal contents from returning to the stomach.

Most Digestion Occurs in the Small Intestine As chyme passes through the pyloric sphincter, it enters the long, coiled small intestine. This organ consists of three segments—the duodenum, jejunum, and ileum— and extends from the pyloric sphincter to the ileocecal valve at the beginning of the large intestine (Figure 3.5). The first segment, the duodenum, is approximately 10 inches long. The second, the jejunum, measures about 8 feet long, and the third, the ileum, is about 12 feet long. The “small” in “small intestine” refers to its diameter (about 1 inch), not its extended length.

As in the stomach, both mechanical and chemical diges- tion occur in the small intestine. Muscular contractions squeeze chyme forward while digestive secretions from the pancreas, gallbladder, and intestinal lining chemically break down the nutrients.

Numerous fingerlike projections, called villi, line the small intestine. They increase the surface area to maximize absorption and help mix the partially digested chyme with intestinal secretions (Focus Figure 3.6). Each villus contains capillaries and lymphatic vessels called lacteals that pick up digested nutrients during absorption. The villi extend about 1 millimeter into the lumen, creating a velvety appearance, and are arranged into hundreds of overlapping, circular folds. The circular folds cause chyme to spiral forward through the small intestine, further increasing its exposure to the villi.

chyme Semiliquid, partially digested food mass that leaves the stomach and enters the small intestine.

small intestine Long coiled chamber that is the major site of food digestion and nutrient absorption.

villi Small, fingerlike projections that line the lumen of the small intestine.

▲ Figure 3.5 Anatomy of the Small Intestine The small intestine is highly adapted for absorbing nutrients. Its length— about 20 feet—provides a huge surface area, and its wall has three structural features—circular folds, villi, and microvilli—that increase its surface area by a factor of more than 600.

Duodenum

Ileum

Jejunum

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80 Chapter 3 | Digestion, Absorption, and Transport

Head to Mastering Nutrition and watch a narrated video tour of this figure by author Joan Salge Blake.

Figure 3.6 Structures of the Small Intestinal WallFOCUS

The lining of the small intestine is heavily folded, resulting in increased surface area for the absorption of nutrients.

CIRCULAR FOLDS

The folds are covered with villi, thousands of fingerlike projections that increase the surface area even further. Each villus contains capillaries and a lacteal for picking up nutrients absorbed through the enterocytes and transporting them throughout the body.

VILLI

The cells on the surface of the villi, enterocytes, end in hairlike projections called microvilli that together form the brush border through which nutrients are absorbed.

MICROVILLI

The small intestine is highly adapted for absorbing nutrients. Its length—about 20 feet—provides a huge surface area, and its wall has three structural features—circular folds, villi, and microvilli— that increase its surface area by a factor of more than 600.

Crypt

Goblet cell

Enterocyte

Microvilli (brush border)

Lacteal

Enterocyte

Capillaries

Small Intestine

Villi

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What Are the Processes and Organs Involved in Digestion? 81

Epithelial cells called enterocytes cover the villi. They have smaller projections called microvilli, referred to collectively as the brush border, which trap nutrients and absorb them into the enterocyte interior. There, the nutrients enter blood and lymphatic vessels that transport them throughout the body. Enterocytes also secrete several enzymes that help digest specific nutrients. We discuss these secretions in more detail later in the chapter.

Goblet cells scattered along the villi secrete lubricating mucus into the intestine. Between the villi lie glands called crypts that secrete intestinal juices. Within the crypts, stem cells continually divide, producing younger cells that travel up the villi to replace mature cells when they die. A constant source of nutrients is needed to replace these cells and maintain a healthy absorptive surface. Without the proper nutrients, the villi deteriorate and flatten, result- ing in malabsorption.

Depending on the amount and type of food eaten, the contact time in the small intes- tine is between 3 and 10 hours. Usually, by the time you sit down to dinner, your breakfast is just about reaching the end of the small intestine.

The Large Intestine Absorbs Water and Some Nutrients The large intestine is only about 5 feet long—much shorter than the small intestine— but its lumen is larger, about 2.5 inches in diameter. It has three segments: the cecum, colon, and rectum (Figure 3.7). The cecum (SEE-kum) marks the beginning of the large intestine. Chyme from the small intestine passes through the ileocecal valve into the cecum before entering the colon. The colon is the largest portion of the large intes- tine, and it is further subdivided into the ascending, transverse, descending, and sigmoid regions. These regions are relatively long and straight. Note that though the terms colon and large intestine are often used interchangeably, technically they’re not the same thing.

By the time chyme enters the large intestine, the majority of the nutrients, except water and the electrolytes sodium, potassium, and chloride, have been absorbed. The cells of the large intestine absorb water and these electrolytes much more efficiently than the cells of the small intestine. The large intestine also produces mucus that protects the cells and acts as a lubricant for fecal matter.

Helpful bacteria that colonize the colon are collectively known as the GI flora or microflora. These GI flora produce some vitamins, including vitamin K, thiamin, ribo- flavin, biotin, and vitamin B12. Only biotin and vitamin K can be absorbed in the colon, however. Bacteria also ferment some of the undigested and unabsorbed dietary carbohy- drates into simpler compounds, methane gas, carbon dioxide, and hydrogen. This fermen- tation process is the major source of intestinal gas. Similarly, some of the colon’s bacteria break down undigested fiber and produce various short-chain fatty acids. Amino acids that reach the colon are converted to hydrogen, sulfide, some fatty acids, and other chemi- cal compounds. Given the importance of bacteria in the healthy functioning of the colon, it isn’t surprising that foods containing such beneficial living microorganisms, called pro- biotics, have become popular in recent years. The Examining the Evidence feature evalu- ates the claims of health benefits associated with consumption of probiotic foods.

enterocytes Absorptive epithelial cells that line the lumen of the small intestine.

microvilli Tiny projections on the villi in the small intestine.

crypts Glands at the base of the villi; they contain stem cells that manufacture young cells to replace the cells of the villi when they die.

large intestine Lowest portion of the GI tract, where water and electrolytes are absorbed and waste is eliminated.

cecum Pouch at the beginning of the large intestine that receives waste from the small intestine.

ileocecal valve Sphincter that separates the small intestine from the large intestine.

colon Another name for the large intestine.

GI flora Microorganisms that live in the GI tract of humans and animals.

ferment To metabolize sugar into carbon dioxide and other gases.

▲ Figure 3.7 Anatomy of the Large Intestine By the time chyme reaches the large intestine, most of its nutrients have been absorbed. However, water and some electrolytes are absorbed in the colon. The final waste products of digestion pass out of the body through the anus.

Rectum

Anal sphincter

Cecum

Appendix

Ileocecal valve

Ileum

Ascending colon

Transverse colon

Descending colon

Sigmoid colon

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82 Chapter 3 | Digestion, Absorption, and Transport

EXAMINING THE

EVIDENCE

If you’re like most people, the thought of eating food that contains living microorgan- isms is not appealing. But did you know that consuming certain microbes, called probiotics, might improve your health? Pro- biotics (pro = for, bios = life) are “live microorganisms, which, when administered in adequate amounts, confer a health benefit on the host.”1 They are similar to the more than 10 trillion beneficial microbes, mostly strains of bacteria, that colonize your GI tract. Medications, stress, a poor diet, and illness can disrupt the balance of friendly GI flora, and consuming them in probiotic foods can help restore their numbers. In the United States, sales of probiotic supplements and foods reached $1.14 billion in 2014, and sales are expected to nearly double by 2019.2 Why?

Research Supports the Health Benefits of Probiotics Probiotics function in the same way that the native bacteria in your GI tract do. For example, they pro- duce organic acids that inhibit the growth of disease-causing micro- organisms. They also compete with these pathogens for nutrients and receptor sites, keeping the popula- tion of harmful microbes in check. Other proposed benefits of probiotics include:3–9

• May reduce constipation, in part by reducing transit time of chyme through the GI tract.

• May prevent or reduce the symp- toms of diarrhea and the cramping and bloating associated with lactose intolerance.

• May reduce inflammation. • May play a role in preventing food

allergies. • May shorten the duration and reduce

the severity of the symptoms of the common cold, especially in academi- cally stressed students.

• May reduce signs of colic in infants.

Prebiotics and Synbiotics Support the GI Flora Prebiotics, which are present in thou- sands of plant-based foods, are nondi- gestible resistant starches—a form of dietary fiber—that support the growth and health of your GI flora. Because the body can’t digest them, they reach the large bowel intact. There, the GI flora digest them. Eating foods rich in pre- biotics thus helps build and maintain a healthy population of GI flora. The two most common prebiotics are fructo- oligosaccharides (FOS) and inulin.

Products called synbiotics are processed foods and supplements that contain both probiotic bacteria and prebiotic starches. For example, yogurts to which inulin has been added qualify as synbiotics. Such products both supplement your native GI flora and support their growth and activity.

Where Can You Find Probiotics, Prebiotics, and Synbiotics? Probiotics are found primarily in fer- mented dairy and soy products and in dietary supplements (see the accom- panying table). For example, eating Activia yogurt is one way to increase probiotics through diet. The food label should identify the strain of the bac- teria used, when the product expires,

the suggested serving size, and how to store the product to make sure the bacteria are still alive when you eat it. A serving should contain at least one billion CFUs (a measure of live bacteria called colony-forming units) to provide the level of probiotics found to deliver health benefits.1 When choosing yogurt, look for brands low in saturated fat and added sugars. Plain Greek-style yogurt is a healthful and satisfying choice.

The food richest in prebiotics is bananas. Other foods containing prebi- otics include garlic, onions, asparagus, leeks, squash, jicama, beets, carrots, tur- nips, parsnips, sweet potatoes, and arti- chokes. Oats, barley, and certain other whole grains also provide prebiotics.

Some yogurt producers add inulin, producing a synbiotic. You can also create your own synbiotics by eating foods rich in probiotics and prebiotics together. Consider, for example, yogurt with oatmeal and bananas, or tempeh (fermented soy) in a stir-fry with aspara- gus, garlic, and onions.

There’s no downside to increasing your consumption of foods rich in probiotics, prebiotics, and synbiotics. Although there is a great deal we still don’t know about their impact on our health, we do know that the dairy and plant-based foods providing them are nutrient dense and delicious.

Do Probiotics, Prebiotics, and Synbiotics Improve Your Health?

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5. Spaiser, S. J., et al. 2015. Lactobacillus gas- seri KS-13, Bifidobacterium bifidum G9-1, and Bifidobacterium longum MM-2 Inges- tion Induces a Less Inflammatory Cytokine Profile and a Potentially Beneficial Shift in Gut Microbiota in Older Adults: A Random- ized, Double-Blind, Placebo-Controlled, Crossover Study. Journal of the American College of Nutrition 34(6):459–469. doi: 10.1080/07315724.2014.983249.

6. Savaiano, D. A., A. J. Ritter, et al. 2013. Improving Lactose Digestion and Symptoms of Lactose Intolerance with a Novel Galacto- Oligosaccharide (RP-G28): A Randomized, Double-Blind Clinical Trial. Nutrition Journal 12:160. doi: 10.1186/1475-2891-12-160.

7. Ritz, B. W. 2011. Probiotics for the Prevention of Childhood Eczema. Available at http://natu- ralmedicinejournal.com. Accessed January 2017.

8. Langkamp-Henken, B., et al. 2015. Bifidobac- terium bifidum R0071 Results in a Greater Proportion of Healthy Days and a Lower Percentage of Academically Stressed Students

Reporting a Day of Cold/Flu: A Ran- domised, Double-blind, Placebo-controlled Study. British Journal of Nutrition 113(3):426– 434. doi: 10.1017/S0007114514003997.

9. Anabrees, J., F. Indrio, et al. 2013. Probiotics for Infantile Colic: A Sys- tematic Review. BMC Pediatrics 13:186. doi:10.1186/1471-2431-13-186.

References 1. Kechagia, M. D., S. Basoulis, et al. 2013.

Health Benefits of Probiotics: A Review. ISRN Nutrition, Article ID 481651, doi:10.5402/ 2013/481651. Accessed January 2017.

2. Statista: The Statistics Portal. 2016. Dollar Sales of Probiotic Supplements in the United States in 2015, By Channel (in million U.S. dollars). Available at https://www.statista. com/statistics/493536/dollar-sales-of- probiotic-supplement-in-the-us-by-channel. Accessed January 2017.

3. Ritchie, M. L., and T. N. Romanuk. 2012. A Meta-Analysis of Probiotic Efficacy for Gastrointestinal Diseases. PLoS ONE 7(4):e34938. doi:10.1371/journal. pone.0034938.

4. Beserra, B. T., et al. 2014. A Systematic Review and Meta-analysis of the Prebiotics and Synbiotics Effects on Glycaemia, Insulin Concentrations, and Lipid Parameters in Adult Patients with Overweight or Obe- sity. Clinical Nutrition 34(5):845–858. doi: 10.1016/j.clnu.2014.10.004.

probiotics Live microorganisms that, when consumed in adequate amounts, confer a health benefit on the host.

Beneficial Bacteria

If You Have This Problem Try This Probiotic Found in These Foods and Dietary Supplements

Diarrhea ■■ Lactobacillus reuteri 55730 ■■ Saccharomyces boulardii yeast

■■ BioGai tablets, drops, and lozenges ■■ Florastor dietary supplement

Constipation, irritable bowel syndrome, and overall digestion problems

■■ Bifidobacterium animalis DN-173 010 ■■ Bifidobacterium infantis 35624

■■ Dannon Activia yogurt ■■ Align supplement

Poor immune system ■■ Bifidobacterium lactis Bb-12 ■■ Lactobacillus casei Shirota ■■ Lactobacillus casei DN-114 001

■■ Yo-Plus Yogurt ■■ Yakult fermented dairy drink ■■ Dannon’s DanActive dairy drink

Vaginal infections ■■ Lactobacillus rhamnosus GR-1 combined with Lactobacillus reuteri

■■ Fem Dophilus dietary supplement

About l liter of fluid material—consisting of water, undigested or unabsorbed food particles, indigestible residue, and electrolytes—passes into the colon each day. Gradually, the material is reduced to about 200 grams of brown fecal matter (also called stool, or feces). Stool consists of the undigested food residue, as well as sloughed-off cells from the GI tract and a large quantity of bacteria. The brown color is due to unabsorbed iron mix- ing with a yellowish-orange substance called bilirubin. The greater the iron content, the darker the feces. The intestinal matter passes through the colon within 12–70 hours, depending on a person’s age, health, diet, and fiber intake.

Stool is propelled through the large intestine until it reaches the final 8-inch portion called the rectum. The anus is connected to the rectum and controlled by an internal and an external sphincter. Under normal conditions, the anal sphincters are closed. When stool distends the rectum, the action stimulates stretch receptors that in turn stimulate the internal anal sphincters to relax, allowing the stool to enter the anal canal. This causes nerve impulses of the rectum to communicate with the rectum’s muscles, resulting in defecation. The final stage of defecation is under voluntary control and influenced by age, diet, prescription medicines, health, and abdominal muscle tone.

stool Waste produced in the large intestine; also called feces.

prebiotics Nondigestible starch found in plant foods that promotes the growth and health of your GI flora.

rectum Final 8-inch portion of the large intestine.

anus Opening of the rectum, or end of the GI tract.

What Are the Processes and Organs Involved in Digestion? 83

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84 Chapter 3 | Digestion, Absorption, and Transport

The Accessory Organs Secrete Digestive Juices The salivary glands, liver, gallbladder, and pancreas are considered accessory organs because food does not pass through them (Figure 3.8). These organs are still key to the digestive process, however. They contribute digestive secretions such as saliva, bile, and enzymes that help with breakdown and transport of nutrients.

The salivary glands, located beneath the jaw and under and behind the tongue, produce about 1 quart of saliva per day.3 In addition to water and electrolytes, saliva

contains several enzymes, including salivary amylase and lysozyme, an enzyme that destroys certain oral bacteria. It also contains mucus, which helps lubricate food,

helps it stick together, and protects the inside of the mouth. Weighing in at about 3 pounds, the liver is the largest organ in the

body. It is located just beneath the rib cage and functions as a major player in the digestion of food and the absorption and transport of nutrients. The liver plays an essential role in carbohydrate metabolism, produces

proteins, and manufactures bile salts, which contribute to the break- down of fats. The bile produced by the liver is secreted into the gall- bladder for storage. The liver also metabolizes alcohol and removes and degrades toxins and excess hormones from the circulation.

The gallbladder is located beneath the right side of the liver. This pear-shaped organ receives bile from the liver through the

common hepatic duct, concentrates it, and secretes bile into the small intestine through the common bile duct.

The pancreas is a flat organ about 10–15 centimeters long that rests behind the stomach in the bend of the duodenum. The function of the pan- creas is both endocrine (endo = inside) and exocrine (exo = outside). As an endocrine organ, the pancreas releases into the bloodstream hormones that help regulate blood glucose levels. As an exocrine organ, the pancreas produces and secretes digestive enzymes through the pancreatic duct into the small intestine.

salivary glands Cluster of glands located underneath and behind the tongue that release saliva in response to the sight, smell, and taste of food.

liver Accessory organ of digestion located in the upper abdomen and responsible for the synthesis of bile, the processing of nutrients, the metabolism of alcohol, and other functions.

gallbladder Pear-shaped organ that stores and concentrates bile produced by the liver and secretes it through the common bile duct into the small intestine.

pancreas Large gland located behind the stomach that releases digestive enzymes and bicarbonate after a meal. Also secretes the hormones insulin and glucagon, which control blood glucose.

Duodenum

Liver

Salivary glands

Gallbladder

Pancreas

Common bile duct

▲ Figure 3.8 The Accessory Organs The salivary glands, liver, gallbladder, and pancreas produce digestive secretions that flow into the GI tract through various ducts.

LO 3.1: THE TAKE-HOME MESSAGE Digestion takes place in the GI tract. Chemicals break the molecular bonds in food so that nutrients can be absorbed and transported throughout the body. Saliva mixes with and moist- ens food in the mouth, making it easier to swallow. Once a bolus of food mixes with gastric juices in the stomach, it becomes chyme. Maximum digestion and absorption occur in the small intestine. Undigested residue enters the large intestine, where water is removed from the chyme as it is prepared for elimina- tion. Eventually, the remnants of digestion reach the anus and exit the body in the feces. The salivary glands, liver, gallbladder, and pancreas are important accessory organs. The salivary glands produce saliva. The liver produces bile, which the gallbladder concentrates and stores. The pancreas produces diges- tive enzymes and hormones.

How Is Food Propelled through the GI Tract? LO 3.2 Explain how food is propelled through the gastrointestinal tract.

Wavelike movements of the muscles throughout the GI tract propel food and liquid for- ward. These contractions, which depend on coordination between the muscles, nerves, and hormones in the GI tract, help mechanically digest the food by mixing and pushing

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How Is Food Propelled through the GI Tract? 85

it at just the right pace through each organ. The two primary types of contractions are called peristalsis and segmentation (Figure 3.9).

Peristalsis begins in the esophagus, as contractions of circular muscles prevent food from moving backward. Repeated waves of contractions follow as the circular muscles relax and the longitudinal muscles constrict and push the food forward.

In the stomach, circular, longitudinal, and diagonal muscles move the food from the top of the stomach toward the pyloric sphincter at the base of the stomach. The waves of contractions in the stomach are slower than in other GI organs, as peristalsis mixes and churns the stomach contents with gastric juices until the food is liquefied.

As the partially digested food leaves the stomach, the second form of mechanical digestion, called segmentation, begins. Segmentation moves the food back and forth, helping to break it down into smaller pieces while mixing it with the chemical secretions of the intestine. Segmentation differs from peristalsis in that food is shifted (rather than squeezed) back and forth along the intestinal walls. This shifting action increases the time food is in contact with the intestinal lining, moving food through the small intestine at a rate of 1 centimeter per minute.4

Mass movement, also known as mass peristalsis, is a series of strong, slow contrac- tions in the large intestine that move the chyme through the colon. Three or four times a day, these slow but powerful muscular contractions force the waste products toward the rectum. Meanwhile, segmentation within the colon helps dry out the feces, allowing for the maximum amount of water to be absorbed. These contractions often occur shortly after eating and are stronger when the diet contains more fiber.

peristalsis Forward, rhythmic muscular con- tractions that move food through the GI tract.

segmentation Muscular contractions of the small intestine that move food back and forth, breaking the mixture into smaller and smaller pieces and combining it with digestive juices.

mass movement (mass peristalsis) Strong, slow peristaltic movements, occurring only three or four times a day within the colon, that force waste toward the rectum.

▲ Figure 3.9 Peristalsis and Segmentation

Longitudinal, circular, and diagonal muscles constrict in wavelike motions to propel food through the GI tract.

1

Longitudinal and circular muscles in the small intestine mix and squeeze food back and forth along the intestinal wall.

2

Stomach

Small intestine Segmentation

Peristalsis

Longitudinal

Circular

Longitudinal

Circular

Diagonal

LO 3.2: THE TAKE-HOME MESSAGE Food is propelled through the GI tract by strong muscular contractions. Peristalsis in the esophagus, stomach, and small intestine squeezes the food and propels it forward, while segmentation in the small and large intestines shifts food back and forth along the intestinal walls. Mass movement moves waste slowly and powerfully toward the rectum.

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How Is Food Chemically Digested? LO 3.3 Identify the role of enzymes and other secretions in chemical digestion.

While food travels through the organs of the GI tract, digestive enzymes and other chemicals break it down into nutrients. How do these chemicals work?

Enzymes Drive the Process of Digestion Enzymes are compounds, most of which are proteins, that catalyze or speed up chemi- cal reactions. One of the most important roles of enzymes is to accelerate hydrolysis (hydro = water, lysis = break) reactions, in which water breaks the bonds of digestible carbohydrates, fats, proteins, and alcohol. Hydrolysis produces single molecules small enough to be absorbed by the intestines. During hydrolysis, the hydroxyl (OH) group from water joins one of the molecules, while the hydrogen ion (H) joins the other molecule, forming two new molecules. This is illustrated in the Chemistry Boost. Enzymes aren’t changed in the reaction and can thus be used over and over again.

enzymes Substances, mostly proteins, that increase the rate of chemical reactions; also called biological catalysts.

hydrolysis Chemical reaction that breaks the bond between two molecules with water. A hydroxyl group is added to one molecule and a hydrogen ion is added to the other molecule.

Once food enters the mouth, enzymes begin to chemically break the bonds that bind the nutrients.

Hydrolysis The process known as

hydrolysis digests most food molecules, in which the addition of water and a specific enzyme breaks down the corresponding molecule. In the figure below, a molecule of sucrose (sugar) is digested by the enzyme sucrase when it breaks the bond with the addition of water by adding a hydroxyl group (OH) to form glucose and hydrogen (H) to form fructose.

Chemistry Boost

H2O

H

H

OH

O

OH

H

HO

H OH

CH2OH

C

C C

C

Glucose (C6H12O6) Fructose (C6H12O6)

+

H

HO

OH

H

HO

H OH

CH2OH

C

C

CH

CH

C O

C

Sucrose (C12H22O11)

Sucrase

OH H

O

H HO

OH H

HOCH2 CH2OH

C

C C

C

OH H

O

H

OH H

HOCH2 CH2OH

C

C C

C

In order for enzymes to catalyze hydrolytic reactions, three conditions must be present.

1. The compatible enzyme and nutrient must both be present. Enzymes are compatible only with a specific compound or nutrient, referred to as a substrate. Each enzyme has a binding site that only fits certain substrates, much like a key fits a specific lock. When the substrate binds to the active site of the enzyme, the bond is hydrolyzed. This reaction is illustrated in Figure 3.10. Enzymes are often named according to the type of substrate they act upon, plus the suffix -ase. So, sucrase hydrolyzes the sugar sucrose, and maltase hydrolyzes maltose. Some enzymes, such as pepsin, were named before this new nomenclature was devel- oped and don’t follow these naming rules.

substrate Substance or compound that is altered by an enzyme.

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How Is Food Chemically Digested? 87

2. The pH of the environment must fall in the appropriate range. Enzymes are most effective when the fluid environment falls within a certain pH range, a range of acidity or alkalinity (see the Chemistry Boost). When the pH falls outside of that range, the activity of the enzyme is decreased or even halted. For example, saliva contains bicarbonate, which neutralizes acids in foods. It has a pH of about 6.4, which is optimal for the effective action of salivary amylase. When the bolus containing the salivary enzymes reaches the stomach, where the pH is closer to 1, salivary amylase activity is stopped. However, another enzyme, pepsin, becomes activated in this acidic environment. As chyme continues to travel through the GI tract, various organs secrete digestive juices that produce the optimal range of pH for the enzymes to function.

3. The temperature of the environment must fall within the appropriate range. As temperature falls below optimal levels, enzyme activity slows. If the temperature becomes too high, the enzyme is inactivated. In the body, the opti- mal temperature for enzymatic activity is 98.6°F (35.7°C), which is considered normal body temperature. Temperature also influences enzyme activity in foods. Cooling food in a refrigerator slows enzyme activity, and cooking food com- pletely inactivates any enzymes it contains.

Digestive enzymes are secreted all along the GI tract, but most are produced in the pancreas. The pancreas secretes digestive enzymes into the duodenum through the pancreatic duct. The brush border of the small intestine releases the last of the digestive enzymes. Table 3.1 summarizes the digestive enzymes, the organs that secrete them, and their actions.

pH Measure of the acidity or alkalinity of a solution.

▲ Figure 3.10 An Enzyme in Action Enzymes increase the rate of digestion without altering their shape.

Enzyme

Substrate

Bond

H2O

The substrate binds to the active site of the enzyme.

1 The individual products are released and the enzyme is ready to take action again.

3 The bond is broken between the two molecules by hydrolysis.

2

Organ or Gland Enzyme Action Nutrient

Salivary glands Salivary amylase Begins the digestion of starch Carbohydrates

Stomach Pepsinogen S Pepsin Begins the hydrolysis of polypeptides Protein Gastric lipase Begins digestion of lipids Lipids

TABLE 3.1 Digestive Enzymes and Their Actions

(continued)

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88 Chapter 3 | Digestion, Absorption, and Transport

pH Scale Acids and bases describe the chemical

properties of a substance. Acidity level is expressed using a pH scale that measures the hydrogen ion concentration. The range of a pH scale is 0–14, with 7—the pH of pure water—considered neutral. A solution that has a pH lower than 7 is considered acidic (0 is the most acidic); one higher than 7 is basic (14 is the most basic).

The lower the number on the pH scale, the greater the concentration of hydro- gen ions (H+) in a solution. The more H+ in a solution, the stronger the acid. For example, gastric juice, with a pH of 1, has a higher concentration of H+ and is much more acidic than pancreatic juice, which has a pH of 8.

Bases—alkaline compounds—contain more hydroxide ions (OH-) and have a low concentration of H+. The more OH- in a solution, the stronger the base. For example, sodium hydroxide, an acid, has a pH of 1, whereas bile, which is a base, has a pH of 6.8 to 8.5.

Each pH unit below 7 is 10 times more acidic than the next pH unit. For example, tomato juice, with a pH of 4, contains 10 times more H+ than coffee, with a pH of 5, and over 100 times more H+ than saliva, with a pH of 6.4. Each pH unit above 7 is 10 times more basic than the next pH unit.

Chemistry Boost

14

pH of Common Substances (numbers are approximate)

Basic

Acidic

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Concentrated lye

Low concentration of hydrogen ions

High concentration of hydrogen ions

Oven cleaner

Chlorine bleach

Household ammonia

Baking soda

Toothpaste

Bile (6.8 to 8.5) Pancreatic juice (7 to 8) Blood WaterpH neutral

Saliva (6.4)

Urine

Coˆee

Tomato juice

Lemon juice

Gastric juice (1 to 1.5)

Orange juice Soda

Battery acid

Organ or Gland Enzyme Action Nutrient

Pancreas Pancreatic amylase Digests starch Carbohydrates

Trypsinogen S Trypsin Catalyzes the hydrolysis of proteins in the small intestine to form smaller polypeptides

Protein

Chymotrypsinogen S

Chymotrypsin

Catalyzes the hydrolysis of proteins in the small intestine into polypeptides and amino acids

Protein

Procarboxypeptidase S Carboxypeptidase

Hydrolyzes the carboxyl end of a peptide, releas- ing the last amino acid in the peptide chain

Protein

Pancreatic lipase Digests triglycerides Lipids

Small Intestine Sucrase Digests sucrose Carbohydrates

Maltase Digests maltose Carbohydrates

Lactase Digests lactose Carbohydrates

Dipeptidase Digests dipeptides Protein

Tripeptidase Digests tripeptides Protein

Lipase Digests monoglycerides Lipids

TABLE 3.1 Digestive Enzymes and Their Actions (continued)

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How Is Food Chemically Digested? 89

Certain Secretions Are Essential for Digestion Enzymes and other essential compounds are often contained in fluids that are secreted throughout the digestion process. These secretions include saliva, which has already been discussed, as well as gastric juices, bile, and bicarbonate ions, each of which contributes to optimal conditions for digestion to occur.

Gastric Juices The specialized parietal and chief cells introduced earlier in the chapter produce the gas- tric juices secreted by the stomach. When food enters the stomach, the parietal cells produce hydrochloric acid (HCl) and a protein called intrinsic factor, which is important for the absorption of vitamin B12 in the ileum.

Hydrochloric acid is unique in that it can impair the activity of some proteins while activating others. It is essential for digestion because of its ability to lower the pH of gastric juice close to 1. This acidic pH denatures proteins, which means it inactivates the protein by uncoiling its strands. Once the protein is denatured, proteases, or protein-digesting enzymes, hydrolyze the bonds, breaking the proteins into shorter chains. Denaturing applies to all proteins, including hormones, and to bacteria found in food, which are destroyed before they can be absorbed intact.

Hydrochloric acid can also activate proteins, such as pepsinogen, a protein-digesting enzyme secreted from the chief cells lining the gastric glands. In the presence of HCl, pepsinogen is converted to its active form, pepsin, which begins the digestion of protein. HCl also enhances the absorption of certain minerals, such as calcium. In addition to pepsinogen, the chief cells also secrete gastric lipase, which begins to digest fats, although this enzyme is not particularly active in adults.

You might think that an acid as strong as HCl would “digest” the stomach itself, but mucus secreted by the goblet cells acts as a barrier between the HCl and the stomach lining, protecting the lining from irritation or damage. This slippery secretion is also produced in the mucous membranes lining the esophagus to lubricate food as it passes down the GI tract.

Bile Bile, the yellowish-green substance synthesized in the liver, helps break apart dietary fats. This dilute, alkaline liquid is stored in concentrated form—up to five times its original composition—in the gallbladder. Bile, which is composed of water, bile salts, bile pig- ments, fat, and cholesterol, emulsifies fat by breaking down large fat globules into smaller globules, much like dishwashing detergent breaks up the grease in a frying pan. Emulsification increases the surface area of the fat globule, making it far more accessible to pancreatic lipase, the fat-digesting enzyme secreted by the pancreas. Pancreatic lipase is water-soluble and only works on the surface of the fat globule.

In addition to dietary fat, bile also increases the absorption of the fat-soluble vitamins A, D, E, and K. Because bile has a slightly acidic to alkaline pH (between 6.8 and 8.5), it helps neutralize excess HCl. Finally, bile salts exhibit antibacterial properties.

Unlike other digestive secretions, bile can be reused. From the large intestine, bile is recycled back to the liver through enterohepatic (entero = intestine, hepatic = liver) circulation. This recycling allows bile to be reused up to 20 times.

Bicarbonate Bicarbonate ions alter the pH of food at various points along the GI tract. As noted earlier, the salivary glands produce enough bicarbonate to neutralize the food you eat and produce a favorable pH for salivary enzymes to hydrolyze starch. The pancreas secretes bicarbonate ions that flow into the duodenum via the pancreatic duct. The bicarbonate helps neutralize chyme as it arrives in the small intestine. The alkaline pH is critical to

hydrochloric acid (HCl) Strong acid pro- duced in the stomach that aids in digestion.

proteases Classification of enzymes that catalyze the hydrolysis of proteins.

pepsinogen Inactive protease secreted by the chief cells in the stomach; it is converted to the active enzyme pepsin in the presence of HCl.

pepsin Active protease that begins the diges- tion of proteins in the stomach.

bile Secretion produced by the liver, stored in the gallbladder, and released into the duode- num to emulsify dietary fat.

emulsify To break large fat globules into smaller droplets.

enterohepatic circulation Process of recy- cling bile from the large intestine back to the liver to be reused during fat digestion.

bicarbonate Negatively charged alkali ion produced from bicarbonate salts; during diges- tion, bicarbonate ions released from the pan- creas neutralize HCl in the duodenum.

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90 Chapter 3 | Digestion, Absorption, and Transport

protect the cells lining the duodenum, which are not resistant to damage by HCl, and to provide a favorable pH for the pancreatic enzymes and the brush border enzymes sucrase, maltase, and lactase.

Table 3.2 summarizes the important digestive compounds and the organs that secrete them.

Secretion Secretion Pathway Action(s)

Saliva Secreted by salivary glands into mouth Moistens food, eases swallowing; contains the enzyme salivary amylase

Hydrochloric acid (HCl) Secreted by parietal cells into stomach Denatures protein; activates pepsinogen S pepsin Intrinsic factor Secreted by parietal cells into stomach Needed for vitamin B12 absorption

Mucus Secreted throughout the GI tract, including in the stomach and intestinal glands

Lubricates and coats the internal mucosa to protect it from chemical or mechanical damage

Intestinal juice Secreted by the crypts into small intestine Contains enzymes that digest carbohydrate, protein, and lipid

Bile Secreted by liver into gallbladder for storage; released from gallbladder into small intestine via com- mon bile duct

Emulsifies large globules of lipid into smaller droplets

Bicarbonate ions Secreted by pancreas through the pancreatic duct into the small intestine

Raise pH and neutralize stomach acid

TABLE 3.2 Secretions of the GI Tract and Their Actions

LO 3.3: THE TAKE-HOME MESSAGE Foods are chemically digested by hydro- lysis, which is catalyzed by enzymes. The three conditions that govern enzyme action are specific substrates (nutrients), pH, and optimal tempera- ture. Other secretions produced in the GI tract, such as saliva, gastric juices, bile, and bicarbonate, contribute to the optimal environment for digestion to occur.

How Are Digested Nutrients Absorbed? LO 3.4 Describe how digested nutrients are absorbed.

Although some absorption occurs in the stomach and large intestine, the small intestine absorbs most nutrients via the enterocytes lining its wall. Nutrients that are digested by the time they reach the duodenum are absorbed quickly. Nutrients that need more time to be disassembled are absorbed mainly in the jejunum. Absorption is remarkably efficient. Under normal conditions, you digest and absorb 92–97 percent of the nutrients in food. In some individuals, the tight junctions between cell membranes of the enterocytes are dis- rupted, inappropriately increasing the permeability of the GI tract. Read the Examining the Evidence box for more information.

There Are Four Mechanisms of Nutrient Absorption The remarkable surface area provided by the folds and crevices in the lining of the small intestine allows for continuous, efficient absorption of most nutrients. Recall that the villi are covered with mature enterocytes—cells that absorb digested nutri- ents. These cells live only a few days before they are sloughed off and themselves digested.

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Is Increased Intestinal Permeability (aka Leaky Gut Syndrome) a Real Disorder? 91

In increased intestinal permeability, the tight junctions between the entero- cytes become inflamed, loosen, and allow large molecules, including undi- gested food particles, bacteria, and even toxins, to pass from the lumen into the bloodstream.

Undigested foods, bacteria, and allergens

Healthy junctionDisrupted junction

Blood

Have you ever heard the term leaky gut syndrome? This proposed medical condition, clinically known as increased intestinal permeability, is characterized by a reduced ability of the GI tract to regulate the absorption of nutrients. Some researchers theo- rize that restoring normal functioning of the lining of the GI tract may be effec- tive in curing many health problems. In the past, there has been little evidence to support this theory; how- ever, new research suggests that increased intestinal permeability may exist and might contribute to a variety of conditions from simple intestinal gas and bloating, to eczema, chronic fatigue syndrome, and even cardio- vascular disease and can- cer. Before we explore why, let’s take a closer look at how increased intestinal permeability occurs.

Physiology of Increased Intestinal Permeability As you’ve learned in this chapter, the healthy lining of the GI tract is a bar- rier that only allows nutrients that have been properly digested to pass through the enterocyte cell membrane and enter the bloodstream. This bar- rier blocks the entry of allergens and microorganisms, for example, into the bloodstream.1 As illustrated in the accompanying figure, when the tight junctions between the enterocytes become inflamed,2 they loosen and allow large molecules to pass through into the blood. These large molecules include undigested food particles, bac- teria, and even toxins. As they enter the bloodstream, the immune system releases antibodies and signaling mol- ecules called cytokines that stimulate white blood cells to fight the perceived invaders. This response can trigger

inflammation throughout the body, which is a risk factor for cardiovascular disease,3 celiac disease, Crohn’s disease,4, 5 diabetes,6 and other disorders.7, 8, 9

Causes and Symptoms of Increased Intestinal Permeability Little is known about the causes of increased intestinal permeability, and tests often fail to uncover the problem. The disruption of the intestinal barrier may be caused by chronic stress,10 intestinal infections including bacterial overgrowth,11 excessive use of alco- hol,12 poor diet,13 or the use of certain medications.14

The disruption of the intestinal barrier may result in a variety of gas- trointestinal symptoms, including flatulence, indigestion, constipation, bloating, and abdominal pain.15 These symptoms aren’t unique, however, and are often shared by other condi- tions. Symptoms beyond the GI tract have also been reported, including difficulty breathing, chronic joint and muscle pain, confusion, mood swings, poor memory, and anxiety. Moreover, asthma, recurrent infections, and chronic fatigue syndrome may be

associated with increased intestinal permeability.16, 17

Treatment for Intestinal Permeability

Diet is often the first approach to treating increased intestinal per- meability, especially the elimination of processed foods and sugars. How- ever, no treatment is known to restore the intestinal barrier. Moreover, it is not known whether restora- tion will actually resolve the patient’s symptoms. Human and animal stud- ies have not shown that intestinal barrier loss alone causes disease or that repairing the loss improves the disease state. Treat- ments such as nutritional supplements containing

pre- and probiotics, herbal remedies, gluten-free foods, and a low-sugar diet have not been shown to be beneficial.18 Additionally, most physicians don’t know enough about the GI tract to even begin to treat increased intestinal permeability. Clearly, more research evidence is needed before an effective treatment can be found.

References 1. Brandtzaeg, P. 2011. The Gut as Communi-

cator Between Environment and Host: Immunological Consequences. Eur J Pharmacol 668(Suppl 1):S16–S32.

2. Al-Sadi, R., et al. 2014. Interleukin-6 Modulation of Intestinal Epithelial Tight Junction Permeability is Mediated by JNK Pathway Activation of Claudin-2 Gene. PLoS One 9(3):e85345. doi: 10.1371/journal. pone.0085345

3. Schicho, R., G. Marsche, et al. 2015. Car- diovascular Complications in Inflammatory

intestinal permeability Condition in which the junctions between enterocytes allow large molecules to enter the bloodstream; also called leaky gut syndrome.

Is Increased Intestinal Permeability (aka Leaky Gut Syndrome) a Real Disorder?

EXAMINING THE

EVIDENCE

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92 Chapter 3 | Digestion, Absorption, and Transport

Bowel Disease. Current Drug Targets 16(3):181–188.

4. Bischof, S., et al. 2014. Intestinal Perme- ability: A New Target for Disease Prevention and Therapy. BMC Gastroenterolog y 14:189.

5. Sapone, A., et al. 2011. Divergence of Gut Permeability and Mucosal Immune Gene Expression in Two Gluten-associated Conditions: Celiac Disease and Glu- ten Sensitivity. BMC Medicine 9:23. doi: 10.1186/1741-7015-9-23.

6. Li, X., and M. A. Atkinson. 2015. The Role of Gut Permeability in the Pathogenesis of Type 1 Diabetes—A Solid or Leaky Con- cept? Pediatric Diabetes 16(7):485–492. doi: 10.1111/pedi.12305

7. Schulberg, J., and P. DeCruz. 2016. Charac- terisation and Therapeutic Manipulation of the Gut Microbiome in Inflammatory Bowel Disease. Internal Medicine Journal 46(3):266– 273. doi: 10.1111/imj.13003.

8. Johansson, M.E., et al. 2014. Bacteria Pen- etrate the Normally Impenetrable Inner Colon Mucus Layer in Both Murine Colitis Models and Patients with Ulcerative Colitis. Gut 63(2):281–291.

9. Campbell, A. W. 2016. The Gut, Intesti- nal Permeabil ity, and Autoimmunity. Alternative Therapy in Health and Medicine 21(1):6–7.

10. Yu, C., et al. 2016. Chronic Kidney Disease Induced Intestinal Mucosal Barrier Damage Associated with Intestinal Oxidative Stress Injury. Gastroenterological Research and Practice. doi: 10.1155/2016/6720575.

11. Sorobetea, D., et al. 2016. Acute Infection with the Intestinal Parasite Trichuris Muris Has Long-term Consequences on Musco- sal Mast Cell Homeostasis and Epithelial Integrity. European Journal of Immunolog y. doi: 10.1002/eji.201646738.

12. Swanson, G. R., et al. 2016. Night Workers with Circadian Misalignment are Susceptible to Alcohol-induced Intestinal Hyperpermeability with Social Drinking. American Journal of Physiolog y and Gatrointes- tinal Liver Physiolog y 311(1):G192–201. doi: 10.1152/ajpgi.00087.2016.

13. Odenwald, M. A., and J. R. Turner. 2013. Intestinal Permeability Defects: Is It Time to Treat? Clinical Gatroenterolog y and

Hepatolog y 11(9):1075–1083. doi: 10.1016/j. cgh.2013.07.001.

14. Pavlidis, P., and I. Bjarnason. 2015. Aspirin Induced Adverse Effects on the Small and Large Intestine. Current Pharmaceutical Design 21(35):5089–5093. Review.

15. Farré, R., and M. Vicario. 2016. Abnormal Barrier Function in Gastrointestinal Disor- ders. In Handbook of Experimental Pharmacol- og y. doi: 10.1007/164_2016_107.

16. Claesson, M. J., Jeffery, I. B., et al. 2012. Gut Microbiota Composition Correlates with Diet and Health in the Elderly. Nature 488:178–184.

17. Slyepchenko, A., et al. 2017. Gut Microbiota, Bacterial Translocation, and Interactions with Diet: Pathophysiological Links Between Major Depressive Disorder and Non-Communicable Medical Comorbidities. Psychotherapy and Psycho- somantics 86(1):31–46. doi: 10.1159/000448957.

18. NHS Choices. 2015. Leaky Gut Syndrome. Available at http://www.nhs.uk/conditions/ leaky-gut-syndrome/Pages/Introduction. aspx. Accessed January 2017.

Nutrients move across cell membranes in the small intestine via one of four mecha- nisms: passive diffusion, facilitated diffusion, active transport, or endocytosis. Figure 3.11 illustrates these four processes.

Passive Diffusion Passive diffusion is a process in which nutrients are absorbed along their concentration gradient. When the concentration of a nutrient is greater in the GI lumen than within the enterocyte, the nutrient flows passively across the cell membrane. Thus, the nutrient

passive diffusion Movement of substances across a cell membrane along their concentra- tion gradient.

▲ Figure 3.11 Four Methods of Nutrient Absorption in the Small Intestine

Passive di�usion: Nutrients pass through the cell membrane.

Facilitated di�usion: Requires a specific carrier but no energy is needed to cross the membrane.

Active transport: Requires both a carrier protein and energy to cross the cell membrane.

Endocytosis: Whole molecules are engulfed by the cell membrane.

Nutrient Nutrient

High concentration High concentration

Low concentration Low concentration

Low concentration

High concentration

Cell membrane

Carrier protein

Carrier protein

Nutrient

ATP

Nutrient

a b c d

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How Do Hormones and the Nervous System Regulate Digestion? 93

moves from a high concentration to a low concentration. This simple process requires neither energy nor a special carrier molecule. Water, small lipids, a few minerals, and vitamin C are examples of nutrients absorbed via passive diffusion.

Facilitated Diffusion The cell membrane of enterocytes has a lipid layer that is impermeable to most nutrients. Thus, most nutrients require an alternate method for absorption. In facilitated diffusion, nutrients are helped across the membrane by specific carrier proteins. This form of absorption is similar to passive diffusion in that it does not require energy, and nutrients flow from an area of higher to lower concentration. Fructose is an example of a nutrient that is absorbed with the help of a carrier protein.

Active Transport In active transport, a carrier protein and energy in the form of ATP shuttle nutrients across the cell membrane from an area of lower to higher concentration. This transport mechanism allows absorption even when the concentration of nutrients outside the enterocyte is lower than the concentration inside the cell. Glucose and amino acids are examples of nutrients absorbed by active transport.

Endocytosis Endocytosis occurs when molecules are too big to cross through a membrane using passive or active transport. This process occurs when a cell membrane folds back on itself to form a vesicle or pouch to surround and engulf the nutrient and bring it inside the cell using energy. Once inside the vesicle, the nutrient is dissolved in water. The mem- brane then disassembles and the contents are released on the other side. This type of absorption allows whole proteins, such as an immunoglobulin from breast milk, to be absorbed intact.

Fluid Absorption Occurs in the Large Intestine By the time chyme enters the large intestine, the majority of the nutrients have been absorbed. The large intestine absorbs water and certain minerals (mainly sodium) before the remaining mass reaches the rectum for excretion. The water is absorbed via passive diffusion and sodium is absorbed via active transport.

facilitated diffusion Movement of sub- stances across a cell membrane with the help of a carrier protein along their concentration gradient.

active transport Movement of substances across a cell membrane against their con- centration gradient with the help of a carrier protein and energy expenditure.

endocytosis Type of active transport in which the cell membrane forms an indentation, engulfs the substance to be absorbed, and releases it into the interior of the cell.

LO 3.4: THE TAKE-HOME MESSAGE The brush border of the small intestine is the major site of absorption for digested nutrients. Water and minerals not absorbed in the small intestine are absorbed in the large intestine. Nutrients are absorbed by passive diffusion, facilitated diffusion, active transport, or endocytosis.

How Do Hormones and the Nervous System Regulate Digestion? LO 3.5 Explain how hormones and the nervous system regulate digestion.

The endocrine and nervous systems work together to control and coordinate digestion, absorption, and elimination of waste products. Endocrine glands, which are found in the stomach, small intestine, and pancreas, are specialized cells that secrete hormones into the bloodstream in response to a stimulus. For example, when the food you eat reaches your stomach, endocrine glands release gastric hormones to signal the rest of the GI tract

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94 Chapter 3 | Digestion, Absorption, and Transport

to prepare for digestion. When food is not present, these hormones are not released. Digestion, absorption, and elimination are also regulated by the enteric nervous system, a network of nerve fibers embedded in the layers of the GI tract. Let’s take a closer look at how these two systems communicate and ultimately control the digestive process.

Hormones in the GI Tract Regulate Digestion Hormones secreted throughout the GI tract regulate digestion by controlling the release of gastric and pancreatic secretions, peristalsis, and enzyme activity. Enterogastrones, for example, are hormones produced and secreted by the cells lining the stomach and small intestine. These hormones—including gastrin, secretin, cholecystokinin, and gastric inhib- itory peptide (GIP)—have a powerful influence on gastrointestinal motility (the pace of digestion), stomach emptying, gallbladder contraction, intestinal absorption, and even hunger.

Certain types of food passing through the GI tract stimulate the release of hor- mones. For example, when a protein-containing bolus passes through the lower esoph- ageal sphincter (LES), gastrin is secreted from the stomach. This hormone triggers the release of gastric secretions that contain gastric lipase and stimulates the secretion of HCl. Gastrin also increases gastric motility and emptying and increases the tone of the LES.

Cells of the duodenum release secretin when the acidic chyme passes through the pyloric sphincter. The release of secretin in turn stimulates the pancreas to send bicarbonate ions through the pancreatic duct to neutralize the acid. Duodenal cells also secrete cholecystokinin (CCK) (koli-sis-te-KI-nin) in response to the entry of partially digested protein and fat. This powerful hormone stimulates the pancreas to release lipase and the gallbladder to contract and release bile, while it slows down gastric motility and contributes to meal satisfaction. A third hormone released by duodenal cells is gastric inhibitory peptide (GIP). Triggered by the presence of fatty acids or glucose, GIP inhibits gastric activities, allowing time for the digestive process to proceed in the duodenum before it receives more chyme. GIP also signals the pancreas to increase its secretion of insulin, which facilitates the uptake of blood glucose by body cells.

This synchronized effort by the GI tract hormones ensures the efficiency of digestion and maintains homeostasis in the body. Refer to Table 3.3 for a summary of the individual hormones, the tissues that secrete them, and their actions.

enteric nervous system Section of the peripheral nervous system that directly controls the gastrointestinal system.

enterogastrones Group of GI tract hor- mones, produced in the stomach and small intestine, that controls gastric motility and secretions.

gastrin Hormone released from the stomach that stimulates the release of acid.

secretin A hormone secreted from the duo- denum that stimulates the stomach to release pepsin, the liver to make bile, and the pan- creas to release digestive juices.

cholecystokinin (CCK) Hormone released by the duodenum that stimulates the gallblad- der to release bile.

gastric inhibitory peptide (GIP) Hormone produced by the small intestine that slows the release of chyme from the stomach.

Organ Hormone Stimulus Secreted from Action(s)

Stomach Ghrelin Empty stomach Gastric cells Stimulates gastric motility; stimulates hunger

Gastrin Food in the stomach Gastric cells Stimulates parietal cells to release HCl

Small Intestine Secretin Acidic chyme in duodenum

Duodenum Stimulates the pancreas to release bicar- bonate ions

Cholecystokinin (CCK) Fats and proteins in duodenum

Duodenum Stimulates the gallbladder to secrete bile and the pancreas to secrete bicarbonate ions and enzymes

Gastric inhibitory peptide (GIP)

Nutrients in the small intestine

Duodenum Stimulates secretions from the intestines and pancreas; inhibits stomach motility

Peptide YY Nutrients in the small intestine

Ileum Slows stomach motility

TABLE 3.3 Hormones of the GI Tract and Their Actions

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How Do Hormones and the Nervous System Regulate Digestion? 95

The Enteric Nervous System Communicates Within and Beyond the GI Tract The neurons in the enteric nervous system communicate with one another to perform certain GI functions autonomously. For example, within the walls of the GI tract, recep- tors of the enteric neurons sense cellular changes and trigger motor responses, which include stimulating and regulating contractions of the GI smooth muscle, motility of the GI tract, and gastrointestinal blood flow.

The enteric nervous system also works collaboratively with other branches of the nervous system, such as the central nervous system. For example, together with hormones secreted from the GI tract, enteric nerves generate signals about the fullness or empti- ness of the GI tract. These signals travel to the brain, which, with the help of hormones, communicates and interprets messages of hunger or satiation and encourages you to seek food or stop eating.

The hormones ghrelin and peptide YY communicate feelings of hunger or fullness to the brain (see Figure 3.12). Ghrelin, which is referred to as the “hormone of hunger,” is released from the gastric cells when the stomach is empty. It travels to the brain, where it prompts a desire to eat. Peptide YY is secreted by cells of the ileum and colon in response to the presence of food. It travels through the bloodstream to the brain and signals that you’ve eaten.

The central nervous system can also trigger enteric responses. Imagine, for example, that you walk by a bakery and smell freshly baked bread. Your brain receives this sensory data, signals the GI tract, and the enteric nerves interpret the signal and respond by stimu- lating the release of digestive juices.

In addition, the enteric nerves excite the muscles stimulating peristalsis, pushing the food and digestive juices through the GI tract. Once the stomach has emptied, the release of hormones and digestive juices ceases.

ghrelin Hormone produced in the stomach that stimulates hunger.

peptide YY Hormone produced in the small intestine that reduces hunger.

▲ Figure 3.12 Digestive Hormones Digestive hormones from the stomach and small intestine control and coordinate digestive pro- cesses and hunger.

Ghrelin

Gastrin

hunger

secretions from stomach stomach motility

Stomach

Secretin

CCK

GIP

Peptide YY

bicarbonate ions from pancreas

enzymes from pancreas bile from gallbladder

secretions from pancreas and small intestine stomach motility

hunger

Small intestine

Key Increases

Decreases

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How Are Nutrients Transported throughout the Body? LO 3.6 Explain how absorbed nutrients are transported throughout the body.

Once the nutrients have been absorbed through the lining of the small intestine, they are carried either through the cardiovascular or lymphatic system to other parts of the body (Figure 3.13). These two transportation systems consist of varying levels of pathways.

The Cardiovascular System Distributes Nutrients through Blood The blood is the body’s primary transport system, shuttling oxygen, nutrients, hormones, and waste products throughout the body. The heart pumps blood through this closed system of vessels through the entire body and back to the heart.

LO 3.5: THE TAKE-HOME MESSAGE The enterogastrones gastrin, secretin, cho- lecystokinin, and gastric inhibitory peptide regulate digestion by stimulating or inhibiting the release of secretions from the stomach, small intestine, pancreas, and gallbladder. They also influence gastric motility, which controls the pace of digestion. The enteric nervous system is a network of nerves embedded in the wall of the GI tract. It functions both autonomously and in collaboration with the central nervous system and other nervous system branches to achieve GI func- tions. Two hormones, ghrelin and peptide YY, communicate with the brain to help you decide when to eat and when to stop eating.

Figure 3.13 The Cardiovascular and Lymphatic Systems Blood and lymph are fluids that circulate throughout the body. They both pick up nutri- ents absorbed by the villi of the small intes- tine. Water-soluble nutrients travel through the bloodstream via the hepatic portal vein to the liver for processing. Fat-soluble nutrients travel through lymphatic vessels until they are released through the thoracic duct into the bloodstream.

Villus

Blood capillaries

Water-soluble nutrients

Lymph

Liver

Heart

Hepatic portal vein

Fat-soluble nutrients transported in the lymph move through the thoracic duct into the blood through the left subclavian vein

Blood

Fat-soluble nutrients

Lacteal

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How Are Nutrients Transported throughout the Body? 97

The heart is divided into four chambers: two upper atria and two lower ventricles. The oxygen-poor blood the heart receives from the body flows into the right atrium, through the right ventricle, and into the lungs, where it is replenished with oxygen. The oxygen-rich blood flows from the lungs to the left atrium and then into the left ventricle. As the left ventricle contracts, blood is pumped through the aorta and arteries to the capillaries, where it exchanges a variety of substances—including oxygen, water, glucose, and amino acids—with the cells. Carbon dioxide and other substances picked up by the blood are eventually returned to the heart via the veins as the blood completes its route. Carbon dioxide, a metabolic waste, is taken to the lungs and expelled.

Water-soluble nutrients, including carbohydrates, amino acids, and water-soluble vitamins, are absorbed into the enterocytes, cross into the capillaries, and travel via the hepatic portal vein to the liver. The portal vein branches out into capillaries, supplying all the liver cells with nutrient-rich blood. The liver further breaks down the nutrients and repackages them before releasing them into the blood. Blood leaves the liver and contin- ues on its journey through the hepatic vein back to the heart. Thus, the liver plays a key role in nutrition—it is the first organ to receive water-soluble nutrients absorbed through the intestines via the hepatic portal vein.

The Lymphatic System Distributes Some Nutrients through the Lymph The lymphatic system is a complex network of capillaries, small vessels, valves, nodes, and ducts that transport fat-soluble nutrients throughout the body. Lacteals in the villi collect fat-soluble vitamins, long-chain fatty acids, and some proteins too large to be trans- ported via the blood capillaries. The vessels of the lymph system are different from those of the circulatory system. The cells of the lymph capillary overlap one another, allowing the contents of the lymph vessels to seep out under pressure and circulate between the cells. The fat-soluble nutrients are transported from the lymph capillaries through the lymphatic vessels and eventually arrive at the thoracic duct. At the junction of the thoracic duct is a valve that allows the lymph fluid to flow into the subclavian vein, where it finally enters the blood. The nutrients can then be circulated by the blood to be picked up and used by cells.

The Excretory System Eliminates Waste After the cells have gleaned the nutrients and other useful metabolic components they need from the blood, their waste must be eliminated through the excretory system. For instance, the breakdown of proteins releases a nitrogenous waste called urea. The kidneys filter urea from the blood, allowing it to be excreted in urine. The nearby Table Tips provide strategies for improving your digestion.

hepatic portal vein Large vein that connects the GI tract to the liver and transports newly absorbed water-soluble nutrients.

hepatic vein Vein that carries the blood received from the hepatic portal vein away from the liver.

lymphatic system System of interconnected vessels that contains lymph fluid in which fat-soluble nutrients are carried; also includes bone marrow, lymph nodes, and other tissues and organs that produce and store defensive cells.

TABLE TIPS

Improve Your Digestion

Eat and drink slowly and thoroughly chew your food. This will reduce the amount of air you swallow and may reduce the need for belching later on.

Drink plenty of fluids and add more fiber to increase the bulk of feces and prevent constipation.

Include probiotic bacteria, found in yogurt, kefir, tempeh, kimchi, and sauerkraut, in your diet to help maintain your GI tract’s population of GI flora.

Practice mindful eating. Savor the flavor of food and enjoy every bite to allow the brain time to receive signals from digestive hormones that you are becoming full. This reduces overeating and improves portion control.

Identify foods that cause irritation or sensitivities. Reduce or eliminate these foods if necessary to improve your digestion.

LO 3.6: THE TAKE-HOME MESSAGE The cardiovascular and lymphatic systems pick up absorbed nutrients in the villi of the small intestine and transport them throughout the body. Water-soluble nutrients, including carbohydrates, proteins (amino acids), and the water-soluble vitamins, are transported in the blood- stream through the hepatic portal vein to the liver; fat-soluble nutrients, includ- ing the fat-soluble vitamins and long-chain fatty acids, are transported via the lymphatic system, which eventually drains into the subclavian vein. The excre- tory system filters the blood and eliminates waste.

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98 Chapter 3 | Digestion, Absorption, and Transport

HEALTHCONNECTION Treatments include surgery, radiation, and chemotherapy.7

Disorders of the Stomach Stomach problems can range from the trivial, such as belching or an occasional stomachache, to life-threatening compli- cations such as bleeding ulcers or stomach cancer. Common causes of stomachache include overeating or eating too fast. Other possible causes include eating foods that are high in fat or fiber, lactose intolerance, or swallowing air while eating.

Belching Swallowing air usually causes belching (also called burping). Often, the air dis- tends the stomach, and is then expelled up through the esophagus, pharynx, and mouth to relieve the discomfort. Swallowing large amounts of air (called aerophagia) is most often due to eating or drinking too fast, consuming carbon- ated beverages, or anxiety. However, aerophagia can occur without any act of swallowing, such as while chewing gum or smoking.

Gastroenteritis and Foodborne Illness A stomachache can be due to a number of causes, including gastroenteritis, commonly known as the “stomach flu.” Despite its name, stomach flu is not a form of influenza. It can be caused by a variety of bacteria and viruses, the most common of which is the norovi- rus. Infection causes an inflammation of the stomach or intestines. Symptoms include nausea and vomiting, diarrhea, and abdominal cramping. Sometimes the problem requires medical intervention, but usually rest, oral rehydration therapy to replace lost fluids and electrolytes, and a soft-food diet will help with the symp- toms of this type of illness.

Eating food or drinking fluid that is con- taminated with a pathogenic microbe usu- ally results in foodborne illnesses. Raw meat and poultry, raw eggs, unpasteurized milk,

in allowing this backflow. Certain foods, including chocolate, fried or fatty foods, coffee, soda, onions, and garlic, seem to be associated with this condition.6 Lifestyle factors also play a role. For example, smok- ing cigarettes, drinking alcohol, wearing tight-fitting clothes, being overweight or obese, eating large evening meals, and reclining after eating tend to cause or worsen the condition.

If dietary changes and behavior modifi- cation are insufficient to relieve heartburn, over-the-counter antacids or prescription drugs may help. In rare circumstances, sur- gical intervention is required to treat severe, unrelenting GERD.

Esophageal Cancer Esophageal cancer is another medical condition that has serious consequences. According to the National Cancer Insti- tute, esophageal cancer is one of the most common cancers of the GI tract, and the seventh leading cause of cancer-related deaths worldwide. In the United States, this type of cancer is typically found among individuals older than 50 years, males, those who live in urban areas, long-term smokers, and heavy drinkers.

gastroesophageal reflux disease (GERD) Chronic condition characterized by the backward flow of stomach contents into the esophagus, resulting in heartburn.

gastroenteritis Inflammation of the lining of the stomach and intestines; also known as stomach flu.

ulcer Sore or erosion of the stomach or intestinal lining.

What Are Some Common Digestive Disorders? LO 3.7 Discuss the most common

digestive disorders.

When the digestive system gets “off track,” the resulting symptoms can quickly catch your attention. Some of the problems are minor, like occasional heart- burn or indigestion; other problems such as ulcers or colon cancer are very serious and require medical treatment.

Esophageal Problems Several minor esophageal problems can lead to annoying symptoms such as belching, hiccups, burning sensations, or uncomfortable feelings of fullness. More serious esophageal problems include can- cer, obstruction from tumors, faulty nerve impulses, severe inflammation, abnormal sphincter function, and even death.

Gastroesophageal Reflux Disease One of the most common problems involving the esophagus is the burn- ing sensation in the middle of the chest known as heartburn (also called indigestion or acid reflux). About 60 million Ameri- cans experience heartburn once a month; about 15 million report daily heartburn.5

Heartburn generally occurs when the lower esophageal sphincter (LES) doesn’t close properly and acidic gastric juice from the stomach flows back into the esophagus and irritates its lining. Chronic heartburn is the hallmark symptom of gastroesophageal reflux disease (GERD). A weak LES is often the culprit

Obesity increases the risk of GERD.

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HEALTHCONNECTION (CONTINUED)

What Are Some Common Digestive Disorders? 99

Medical treatment for gallstones may involve surgery to remove the gallbladder, prescription medicine to dissolve the stones, shock-wave therapy (a type of ultrasound treatment) to break them up, or a combina- tion of therapies. If surgery is required to remove the gallbladder, patients typically recover quickly. After gallbladder removal surgery, the anatomy of the biliary tract adapts. The liver continues to produce the bile and secretes it directly into the duodenum.

Celiac Disease One of the more serious malabsorption conditions to occur in the small intestine is celiac disease. In a healthy small intestine, millions of villi and microvilli efficiently absorb nutrients from food. In celiac disease, the lining of the small intestine flattens out, and it can no longer absorb nutrients (Figure 3.16). The flattening is caused by an abnormal reaction to the pro- tein gluten, found in wheat, rye, and barley.

Celiac disease is a genetic, autoimmune disease that causes a person’s own immune system to damage the small intestines when gluten is consumed. Note that celiac disease is not the same thing as gluten intolerance, which does not involve the immune system or damage the wall of the small intestine. However, individuals with gluten intoler- ance can experience similar symptoms such as stomachaches, diarrhea, bloating, and tiredness if gluten is consumed.

Celiac disease is most common among genetically susceptible people of European descent. The most recent estimates suggest that 1 in 133 people (less than 1 percent) have been diagnosed with celiac disease in the United States. When an individual has celiac disease, the incidence among close family members is estimated to be as high

celiac disease Genetic disease in which a hyperimmune response damages the villi of the small intestine when gluten is consumed.

formations, although these factors may not be directly involved.

Burning pain is the most common symptom of an ulcer, along with vomiting, fatigue, bleeding, and general weakness. Medical treatments may consist of prescrip- tion drugs and dietary changes, such as limiting alcohol and caffeine-containing bev- erages, and/or restricting spices and acidic foods. Surgery is necessary only when an ulcer does not respond to drug treatment. Left untreated, ulcers can result in internal bleeding and perforation of the stomach or intestinal lining, causing peritonitis, or infec- tion of the abdominal cavity. Scar tissue can also form in the GI tract, obstructing food and causing vomiting and weight loss. Peo- ple who have ulcers caused by H. pylori have a greater risk of developing stomach cancer.

Gallbladder Disease The incidence of gallbladder disease is high in the United States, especially in women and older Americans. Obesity is one of the major risk factors, and this risk is even greater following rapid weight loss.9

A common gallbladder disorder is the presence of gallstones (Figure 3.15). Most people with gallstones have abnor- mally thick bile, and the bile is high in cholesterol and low in bile acids. Over time, the high-cholesterol bile forms crystals, then sludge, and finally gallstones. Some individuals with gallstones experience no pain or mild pain. Others have severe pain accompanied by fever, nausea and vomiting, cramps, and obstruction of the bile duct.

gallstones Stones formed from cholesterol in the gallbladder or bile duct.

and raw shellfish are the foods most likely to be contaminated.8 The pathogen most commonly involved is the norovirus. The most common bacterial culprit is Salmonella. Symptoms such as vomiting, abdominal cramps, diarrhea, and fever occur when enough of the pathogen has been ingested to trigger the body’s immune response. Most foodborne illnesses are self-limiting and may require only fluids and rest. More severe foodborne infections may require medical intervention. (You will read more about foodborne illness in Chapter 20.)

Ulcers An ulcer is a sore or erosion in the lin- ing of the lower region of the stomach or the upper part of the duodenum (Figure 3.14). Ulcers are named according to their location, such as gastric ulcers, duodenal ulcers, and esophageal ulcers. Whereas spicy foods and stress were once thought to cause most ulcers, researchers have since discovered that a bacterium, Helicobacter pylori, is often involved. The use of nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin, ibu- profen, naproxen, and ketoprofen, may also cause or aggravate ulcers. These pain relievers inhibit the hormonelike sub- stances that protect the stomach lining from HCl, which results in bleeding and ulceration. Nicotine increases the produc- tion of HCl, which increases the risk of developing an ulcer and slows the heal- ing process of ulcers that have already developed. Both excess consumption of alcohol and stress can contribute to ulcer

in allowing this backflow. Certain foods, including chocolate, fried or fatty foods, coffee, soda, onions, and garlic, seem to be associated with this condition.6 Lifestyle factors also play a role. For example, smok- ing cigarettes, drinking alcohol, wearing tight-fitting clothes, being overweight or obese, eating large evening meals, and reclining after eating tend to cause or worsen the condition.

If dietary changes and behavior modifi- cation are insufficient to relieve heartburn, over-the-counter antacids or prescription drugs may help. In rare circumstances, sur- gical intervention is required to treat severe, unrelenting GERD.

Esophageal Cancer Esophageal cancer is another medical condition that has serious consequences. According to the National Cancer Insti- tute, esophageal cancer is one of the most common cancers of the GI tract, and the seventh leading cause of cancer-related deaths worldwide. In the United States, this type of cancer is typically found among individuals older than 50 years, males, those who live in urban areas, long-term smokers, and heavy drinkers.

gastroesophageal reflux disease (GERD) Chronic condition characterized by the backward flow of stomach contents into the esophagus, resulting in heartburn.

gastroenteritis Inflammation of the lining of the stomach and intestines; also known as stomach flu.

ulcer Sore or erosion of the stomach or intestinal lining.

▲ Figure 3.14 Ulcer An ulcer is created when the mucosal lining of the GI tract erodes.

▲ Figure 3.15 Gallstones The size and composition of gallstones vary.

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100 Chapter 3 | Digestion, Absorption, and Transport

HEALTHCONNECTION (CONTINUED)

wheat, rye, or barley must be eliminated. Gluten-free grain products made with rice flour, bean flours, corn, quinoa, amaranth, and millet are fortunately widely available. Whole foods such as meat, milk, eggs, fruits, and vegetables are of course per- missible. The Academy of Nutrition and Dietetics maintains a comprehensive list of

diagnosis, the complications from celiac disease can be serious. Celiac patients have an increased incidence of osteoporosis from poor calcium absorption, diminished growth because of nutrient malabsorp- tion, and even seizures due to inadequate folate absorption. Although cancers associ- ated with celiac disease are rare, a history of celiac disease does increase the risk of developing certain types of cancer, includ- ing small intestinal adenocarcinoma, esoph- ageal cancer, and melanoma. 12

Treatment of Celiac Disease The only treatment for celiac disease is a strict, lifelong gluten-free diet.13 This should stop the symptoms from pro- gressing, allow the intestine to heal, and prevent further damage. The symptoms often improve within a few days after beginning the gluten-free diet. If the diet is followed faithfully, the absorption area of the GI tract often returns to normal status within 3–6 months.

Avoiding all gluten-containing foods can be challenging. All breads, pasta, cere- als, and other grain products made with

as 4.5 percent.10 Because of this genetic link, celiac disease is classified as a lifelong disorder.

Diagnosis of Celiac Disease The symptoms for celiac disease vary. Classic symptoms include reoccurring abdominal bloating, cramping, diarrhea, gas, fatty and foul-smelling stools, weight loss, anemia, fatigue, bone or joint pain, and even a painful skin rash called derma- titis herpetiformis. Some people develop the symptoms of celiac disease in infancy or childhood. Others are diagnosed later in life, after being misdiagnosed with irritable bowel syndrome or various food intolerances.

In the past, diagnosing celiac disease was sometimes difficult because it resembles other similar malabsorption diseases. The contemporary method of diagnosis begins with a simple blood test measuring gluten- reactive T cells. If the test is positive, the next step is a tissue biopsy of the small intestine to confirm the diagnosis. 11

Depending on the length of time between symptom development and

▼ Figure 3.16 Celiac Disease The autoimmune response in celiac disease erodes and flattens the villi of the intestinal lining.

Small intestine

Normal small intestinal wall with healthy villi

Small intestinal wall with flattened villi caused by celiac disease

TABLE TIPS

Gluten Free—What Can I Eat?

Be a label reader to avoid products that contain all forms of wheat, barley, rye, triticale, or their hybrids.

Substitute whole-wheat bread with gluten- free whole grains, such as quinoa, buck- wheat, amaranth, sorghum, and millet.

If you like a hot cereal for breakfast, try teff, a nutritious cereal grain from Ethio- pia with a nutty, chewy texture.

Avoid packaged foods that contain modified food starch, dextrin, malt, or malt syrup, as they may be sources of gluten-containing grains.

Enjoy coffee, tea, sodas, fruit juices, and even fermented or distilled bever- ages, such as wine, sake, and distilled spirits on a gluten-free diet. But avoid coffee flavorings or creamers.

Look for either the GFCO or CSA Seal of Recognition marks on labels to en- sure the product is gluten free.

Maintain a high fiber intake by in- corporating gluten-free fresh fruits, vegetables, beans, peas, and lentils in your diet.

Consume calcium-rich milk, calcium- fortified orange juice, or calcium-forti- fied beverages made from soy, rice, or nuts.

Snack on gluten-free nuts, fruits, popcorn, raisins, rice cakes, and fruit smoothies.

Enjoy homemade meals to control glu- ten intake and eat a variety of nutrient- dense foods.

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HEALTHCONNECTION (CONTINUED)

Flatus is a mixture of carbon dioxide, hydrogen, nitrogen, oxygen, and methane. The offending odor comes from gases that contain sulfur, chiefly hydrogen sulfide and methylmercaptan. Foods high in fiber and starches tend to produce more intestinal gas. Using products such as Beano or eating smaller meals will help reduce the amount of gas produced.

Diarrhea and Constipation Two of the most common intestinal disorders are diarrhea and constipa- tion. Diarrhea is the passage of watery, loose stools more than three times a day. Acute diarrhea lasts for up to 2 days and is usually the result of bacterial, viral, or parasitic infection. The infection results in the enterocytes becoming inflamed and secreting, rather than absorbing, fluid into the GI tract. The result is that food and fluids pass too quickly through the colon and out through the rectum.

Whereas brief episodes of diarrhea may be a sign of infection or an adverse reac- tion to a specific food, medication, or other compound (such as the sugar substitute sorbitol), chronic diarrhea may be a sign of celiac disease, irritable bowel syndrome (see below), or colitis, conditions that require diagnosis by a health care provider.

The loss of fluids and electrolytes in diarrhea can be mild or life-threatening. Mild diarrhea is generally treated with fluid and electrolyte replacement. Diarrhea that is severe or that lasts for an extended period can lead to dehydration, malabsorption— which in turn can lead to malnutrition—and potentially even death. The condition can be particularly dangerous for children and older adults, who are more susceptible to dehydration.

immune system.19 For this reason, there are no real benefits to eating a gluten-free diet if you are not gluten sensitive, have not been diagnosed with celiac disease, or do not have other autoimmune disorders.

Other Intestinal Disorders Other disorders of the intestines can occur anywhere along the length of the small and large intestines.

Flatulence Flatulence is an uncomfortable and sometimes embarrassing (but normal) condition that results from the forma- tion of intestinal gas ( flatus). Intestinal gas is produced for a variety of reasons, and most adults release it 10–20 times a day. Eating too fast or drinking beverages with added air such as beer or carbonated beverages can result in the intake of inci- dental air that makes its way through the GI tract. Beans, lentils, and other legumes can lead to gas production because they contain indigestible carbohydrates that are fermented by intestinal bacteria. The bacteria produce the gas as a by-product. Lack of exercise and smoking have also

been identified as culprits in flatulence.

non-celiac gluten sensitivity (NCGS) Reaction to eating foods that contain gluten when celiac disease has been ruled out. Symptoms vary widely but may include abdominal pain, fatigue, headaches, rashes, or mental confusion.

flatulence Production of excessive gas in the stomach or the intestines.

diarrhea Abnormally frequent passage of watery stools.

foods allowed on a gluten-free diet on its website, www.eatright.org.14 The nearby Table Tips identify strategies for maintain- ing a gluten-free diet.

Until recently, oats had been consid- ered a forbidden food for celiac patients. However, unprocessed oats are gluten-free. Concerns for people with celiac disease are due to the possibility that oats may be con- taminated with hidden sources of gluten during processing. Purchasing oats from companies that advertise their products as gluten-free and avoiding oats sold in bulk bins can allay such concerns.

Non-Celiac Gluten Sensitivity Many people who have tested negative for celiac disease nevertheless report a reduction in both GI and non-GI symp- toms when they remove gluten from their diet. This condition, referred to as non-celiac gluten sensitivity (NCGS), is currently the subject of considerable research.15

People without Celiac Disease or Gluten Sensitivity Should Not Avoid Gluten Gluten-free diets have become a fad that is fueling a market of gluten-free prod- ucts, with sales of more than $2 billion expected by 2020.16 There are currently more people following a gluten-free diet than there are diagnosed with celiac dis- ease.17 Is this healthy?

Gluten itself isn’t an essential nutri- ent, but the foods that contain gluten are typically rich in dietary fiber, vitamins, and minerals. Many gluten-free products are made with refined, unenriched gluten-free grains low in essential nutrients and high in kilocalories. Gluten-free foods are also typically much more expensive than simi- lar foods made with wheat.18 Moreover, gluten itself may provide some health benefits for individuals without celiac dis- ease or NCGS. Recent studies have shown gluten-rich foods may improve blood lip- ids, control blood pressure, and boost the

Rice noodles are a good gluten-free substi- tute for pasta.

What Are Some Common Digestive Disorders? 101

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It causes a great deal of discomfort for the estimated 30 million North Ameri- cans who have it.22 People with IBS do not have tissue damage, inflammation, or immunologic involvement of the colon. They do, however, overrespond to colon stimuli. This results in alternating pat- terns of diarrhea, constipation, bloating, and abdominal pain. The exact cause of IBS is unknown, but low-fiber diets, stress, consumption of foods that trigger the symptoms (such as alcohol, choco- late, and dairy products), and intestinal motility disorders are all suspected factors. Medical management includes increasing dietary fiber, managing stress, and occasional use of prescription drugs.

A diet designed to alleviate the symp- toms of IBS is called the low FODMAP diet. The acronym FODMAP stands for fermented, oligosaccharides, disaccharides, monosaccharides, and polyols. The foods that contain these carbohydrates, which you learn more about in Chapter 4, are not completely digested and thus can trigger cramps, diarrhea, and other symptoms of IBS. They are listed in Table 3.4. The diet limits these carbohydrates but doesn’t completely eliminate them. While the low FODMAP diet may not work for everyone with IBS, a recent study found that some participants with IBS reversed their symptoms after just 7 days on the diet.23 See the Nutrition in Practice for more information on nutritional strate- gies for irritable bowel syndrome.

irritable bowel syndrome (IBS) Intestinal disorder resulting in abdominal discomfort, pain, diarrhea, constipation, and bloating; the cause is unknown.

Hemorrhoids A hemorrhoid is an inflammation and swelling of the veins in the rectum and anus. The walls of the veins dilate, become thin, and bleed. Though the exact cause of hemorrhoids is not known, several factors result in a buildup of pressure within the veins. These include constipation (straining to pass dry stools), pregnancy, and obesity. Chronic diarrhea, anal intercourse, a low- fiber diet, and aging are also risk factors. Whatever the cause, the result is that the walls of the veins dilate, become thin, and bleed. As the pressure builds, the vessels protrude. Hemorrhoids may not be noticed until they begin to bleed (following a bowel movement), itch, or become painful.

The most common treatment for hem- orrhoids is the same as for constipation: increase dietary fiber and fluid intake. Other symptoms, including itching and pain, can be relieved with over-the-counter creams, ice packs to relieve swelling, and soaking in a warm bath. In severe cases of hemor- rhoids, surgery may be necessary.

Irritable Bowel Syndrome Irritable bowel syndrome (IBS) is a general term used to describe changes in colon rhythm, not an actual disease.

hemorrhoid Swelling in the veins of the rectum and anus.

Constipation is caused by excessively slow movements of undigested food residue through the colon, often as a result of insuf- ficient fiber or water intake. Ignoring or put- ting off the need to defecate can also result in more absorption of water from fecal mat- ter in the large intestine, leading to harder, drier stools. Stress, inactivity, cessation of smoking, and various illnesses and medica-

tions can also lead to constipation.20,21

Because fiber attracts water, adds bulk to stool in the colon, and stimulates peri- stalsis, constipation is often treated with a high-fiber, high-liquid diet. Daily exercise, establishing eating and resting routines, and using over-the-counter stool softeners are usually recommended to treat this condition without the use of laxatives or enemas.

If constipation persists, laxatives can provide some relief, but should be used sparingly. A variety of laxatives can be pur- chased over the counter. Bulk-forming or stool-softening laxatives trigger peristalsis by drawing water into the GI tract, which increases the bulk of the feces and stretches the circular muscles in the intestine. Stimu- lant laxatives, such as Ex-lax or Senokot, are the harshest form of laxative and work by irritating the lining of the GI tract to stimu- late peristalsis. Because laxatives can cause dehydration, electrolyte imbalances, and laxative dependency, they should not be used routinely unless a physician supervises.

One harmful and unnecessary practice is colonic cleansing, which involves adminis- tration of an enema to flush water and feces out of the body. The practice can interfere with the absorption of fat-soluble vitamins and can be dangerous if the equipment used to administer the enema isn’t sanitized or if the bowel is perforated when the rub- ber tube is inserted. Other problems may result from electrolyte and water imbalance and dependency.

constipation Infrequent passage of dry, hard- ened stools.

Irritable bowel syndrome can cause signifi- cant abdominal pain.

Food Components to Avoid

Foods Containing Component

Fructose Fruits, honey, high-fruc- tose corn syrup, agave

Lactose Dairy

Fructans Wheat, onions, garlic

Galactans Legumes, beans, lentils, and soybeans

Polyols Sugar alcohols, fruits with pits or seeds such as apples, avocados, cherries, figs, peaches, or plums

TABLE 3.4 FODMAP Foods

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HEALTHCONNECTION (CONTINUED)

NUTRITION in PRACTICE: Nurse Practitioner

College freshman Desiree has always had what her mother called “a nervous stomach.” When she is stressed or worried about something, her bowels seem to respond first with abdominal pain followed by constipation or diarrhea. Her first semes- ter at college has been challenging as she is trying to adjust to the rigorous demands of her pre-med curriculum and the intense schedule of science classes and labs. Her symptoms are worse right before an exam or when she eats large meals. She eats a very light break- fast because she doesn’t want to feel uncomfortably full and bloated during her morning classes. Lately, Desiree has had frequent bouts of abdominal pain that are only relieved by a bowel movement, followed by several loose, watery stools. With mid-terms approaching the abdomi- nal pain and diarrhea appear to be getting worse. She is experiencing difficulty fall- ing asleep at night, which is causing her to become tired by midday. She decides to visit the campus health center.

After meeting with the nurse practitio- ner (NP) at the health center, Desiree is told that she has irritable bowel syndrome (IBS) and that stress and her diet are likely contributing to her health issue. The nurse practitioner referred Desire to the campus health center stress manage- ment program, which is designed to help students manage anxiety and stress while on campus. She also referred Desiree to the campus registered dietitian nutrition- ist (RDN) for an appointment. The RDN meets with Desiree to discuss her diet.

Desiree’s Stats: ❏ Age: 18 ❏ Height: 5 feet 4 inches ❏ Weight: 115 lbs ❏ BMI: 19.7 ❏ Symptoms: abdominal pain;

loose, watery stool with bouts of constipation

Critical Thinking Questions 1. How do you think her eating pattern

may be contributing to her IBS symp- toms? What aspect of Desiree’s diet and lifestyle may be contributing to her IBS symptoms?

2. Which beverages could be contribut- ing to Desiree’s bouts of diarrhea and tiredness?

3. How are Desiree’s food choices con- tributing to her discomfort?

RDN’s Observation and Plan for Desiree:

❏ Reinforce the NP’s recommenda- tion to visit the campus health center

stress management program for guidance on dealing with stress at college. Stress is a trigger for IBS symptoms.

❏ Slowly reduce the caffeine (coffee, energy drinks) in her diet. Caffeine can increase jitteriness (anxiety), cause a spastic colon (diarrhea), and interfere with the ability to fall asleep (fatigue).

❏ Reduce the fat at her meals. Fatty foods can also trigger diarrhea in folks with IBS.

❏ Avoid large meals. Eat three smaller meals and two small snacks through- out the day.

Desiree

DESIREE’S FOOD LOG

Food/Beverage Time

Consumed Hunger Rating* Location

Feelings/ SymptomsLarge black coffee

24 oz. 7:30 a.m. 1 Walking to

class Anxious about classes and upcoming examCheeseburger and fries 1 pm 5 Campus

dining hall Tired

Large diet cola 2 p.m. 1 Library Some diarrheaEnergy drink, 12 oz. 4 p.m. 1 Walking to dorm

Tired

Fried chicken, mashed potato, and corn

6 p.m. 3 In dorm Stressed, stom- ach cramps; feeling bloated*Hunger Rating (1–5): 1 = not hungry; 5 = super-hungry.

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HEALTHCONNECTION (CONTINUED)

chemotherapy, and surgery to remove all or part of the colon. After surgery, patients are given dietary advice regarding the foods that would be the most comfortable to eat. Survival rates vary depending on the indi- vidual’s age, health, treatment response, and stage of cancer at diagnosis.28, 29

Recent research findings suggest that an anti-inflammatory diet similar to the Mediterranean diet might be helpful. Foods thought to have anti- inflammatory properties include fish, olive oil, and fresh fruits and veg- etables. Fish provide omega-3 fatty acids that reduce levels of inflamma- tory proteins in the body, including C-reactive protein, which you learn more about in Chapter 5, and inter- leukin-6, which plays a key role in the acute phase of the body’s inflam- matory response. Fresh fruits and vegetables are rich in certain vitamins and phytochemicals that also reduce inflamma- tion; in addition, probiotics may play a role in reducing inflammation.24,25

Colon Cancer Colon cancer is one of the leading forms of cancer and the second leading cause of cancer death in men and women.26 Fortunately, colon cancer is also one of the most curable forms of cancer, if it is detected in the early stages.

Colon cancer often begins with polyps on the lining of the colon (Figure 3.17). They vary in size from that of a small pea to that of a mushroom or plum. The good news is that polyps are often small and benign and are routinely removed during a screening colonoscopy. This procedure has reduced the incidence of colon cancer during the past two decades.27 If the polyps are not removed and develop into cancer- ous tumors, colon cancer can be difficult to cure.

Individuals diagnosed with colon cancer may require radiation therapy,

inflammatory bowel disease (IBD) Chronic inflammation throughout the GI tract.

ulcerative colitis Chronic inflammation of the colon that results in ulcers forming in the lining.

Crohn’s disease Form of ulcerative colitis in which ulcers form throughout the GI tract and not just in the colon.

Inflammatory Bowel Disease Inflammatory bowel disease (IBD) occurs when inflammation of the GI tract impairs the ability of the colon to compact food waste and form feces. The result is diarrhea. Ulcer- ative colitis and Crohn’s disease are forms of IBD with similar symptoms. Ulcerative colitis is a chronic inflam- mation of the large intestine that results in ulcers in the lining of the colon. The disorder usually begins between the ages of 15 and 30, occurs in both men and women, and tends to run in families, especially in Caucasians and people of Jewish descent. Crohn’s disease is simi- lar to ulcerative colitis except that the ulcers can occur throughout the entire GI tract, from the mouth to the anus, not just in the colon.

The cause of ulcerative colitis and Crohn’s disease is not known and there is no cure. Physical examinations, labora- tory tests, and a colonoscopy are often used to distinguish between the two conditions. Medical treatment includes drug therapy and, in severe cases, surgery.

Three weeks later, Desiree visits the RDN again and reports that she has reduced her caffeine intake by eliminating her daily energy drinks and is sleeping better at night. She has begun to eat a breakfast of a Greek yogurt with sliced

bananas topped with a whole grain cereal. She switched to grilled chicken to reduce the fat at her lunch and is trying to eat a more balanced dinner. She listens to meditation tapes daily, as suggested in the stress management

program, and is feeling less anxious. A dietary analysis of Desiree’s food intake reveals she needs to add more dairy in her diet and a wider variety of vegetables and fruit. The RDN works with Desiree to make these diet changes.

▲ Figure 3.17 Colon Polyp Polyps on the lining of the colon can be one of the first signs of colon cancer.

Polyp

LO 3.7: THE TAKE-HOME MESSAGE Some GI disorders, such as ulcers, GERD, and irritable bowel syn- drome, are not life-threatening, but can significantly reduce quality of life. Others, including celiac disease, ulcerative colitis, Crohn’s disease, and colon or esophageal cancer, may cause nutrient malabsorption, malnutrition, or even death. Less serious conditions, like constipation, diarrhea, hemorrhoids, and stomach flu, are typically temporary.

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Visual Chapter Summary 105

Visual Chapter Summary

Salivary glands

Pharynx

Epiglottis

Esophagus

Gallbladder

Stomach

Pancreas

Large intestine

Small intestine

Rectum

Anus

Liver

Digestion Takes Place in the GI Tract, Helped by the Accessory Organs

Digestion is the process of breaking down whole food into absorb- able nutrients. Digestion takes place in the GI tract, a long tube com- posed of the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Several sphincters control entry and exit of food and chyme through the various organs of the GI tract. Accessory organs, including the salivary glands, liver, gallbladder, and pancreas, secrete bile, hormones, and enzymes that facilitate digestion.

The small intestine is the primary organ for the absorption of digested nutrients. The large intestine absorbs water and some min- erals before pushing waste through the colon and out of the body via the rectum.

Bile produced in the liver emulsifies fat, increasing its access to pancreatic lipase. The liver is the first organ to receive, process, and store absorbed water-soluble nutrients. The pancreas releases pancre- atic digestive enzymes and bicarbonate into the small intestine. The gallbladder stores and concentrates bile.

LO 3.1

Segmentation

Peristalsis

Enzyme

Substrate

Bond

H2O

Peristalsis and Segmentation Propel Food through the Gastrointestinal Tract

Food is propelled along the GI tract by peristalsis and seg- mentation. Peristalsis moves food through the stomach and intestines by rhythmic contractions of longitudinal and circular muscles. Segmentation shifts the mass of food back and forth into smaller pieces while mixing it with the chemical secretions of the intestine. Segmentation in the large intestine facilitates the absorption of water from the feces and mass movements push the mass toward the rectum for excretion.

LO 3.2 LO 3.3 Enzymes and Other Secretions Chemically Digest Food

Chemical digestion includes mixing food with enzymes that break the bonds between molecules via hydrolysis. HCl dena- tures proteins and triggers the conversion of pepsinogen to pepsin, which initiates protein digestion. Bile from the gall- bladder emulsifies large fat globules into smaller pieces to improve enzymatic action. Digestion is completed by the brush border enzymes maltase, sucrase, and lactase, and the prote- ases, breaking nutrients down into single molecules that can be absorbed into the enterocytes.

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106 Chapter 3 | Digestion, Absorption, and Transport

Nutrient Nutrient

High concentration High concentration

Low concentration Low concentration

Low concentration

High concentration

Cell membrane

Carrier protein

Carrier protein

Nutrient

ATP

Nutrient

Passive diffusion: Nutrients pass through the cell membrane.

a Facilitated diffusion: Requires a specific carrier but no energy is needed to cross the membrane.

b Active transport: Requires both a carrier protein and energy to cross the cell membrane.

c Endocytosis: Whole molecules are engulfed by the cell membrane. d

LO 3.4 Digested Nutrients Are Absorbed Primarily through the Small Intestine

Digested nutrients pass through the brush border of the small intestinal lining via passive diffusion, facilitated diffusion, active transport, or endocytosis. The water and salts not absorbed in the small intestine are absorbed in the large intestine.

Digestion Is Regulated by Hormones and the Nervous System

Digestive functions are regulated by hormones, including gas- trin, secretin, cholecystokinin, and gastric inhibitory peptide. These hormones are chemical messengers secreted from the GI tract that direct enzymes and the release of digestive secretions during digestion. The enteric nerves embedded in the wall of the GI tract work autonomously and in collaboration with the central nervous system and other nervous system branches to achieve multiple GI functions. Nerves also interact with the GI tract to stimulate hormone release when certain foods are consumed.

LO 3.5

Stomach Ghrelin Gastrin

Small intestine Secretion CCK GIP Peptide YY

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Terms to Know 107

Digestive Disorders Can Result in Malabsorption and Malnutrition

Several digestive disorders can result in malabsorption and mal- nutrition, including celiac disease, ulcerative colitis, and Crohn’s disease. Celiac disease is an autoimmune disorder that damages the villi of the small intestine following gluten ingestion. Irritable bowel syndrome is a disorder of unknown cause, the symptoms of which may improve on a low-FODMAP diet. Heartburn, constipa- tion, diarrhea, and hemorrhoids are common, less serious digestive disorders. If left unchecked, even diarrhea can be potentially life- threatening. Common cancers of the GI tract include esophageal, stomach, and colon cancer.

LO 3.7

Normal small intestinal wall Small intestinal wall with celiac disease

LO 3.6 Absorbed Nutrients Are Transported throughout the Body by the Cardiovascular and Lymphatic Systems

The cardiovascular system distributes water-soluble nutri- ents throughout the body and carries carbon dioxide and other waste products to be excreted through the lungs and the kidneys. The lymphatic system transports fat-soluble vitamins from the GI tract through the lymphatic system and into the cardiovascular system.

Villus

Blood capillaries

Water-soluble nutrients

Fat-soluble nutrients

Lacteal

Terms to Know

■■ digestion ■■ absorption ■■ elimination ■■ gastrointestinal (GI) tract ■■ propulsion ■■ lumen ■■ sphincters ■■ chemical digestion ■■ mechanical digestion ■■ mastication ■■ saliva ■■ bolus ■■ pharynx ■■ esophagus ■■ epiglottis ■■ upper esophageal

sphincter

■■ lower esophageal sphincter (LES)

■■ stomach ■■ goblet cells ■■ gastric pits ■■ parietal cells ■■ chief cells ■■ mucus ■■ chyme ■■ small intestine ■■ villi ■■ enterocytes ■■ microvilli ■■ crypts ■■ large intestine ■■ cecum ■■ ileocecal valve ■■ colon ■■ GI flora ■■ ferment

■■ probiotics ■■ prebiotics ■■ stool ■■ rectum ■■ anus ■■ intestinal permeability ■■ salivary glands ■■ liver ■■ gallbladder ■■ pancreas ■■ peristalsis ■■ segmentation ■■ mass movement ■■ enzymes ■■ hydrolysis ■■ substrate ■■ pH ■■ hydrochloric acid (HCl) ■■ proteases ■■ pepsinogen

■■ pepsin ■■ bile ■■ emulsify ■■ enterohepatic

circulation ■■ bicarbonate ■■ passive diffusion ■■ facilitated diffusion ■■ active transport ■■ endocytosis ■■ enteric nervous system ■■ enterogastrones ■■ gastrin ■■ secretin ■■ cholecystokinin (CCK) ■■ gastric inhibitory pep-

tide (GIP) ■■ ghrelin ■■ peptide YY ■■ hepatic portal vein

■■ hepatic vein ■■ lymphatic system ■■ gastroesophageal

reflux disease (GERD) ■■ gastroenteritis ■■ ulcer ■■ gallstones ■■ celiac disease ■■ non-celiac gluten sen-

sitivity (NCGS) ■■ flatulence ■■ diarrhea ■■ constipation ■■ hemorrhoid ■■ irritable bowel syn-

drome (IBS) ■■ inflammatory bowel

disease (IBD) ■■ ulcerative colitis ■■ Crohn’s disease

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108 Chapter 3 | Digestion, Absorption, and Transport

Mastering Nutrition Visit the Study Area in Mastering Nutrition to hear an MP3 chapter summary.

Check Your Understanding LO 3.1 1.                is defined as break-

ing apart food by mechanical and enzymatic means in the stomach and the small intestine. a. Absorption b. Transport c. Digestion d. Elimination

LO 3.1 2. The protective tissue that covers the trachea during swallowing is the a. esophagus. b. tongue. c. pharynx. d. epiglottis.

LO 3.1 3. Which of the following state- ments about the small intestine is true? a. It is shorter than the large

intestine. b. It is the primary site of both

digestion and absorption. c. It has access to lymph vessels

but not to the bloodstream. d. It is composed of the ileum,

duodenum, and cecum, in that order.

LO 3.2 4. Food is moved through the GI tract by rhythmic muscular waves called a. segmentation. b. peristalsis. c. bowel movement. d. mastication.

LO 3.3 5. Which of the following secretions chemically breaks apart foods? a. Ghrelin b. Bicarbonate ions c. Cholecystokinin d. Enzymes

LO 3.3 6. Which of the following is a function of hydrochloric acid? a. Triggers the conversion of

pepsinogen to pepsin b. Slows peristalsis c. Neutralizes the pH in the

stomach d. Begins carbohydrate digestion

LO 3.4 7. Glucose and amino acids are absorbed by a. passive diffusion. b. facilitated diffusion. c. active transport. d. endocytosis.

LO 3.5 8. The hormone that is produced in the stomach and stimulates hunger is called a. pepsin. b. gastrin. c. cholecystokinin. d. ghrelin.

LO 3.6 9. Which of the following trans- ports fat-soluble nutrients to the blood? a. Hepatic portal vein b. Lymphatic vessels c. Red blood cells d. The kidneys

LO 3.7 10. Celiac disease is caused by a reaction to gluten found in which foods? a. Citrus fruits b. Wheat, barley, and rye c. Legumes, nuts, and seeds d. Milk, cheese, and yogurt

Answers 1. (c) Digestion is defined as the

process of breaking apart food by mechanical and enzymatic means. Once food is digested it can be absorbed into the blood or lym- phatic system to be transported to the cells. Any waste products are eliminated through urine and feces.

2. (d) The epiglottis covers the tra- chea. The esophagus is a tube that carries food to the stomach. The tongue is a muscle that pushes food to the back of the mouth into the pharynx, a chamber that food passes through before being swallowed.

3. (b) The small intestine is the pri- mary site of both digestion and absorption. With numerous villi and microvilli, it has a vast surface area to enhance absorption. It allows nutrients to pass into both blood and lymph. It consists of the duode- num, then the jejunum, and finally the ileum. It is much longer than the large intestine.

4. (b) Peristalsis is the process that moves food through the GI tract. Segmentation squeezes chyme back and forth along the intestinal walls. A bowel movement involves waves of peristalsis moving feces through the large intestine. Mastication is the process of chewing food.

5. (d) Enzymes secreted from the stom- ach, small intestine, and pancreas chemically break apart food. Grehlin is a hormone that stimulates gastric motility and hunger. Bicarbonate ions neutralize acid. Cholecystokinin is a hormone that slows peristalsis.

6. (a) Hydrochloric acid activates pepsinogen, which is converted to pepsin. It also breaks down connec- tive tissue in meat and destroys some microorganisms. Cholecystokinin and gastric inhibitory hormones slow peristalsis. Bicarbonate ions neutral- ize pH in the duodenum. Salivary amylase begins carbohydrate break- down in the mouth.

7. (c) Glucose and amino acids are absorbed by active transport.

8. (d) Grehlin produced by the gastric cells stimulates gastric motility and hunger. Pepsin is the active form of a protease secreted by the chief cells in the stomach. Gastrin produced in the stomach stimulates parietal cells to release HCl. Cholecystokinin (CCK) is released from the duode- num when fats and proteins enter the duodenum. CCK stimulates the gallbladder to release bile and the pancreas to secrete bicarbonate ions and digestive enzymes.

9. (b) The fat-soluble nutrients are trans- ported through the lymphatic vessels and eventually arrive at the thoracic duct, where they enter the blood- stream at the subclavian vein. The hepatic portal vein transports water- soluble nutrients to the liver. Red blood cells transport oxygen. The kidneys filter the blood to remove and excrete waste into the urine.

10. (b) Wheat, barley, and rye contain gluten, which causes the symptoms associated with celiac disease. Fruits, legumes and other vegetables, dairy foods, meat, nuts, and seeds are free of gluten and are safe to eat on a gluten-free diet.

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References 109

Answers to True or False? 1. True. The enzyme salivary amylase,

found in saliva, begins digesting car- bohydrates during chewing. As they are broken down to their simple sug- ars, these carbohydrates will begin to taste sweet.

2. True. The stomach secretes a power- ful digestive acid, HCl, that is strong enough to damage the stomach lining. A thick layer of mucus protects it.

3. True. Bile emulsifies fat by breaking up the large globules into smaller fat droplets.

4. False. Acid reflux occurs when acidic stomach contents pass back through the lower esophageal sphincter into the esophagus.

5. True. After food has been com- pletely broken down and its nutrients absorbed in the small intestine, the remaining mass passes into the large intestine, where water and electro- lytes continue to be absorbed.

6. True. Fecal matter is about 50 percent bacteria, and the rest is undigested food, water, and sloughed intestinal cells.

7. False. Lymph transports only fat- soluble nutrients. Blood transports water-soluble nutrients.

8. True. Several hormones, including gastrin, secretin, cholecystokinin (CCK), and gastric inhibitory pep- tide, help regulate digestion.

9. False. Though foodborne illness can cause diarrhea, the condition can also result from an adverse reaction to stress or to certain foods, medica- tions, or other compounds.

10. False. The cause of irritable bowel syndrome is not known, but low- fiber diets, stress, consumption of irritating foods, and intestinal motility disorders are all suspected factors.

Web Resources ■■ To learn more about the various

conditions related to digestion,

absorption, and elimination, visit the Center for Digestive Health and Nutrition at www.gihealth.com

■■ Search the National Digestive Dis- eases Information Clearinghouse site for more information about various digestive diseases at http://digestive. niddk.nih.gov

References 1. Marieb, E. N., and K. Hoehn. 2016.

Human Anatomy and Physiology. 10th ed. San Francisco: Benjamin Cummings.

2. Gropper, S. S., and J. L. Smith. 2016. Advanced Nutrition and Human Metabolism. 7th ed. Belmont, CA: Wadsworth Cengage Learning.

3. Marieb, E. N., and K. Hoehn. 2016. Human Anatomy and Physiology. 10th ed. San Francisco: Benjamin Cummings.

4. Ibid. 5. Health Grades. 2013. Prevalence and Incidence

of Heartburn. Available at www.rightdiagno- sis.com. Accessed December 2016.

6. Guandalini, S., and A. Assiri. 2014. Celiac Disease: A Review. Journal of the American Medical Association Pediatrics. doi: 10.1001/ jamapediatrics.2013.3858.

7 National Cancer Institute. 2014. Esopha- geal Cancer. Available at www.cancer.gov. Accessed December 2016.

8. Center for Disease Control and Prevention. 2016. Foodborne Germs and Illnesses. Available at https://www.cdc.gov/foodsafety/food- borne-germs.html. Accessed January 2017.

9. Healthline. 2017. Gallbladder Disease. Avail- able at http://www.healthline.com/health/ gallbladder-disease#Types2. Accessed Janu- ary 2017.

10. Pelkowski, T. D., and A. J. Viera. 2014. Celiac Disease: Diagnosis and Management. Ameri- can Family Physician 89(2):99–105.

11. Shenoy, S. 2016. Genetic Risks and Familial Associations of Small Bowel Carcinoma. World Journal of Gastrointestinal Oncology 8(6):509–519.

12. Pelkowski, T. D., and A. J. Viera, 2014. 13. Lebwohl, B., J. F. Luvigsson, and P. H. R.

Green. 2015. Celiac Disease and Non-Celiac Gluten Sensititivity. British Medical Journal, 351:h4347.

14. Denny, S. 2015. The Gluten-Free Diet: Building the Grocery List. Available at http://www.eatright.org/resource/health/ diseases-and-conditions/celiac-disease/the- gluten-free-diet-building-the-grocery-list. Accessed January 2017.

15. Lebwohl, B., J. F. Luvigsson, and P.H.R. Green. 2015.

16. Ibid.

17. Packaged Facts. 2016. Gluten-Free Foods in the U. S., 6th Edition. Available at https://www.packagedfacts.com/Gluten- Free-Foods-10378213. Accessed January 2017.

18. Jaret, P. 2017. The Truth About Gluten Free. Available at http://www.webmd.com/ diet/healthy-kitchen-11/truth-about-gluten. Accessed January 2017.

19. Stein, K. 2014. Severely Restricted Diets in the Absence of Medical Necessity: The Unintended Consequences. Journal of the Academy of Nutrition and Dietetics 114(7):986-987.

20. Gaesser, G. A., and S. S. Angadi. 2012. Gluten Free Diet: Imprudent Dietary Advice for the General Population? Jour- nal of the Academy of Nutrition and Dietetics 122(9):1330-1333.

21. Fathallah, N., D. Bouchard, et al. 2016. Diet and Lifestyle Rules in Chronic Constipation in Adults: From Fantasy to Reality . . . La Presse Medicale doi: 10.1016/ j.lpm.2016.03.019.

22. What Is Constipation? Available at http:// www.webmd.com/digestive-disorders/ digestive-diseases-constipation#2. Accessed January 2017.

23. Bellini, M., et al. 2016. A Low FODMAP Diet in Irritable Bowel Syndrome Improves Symptoms Without Affecting Body Compo- sition and Extracellular Body Water. Gastroen- terology 150:4, S200.

24. Olendzki, B. C., et al. 2014. An Anti- inflammatory Diet as Treatment for Inflammatory Bowel Disease: A Case Series Report. Nutrition Journal 13:5. doi: 10.1186/1475-2891-13-5

25. Ibid. 26. U.S. Cancer Statistics Working Group. United

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27. Ibid. 28. Baars, A., A. Oosting, et al. 2015. The

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29. Joyce, S. A., and G. M. Cormac. 2016. Bile Acid Modifications at the Microbe-Host Interface: Potential for Nutraceutical and Pharmaceutical Interventions in Host Health. Annual Review of Food Science and Technology 7:313-333. doi: 10.1146/annurev-food- 041715-033159.

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