Chapter 11: DIGESTION AND NUTRITION

11 digestion and Nutrition Chapter Outline OVERVIEW of the DIGESTIVE SYSTEM The digestive tube has four layers The different parts of the digestive system perform five basic functions Chewing and swallowing: FOOD PROCESSING BEGINS The teeth tear and grind bulk food into smaller chunks Enzymes in saliva begin the chemical digestion of food Swallowing has voluntary and involuntary phases the stomach: food storage, digestion, and more the Small Intestine: a huge surface FOR DIGESTION AND ABSORPTION ACCESSORY ORGANS: THE PANCREAS, GALLBLADDER, AND LIVER The pancreas produces key digestive enzymes The liver makes bile and the gallbladder stores it The liver is a multipurpose organ DIGESTION AND ABSORPTION IN THE SMALL INTESTINE the Large Intestine controls over digestion DIGESTIVE SYSTEM DISORDERS ”Heartburn” is an upper GI tract disorder Hepatitis and cirrhosis strike the liver Colon problems range from constipation to cancer Malabsorption disorders prevent nutrients from being properly absorbed Infections in the digestive system CONNECTIONS: The digestive system in homeostasis the body’s nutritional requirements Complex carbohydrates are best Some fats are healthier than others Proteins are body-building nutrients There are several guidelines for healthy eating Vitamins and Minerals food energy and body weight Genes, hormones, and activity affect weight FOCUS ON HEALTH: DEALING WITH WEIGHT EXTREMES Surgery can be an option for treating extreme obesity Eating disorders can be life threatening SUMMARY Review questions self-quiz critIcal thinking explore on your own YOUR FUTURE Objectives Understand the structure and function of the component parts of the human digestive system. Describe how digestion functions in homeostasis. Summarize the daily nutritional requirements of an average person. Explain how the human body manages to meet the energy and nutritional needs of the various body parts even though the person may be feasting sometimes and fasting at other times. List several common disorders of the digestive system and explain their causes and potential treatments. Key Terms digestive system sphincters mechanical processing motility secretion digestion absorption elimination salivary glands salivary amylase bolus palate pharynx esophagus peristalsis pepsins gastric juice chyme rugae villus microvillus brush border pancreas liver bile gall bladder hepatic portal system duodenum jejunum ileum segmentation lacteals colon rectum appendix anal canal anus gastroesophageal reflux disease (GERD) hepatitis alcoholic cirrhosis diverticulitis diverticulosis irritable bowel syndrome (IBS) Crohn’s disease colorectal cancer malabsorption disorder lactose intolerance celiac disease gluten intolerance cystic fibrosis (CF) diarrhea rotavirus giardiasis dental caries gingivitis peptic ulcer gastritis stomach cancer glycemic index essential fatty acids essential amino acids vitamins minerals phytochemicals body mass index (BMI) obesity kilocalories basal metabolic rate (BMR) appetite metabolic syndrome binge eating anorexia nervosa bulimia nervosa Lecture Outline Sixty percent of Americans are considered overweight or obese. Adipose cells store fat to help us through times when food might be scarce (which is not often for many of us in today's world!). The hormones leptin and ghrelin may have effects on hunger and eating. The digestive system works with other organ systems to maintain homeostasis. The Digestive System: An Overview The digestive system (gastrointestinal tract or GI tract) is a 6–9 meter long tube where food processing takes place to bring nutrients into the body. Food is transported through the lumen of the tube from the mouth to the anus. Specialized regions include the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus. The digestive tube has four layers. The mucosa is the epithelial lining that faces the lumen of the gut. Underlying the mucosa is the submucosa, a layer of connective tissue containing blood and lymph vessels plus nerve networks. A smooth muscle layer (longitudinal and circular) lies outside the submucosa. The serosa forms the outer covering; it is a thin serous membrane. Circular arrays of smooth muscle make up the sphincters, which close off a passageway to control forward movement of the tract’s contents. Different parts of the digestive system perform five basic functions. Mechanical processing and motility include movements that break up, mix, and transport ingested food material. Secretion is the release of needed enzymes and other fluids. Digestion is the chemical breakdown of food matter to molecules small enough to cross the gut lining. Absorption is the passage of digested nutrients into the blood and lymph. Elimination is the expulsion of undigested and unabsorbed residues from the end of the GI tract. Chewing and Swallowing: Food Processing Begins The teeth tear and grind bulk food into smaller chunks. Teeth (32 in adults, 20 in children) chew the food. Each tooth has an enamel coat (crown), a dentin core, and an inner pulp. Shape of the tooth determines function: Incisors bite off chunks, cuspids tear, and molars and premolars grind. Enzymes in saliva begin the chemical digestion of food. Salivary glands secrete a fluid (saliva) into the mouth. Parotid glands lie just in front of the ear; submandibular glands are located in the lower jaw; sublinguals are under the tongue. Saliva contains salivary amylase to begin carbohydrate digestion, bicarbonate to neutralize acids, and mucins to help form the food into a bolus. The tongue presses food against the hard palate of the roof of the mouth to aid mixing, then muscle contractions force the bolus into the pharynx (throat) and then into the esophagus. Swallowing has voluntary and involuntary phases. The tongue (with the help of the palate) pushes the bolus into the pharynx where receptors initiate the swallowing reflex into the esophagus. The epiglottis closes off the trachea to prevent choking. Muscle contractions called peristalsis move the food along the tract to the stomach. The Stomach: Food Storage, Digestion, and More The stomach is a muscular sac that stores and mixes food, secretes substances to dissolve and degrade food, and controls the rate at which food enters the small intestine. Cells of the stomach lining secrete intrinsic factor, required for absorption of vitamin B12. Gastric juice includes hydrochloric acid, pepsinogens, and mucus; the hormone gastrin stimulates secretion of these materials. HCl dissolves bits of food to form a thick mixture called chyme; it also converts pepsinogen (inactive) to pepsin (active), and kills microbes. Pepsin begins the digestion of proteins. Normally, mucus and bicarbonate ions protect the stomach lining; however, a bacterium (Helicobacter pylori) can interfere with mucus production, resulting in a peptic ulcer. Peristaltic contractions churn the chyme and keep the pyloric sphincter at the stomach’s exit closed, but small amounts of chyme are released at intervals into the small intestine. Depending on the acidity and fat content of chyme, it can take the stomach 2–6 hours to empty; an empty stomach forms folds called rugae. Alcohol is one of the few substances absorbed across the stomach wall; emptying is slowed when there is food present, which slows alcohol’s effects. The Small Intestine: A Huge Surface Area for Digestion and Absorption The vast majority of nutrients are absorbed in the small intestine. The key to the small intestine’s ability to absorb is the structure of its wall. The mucosa is highly folded. Absorptive surface area is increased by fingerlike projections of the intestine lining called villi, which bear even smaller microvilli, giving each cell a “brush border.” Several types of cells make up the intestinal brush-border lining, including glandular cells that produce enzymes and defensive white blood cells for protection. Accessory Organs: The Pancreas, Gallbladder, and Liver The pancreas produces key digestive enzymes. The pancreas lies near the stomach; it contains both exocrine and endocrine glands. Four kinds of digestive enzymes are produced—one kind for each of the four major categories of food (carbohydrates, proteins, lipids, and nucleic acids). Hormones that regulate blood sugar and bicarbonate to neutralize the pH of chyme are also produced. The liver makes bile and the gallbladder stores it. The liver can make up to 1500 ml of bile per day. The gallbladder is a small sac that stores bile produced by the liver. Bile is emptied into the small intestine when food is present to aid in the digestion and absorption of fats. The liver is a multipurpose organ. The liver is one of the largest organs in the body. The bile secreted by the liver contains bile salts, which are derived from cholesterol. Bile salts help emulsify fats by breaking large fat globules into smaller bits. Excess cholesterol is also emulsified; if the level of cholesterol is much higher than the amount of bile salts present, however, gallstones can form. The liver processes nutrient-laden blood from the small intestine; blood enters the liver via the hepatic portal vein. The liver has many important functions: It converts glucose to glycogen, a storage form. It produces plasma proteins. It makes bile, which is useful in fat emulsification. It detoxifies harmful substances, especially ammonia from protein metabolism. Digestion and Absorption in the Small Intestine Roughly 9 liters of fluid enters the duodenum (the first section of the small intestine) each day. Nutrients are released by chemical and mechanical means. Secretions from the pancreas (pancreatic juice) are added to chyme in the duodenum. Trypsin and chymotrypsin digest proteins to peptide fragments; peptidases (from the intestinal mucosa) break the fragments down into amino acids. Bicarbonate from the pancreas buffers the acid from the stomach. Bile salts speed up fat digestion by emulsification: triglycerides tend to form large globules, but when smaller fat droplets become coated with bile salts, the negative charges on the droplets repel and cause them to stay separated. Repeated segmentation contractions create an oscillating movement that mixes the food. Simple sugars and amino acids are absorbed directly, but fats are absorbed in steps. Monosaccharides (glucose), amino acids, and some nucleotides cross the gut lining by active transport and enter the bloodstream. Absorption of fatty acids and monoglycerides is aided by bile salts. Free fatty acids diffuse into the gut epithelium, aided by micelle formation. Fats combine with proteins to form particles that enter lymph vessels called lacteals, which drain into the blood. The Large Intestine The large intestine begins as a blind, cup-shaped pouch (cecum) where it joins the small intestine. It continues as the colon (ascending, transverse, and descending). It ends in the sigmoid colon, which connects with the rectum. The colon concentrates and stores feces—a mixture of undigested and unabsorbed material, water, and bacteria. Sodium is actively transported out of the colon and water follows by osmosis. About 30 percent of the feces are bacteria that produce vitamins, useful fatty acids, and intestinal gases. The appendix projects from the cecum; it has no known digestive function, but is does house immune system cells. When feces distend the rectal wall, defecation occurs; feces move into the anal canal and eventually out of the body through the anus. Controls over Digestion Nerves and hormones regulate digestion based on the amount of food and its composition. Distention of the gut wall after a meal stimulates the release of signals from sensory receptors, which respond with muscle action and enzyme secretion. Four major gastrointestinal hormones regulate digestion: Gastrin, produced by the stomach lining, stimulates secretion of acids into the stomach to aid digestion of protein; somatostatin inhibits acid production. Secretin, from the intestinal lining, stimulates the pancreas to release bicarbonate. Cholecystokinin (CCK) enhances the actions of secretin and stimulates gallbladder contractions; both CCK and secretin slow the rate of gastric emptying. Blood carries the absorbed nutrients to the liver for delivery throughout the body. Digestive System Disorders ”Heartburn” is an upper GI tract disorder. Called gastroesophageal reflux disease or heartburn, acidic chyme backs up into the esophagus. Mild cases are controlled by diet adjustment or over the counter drugs to limit acid production. Hepatitis and cirrhosis strike the liver. Hepatitis is an inflammation of the liver. This may be caused by viruses or alcohol consumption. In cirrhosis, connective tissue scars replace liver cells. Colon problems range from constipation to cancer. Constipation occurs when food residues remain in the colon for too long, resulting in hard, dry feces that are difficult to eliminate. This is a common cause of hemorrhoids; enlarged rectal blood vessels. Bulk (soluble and insoluble fiber) in the diet is important for moving material through the large intestine at the proper speed. Too little bulk can result in the formation of diverticula, knoblike sacs protruding from the intestinal wall (this condition is called diverticulosis); if the knobs become inflamed this is called diverticulitis and can lead to ruptures of the diverticula. The most common bowel disorder is irritable bowel syndrome (IBS); as a result of disturbances to peristalsis, individuals alternate between diarrhea and constipation. Crohn’s disease is an inflammatory disease that can so severely damage the intestinal lining that portions of the tract must be removed. Colon cancer is the second most common cancer diagnosed in the United States; polyps forming in the colon wall can turn malignant and spread. Malabsorption disorders prevent nutrients from being properly absorbed. A malabsorption disorder is anything that interferes with the uptake of nutrients across the lining of the small intestine. Lactose intolerance results from a deficiency of lactase. Persons suffering from cystic fibrosis do not produce the pancreatic enzymes necessary for normal digestion and absorption of nutrients. Infections in the Digestive System Diarrhea is a common effect of intestinal infection. Stress or other factors speed up peristalsis. Resulting dehydration can interrupt many functions. Bacteria and other types of organisms can infect the GI tract. Common bacterial infections may involve E. coli, which inhabits intestines of cattle and Heliobacteria pylori, which cause ulcers and some forms of gastritis. Giardia is a protozoan in contaminated food or water, causing what is commonly called “traveler’s diarrhea.” Common types of food poisoning involve the Salmonella bacterium and Pseudomonas protozoan. Bacteria can cause gingivitis and lead to periodontal disease. The Digestive System in Homeostasis Many body systems are dependent on the nutrients provided by the digestive system. The digestive system contributes to homeostasis by breaking down the nutrients and providing water, vitamins, and minerals. The Body’s Nutritional Requirements Complex carbohydrates are best. Complex carbohydrates are the main source of energy taken into the body; they are also the best types of carbohydrates to intake. They are degraded to glucose, the main source of energy available to individual cells. Fruits, vegetables, and grains are also high in fiber, needed for bulk in the feces. Refined sugars represent “empty calories” because these types of foods do not have other valuable vitamins or minerals. These foods have a high glycemic index. Eating them leads to surges in blood sugar levels and changes to how fat is used for energy by the body. Too much sugar eventually increases fat storage and increases the risk of heart disease and type 2 diabetes. Some fats are healthier than others. Phospholipids and the steroid cholesterol are important components of membranes; fats are energy reserves, provide insulation and cushioning, and store vitamins. Essential fatty acids are those not made by the body itself and that must be acquired through food; whole foods and vegetable oils provide plenty of them in the diet. Animal fats and trans fats are also common in our food, but as with anything, eating too much of them can increase risk for certain diseases. Proteins are body-building nutrients. Of the 20 different amino acids in proteins, eight are essential amino acids and are not made by the body; they must be supplied in the diet. Most animal proteins are complete, possessing ratios of amino acids that match our needs; nearly all plant proteins are incomplete, meaning they lack one or more of the essentials. There are several guidelines for healthy eating. The classic “food pyramid” has been replaced with a set of nutritional guidelines published by the Food and Drug Administration. These guidelines call for eating less meat, refined grains, trans and saturated fats, and refined sugars. They also suggest that more dark green and orange vegetables, fruits, and milk products should be added to our diets. The Mediterranean diet is an example of an alternative diet that meets the goal of lowering heart disease and other ills; it emphasizes whole grains, fruits and vegetables, and reduced meat intake. Low-carb, high-protein diets can cause quick weight loss, but the effects are not long lasting and they increase the work of the kidneys. Vitamins and Minerals Humans need small amounts of at least 13 organic molecules called vitamins to assist in cellular metabolism; these vitamins must come from a balanced diet. Inorganic substances called minerals (Ca, Mg, K, Fe, for example) are also needed. A balanced diet will normally meet all requirements for these substances; excessive intake is at least wasteful and at worst harmful. Phytochemicals Food Energy and Body Weight Obesity is an excess of fat in the body’s adipose tissue; the World Health Organization has declared obesity a major global health concern. A male is obese if his body is 20% body fat. A female is obese if her body is 24% body fat. One indicator of obesity-related health risk is the body mass index (BMI); the BMI is a number that relates body weight with height. The basal metabolic rate (BMR) is the amount of energy needed to sustain the body at rest; the number of kilocalories needed to maintain “acceptable” body weight can be calculated relative to your amount of physical activity and age. Genes, hormones, and activity affect weight. The hormones leptin and ghrelin influence appetite, hunger, storage of fat in the body, and other weight-related factors; hormone production, as well as other genetic factors, differ between different individuals. To lose weight, caloric intake must be balanced with calories burned; this usually requires dietary controls paired with exercise. Focus on Health: Dealing with Weight Extremes Surgery can be an option for treating extreme obesity Eating disorders can be life threatening Emotions also affect weight Anorexia nervosa and bulimia are eating disorders related to emotional problems. Psychological counseling and some forms of medication can help Suggestions for Presenting the Material This chapter focuses on two main topics: (a) human digestive organs and function, and (b) human nutritional needs and metabolism. The traditional and most logical method of presenting the process of digestion is to follow a mouthful of food as it passes from mouth to anus. Along the way, be as detailed as the course requires. For example, it may be appropriate to ask students to know structures, general secretions, and main functions as presented in Figure 11.1 but add a few, all, or none of the enzymes in Table 11.1. Similarly, inform students as to the amount of material from Tables 11.4 (vitamins) and 11.5 (minerals) as is required. The author of the text did not necessarily intend for these tables to be memorized entirely but provided them for reference and completeness. As each digestive organ is discussed, reference to Figure 11.1 should be made. The animations of the figures in this chapter will also be useful. Although it occupies only one page of the text, the discussion of “nutrient processing” and the accompanying Figure 11.11 present a wealth of information in a very shorthand form. The material may need to be streamlined to accommodate the needs of your students. At some point in class, be sure to emphasize that digestion and absorption are inseparable in the total function of providing nutrition to body cells. The topics of respiration, circulation, and excretion, which are covered in past or future chapters, should be correlated with digestion at some point. See Figures 11.5, 11.8, 11.10 and 11.12. The list of liver functions in Figure 11.9 as they relate to homeostasis can aid you in making the connections between these systems. Students may enjoy the lecture more if some of the “problems along the way” are included that cause us minor, and occasionally major, distress. Nutrition is receiving increasingly more emphasis in our lives. Consider devoting an entire lecture to this timely topic. This is also an excellent opportunity to review the contents of Chapter 2, reinforcing student understanding of the essential chemistry of biology. This is an ideal time to discuss some of the fad weight-loss diets as well as the more legitimate ones. Students are extremely interested in this topic, and it is a beneficial one for them to discuss. It is important for students to realize that not all diets are necessarily healthy or safe. When discussing obesity, introduce the concept of body mass index (BMI) as an indicator of the degree of obesity. Consult a nutrition text for details. Stress to students that this is only one way of analyzing body size and shape. Classroom and Laboratory Enrichment Because the digestive organs lie cramped in a small body space, one upon the other, drawings such as Figure 11.1 are not as useful as life-size models. If the science or similar department has one, use it throughout the journey through the GI tract. Use molecular models to demonstrate the process of digestion. Mix oil and water together in a flask to show their immiscibility. Then add soap to the mixture to illustrate emulsification. It is interesting to point out that, unlike other systems, digestion is regulated based on conditions outside the body, rather than on inside cells or on nutritional needs. To emphasize that the lumen of the GI tract is actually outside the body, ask students what happens if they were to swallow a very long string, leaving one end hanging outside the mouth. Demonstrate peristalsis by placing a ball of suitable size inside a flexible tube (such as a section of old bicycle tire inner tube) and squeezing to move it along. The action of a digestive enzyme (salivary amylase) can be demonstrated using the procedure outlined in the “Enrichment” section for Chapter 3 of this resource manual. Show a video depicting the consequences of vitamin and mineral deficiencies in the human diet. Calculate BMI for several volunteers from the class, and then use the posted height and weight for several professional athletes—male and female if possible—to see if they are characterized as overweight or obese. Does this suggest anything about the validity of the BMI or other such methods for evaluating body size? Impacts, Issues: Classroom Discussion Ideas Millions of people around the world suffer from deficiencies in amount and nutritional composition of food. Surprisingly, dietary disorders of persons in affluent countries result from “too much of good things.” What are the consequences of the American diet? Diet plans for weight reduction are numerous and proliferating daily. How can the wary consumer recognize a plan that could be dangerous? As scientists search for hormones and genes that are influential in appetite, is it possible that overweight persons will dismiss their problem with: “My genes made me do it”? Is this an acceptable response? What is meant by “heartburn”? Is its use in television antacid advertising misleading, especially for young viewers? What would you propose as a better term? What do you think of the programs that call for regimes of fasting and “purification of wastes from the body”? Are they biologically sound? Give some of the reasons that dietary fiber, such as bran, may be important in our diet. Why do some adults, who could drink milk as infants without difficulty, experience intestinal pain (due to gas) and/or dehydrating diarrhea when they drink milk? Is there a potential problem if we send powdered milk in food care packages to other parts of the world? Some people have called for a “fat tax,” a tax that would be attached to high calorie, sugary foods. The idea is to dissuade some individuals from consuming these foods while paying for the medical treatment of those who are not dissuaded and who suffer from obesity related conditions. Is a fat tax fair? After all, we have essentially done the same with a “sin tax” for cigarettes. Another suggested “tax” is to reduce subsidies to farmer who produce “fattening foods.” What foods would be included? What foods are fattening? Is this a good idea? Is it fair? Term Paper Topics, Library Activities, and Special Projects From the library, obtain a “calorie chart.” Monitor and record your caloric intake for a week. Prepare a report. Ulcers are regarded by most people as a badge of success for bank presidents and young stockbrokers. Investigate the serious nature of this medical problem including the ages of the sufferers and what treatments are prescribed. Anorexia and bulimia seem to have appeared only recently as eating disorders. Trace the history of what is known about these conditions. The role of vitamins in human health is a fascinating story. Select one or two vitamins and report on the history of discovery surrounding each. Investigate how the drug cimetidine (Zantac®) suppresses the secretion of acid by the parietal cells of the stomach. Look into the history of the weight loss supplement ephedra, which is now banned in the U.S. Find out how it works, and what sorts of problems can develop from its use. Are there any other such supplements on the market that should be potentially banned? Find out why drinking coffee or other caffeine-containing beverages increases the sensation of hunger. Investigate the technique of gastric bypass surgery. This treatment for obesity is relatively new but has already seen its fair share of controversy, not the least of which is who should pay for it. Videos, Animations, and Websites VIDEOS Films for the Humanities and Sciences Digestive System: Your Personal Power Plant _http://ffh.films.com/id/10134/Digestive_System_Your_Personal_Power_Plant.htm_ Films for the Humanities and Sciences System Breakdown (gastrointestinal disorders) _http://ffh.films.com/id/4121/System_Breakdown.htm_ ANIMATIONS MedTropolis – The Virtual Body Interactive animations of the digestive tract. _http://medtropolis.com/virtual-body/_ PBS NOVA – Nutrients Your Body Needs Interactive animation about nutrition _http://www.pbs.org/wgbh/nova/body/nutrients-body-needs.html_ John Kitses - Digestion Interactive animation on digestion. _http://kitses.com/animation/swfs/digestion.swf_ WEBSITES National Eating Disorder Association Supports individuals and families affected by eating disorders, and serves as a catalyst for prevention, cures and access to quality care. _http://www.nationaleatingdisorders.org/_ Centers for Disease Control (CDC) - Division of Nutrition, Physical Activity, and Obesity This is the CDC’s page on digestion, nutrition, fitness, and obesity. _http://www.cdc.gov/nccdphp/dnpao/_ National Institutes of Health – Weight Control Information Network Provides up-to-date, science-based information on weight control, obesity, physical activity, and related nutritional issues. _http://win.niddk.nih.gov/_ Possible Responses to Review Questions The stomach stores food prior to passing it along to the small intestine. It is also a place where protein digestion starts, which involves the enzyme pepsin; where pathogens are killed; and where appetite and hunger have their beginnings and endings. Gastric hormones are involved in the regulation of stomach functions. The small intestine is where most nutrients are absorbed into the body; it also produces several enzymes involved in digestion. The large intestine concentrates and stores undigested material prior to elimination from the body; water, and some vitamins and electrolytes, are also absorbed through the large intestine. The appendix no longer serves a role in digestion in humans. Both a listing of the organs and accessory organs, as well as their respective functions, are neatly summarized in Figure 11.1 and its associated animation. Peristalsis, the alternating contraction of layers of smooth muscle in the GI tract, essentially occurs throughout the digestive system. Once you swallow food, peristalsis moves it down to the stomach, controls passage out of the stomach, and moves materials all the way out the anus. Sphincters help partition each element of the digestive tract to control flow. Segmentation specifically refers to the bands of smooth muscle in the small intestine that repetitively contract and relax to move material along. Only these organs from the figure should be used for this question: oral cavity, stomach, liver, pancreas, small intestine. Oral cavity: monosaccharides Stomach: amino acids Liver: fatty acids, monosaccharides, amino acids Pancreas: fatty acids, monosaccharides, amino acids Possible Responses to Critical Thinking Questions The breakdown of milk will be as follows: Component Broken down in… By what enzyme? To yield… lactose small intestine lactase glucose + galactose proteins stomach (begin); small intestine (continue) proteases; peptidases amino acids butterfat small intestine lipases fatty acids + glycerol vitamins absorbed as is in small intestine not applicable not applicable minerals absorbed as is in small intestine not applicable not applicable A slightly underweight adult could be the result of several factors, but consider the three mentioned in the textbook. This person is hopefully consuming not only a healthy diet but is matching caloric intake to caloric need. He/she is aware that consuming more calories than are needed will result in storing the excess in adipose tissue. If this adult has a sedentary job, there is a real possibility that even limiting the caloric intake will not be sufficient to keep the pounds off, so physical activity must be increased. Both the amount of food consumption and level of physical activity can be (with will power) controlled by the individual, not so with the genes one has inherited. It is a fact that some persons just seem to be able to neglect their eating and exercise habits and enjoy nice degrees of thinness due to the genes they possess. Of course the term “slightly underweight” is relative and refers to standard published height and weight tables, which may or may not accurately reflect the optimum weight for humans. For an older person’s diet what would they need? Obviously, everyone must have a source of energy, which is usually supplied by carbohydrates but would want to emphasize whole grains and reduce the refined sugars and starches. This would especially be true if the person were at risk for developing diabetes. They would want to make sure the person is receiving adequate amounts of protein and the proper balance of amino acids. If the person is a vegetarian this will be a bit trickier and require supplements, perhaps. The protein levels will not have to be as high as in a growing child (see #4 below). As with anyone, fat intake will have to be carefully regulated, particularly the type of fats. Fats could be used to supply the energy needs if carbs are reduced. Finally, supplements of vitamins and minerals would be a good idea, especially calcium. The nutritional needs for a growing 7-year-old would be similar, but still different from that outlined for the aging adult above. The energy needs would be greater, but we would still want to limit those negative carbs because of the possibility of developing childhood obesity, which is difficult to reverse later on. Encouragement of more physical activity is a necessity also. Protein intake should be plentiful because of the cell-building activities of the body. Proper balance of amino acids must be watched. This could pose a problem if the adults of the family are vegetarians. Fat intake must also be regulated. If the diet does not provide all the needs, supplements of vitamins and minerals should be considered but not “over dosed.” From a health perspective, the product is not overly bad assuming it is consumed in the prescribed quantity. Were it to serve as one of three meals in the day, it is acceptable with regard to fats and carbohydrates. The only question could be the nature of the carbohydrates (are they complex or processed?). The other question is whether the product will be consumed in the recommended amount, or with a host of other foods at a given meal. Possible Responses to Explore on Your Own Questions There are many possible answers to the question of the ultimate fate of your last meal; however, the process of tracing that meal is a very good review for the students, and they should be encouraged to complete this exercise. The mechanics of the process (chewing, swallowing, and movement through the stomach, small intestine, large intestine, and finally out of the body through the anus) is a composite of the various sections of this chapter that outline in detail the components listed in Figure 11.1. About the only part of this process that is voluntary is chewing and possibly control of the anal sphincter. As an example of how food is broken down and how it contributes to nutrition, consider Birds Eye Steam & ServeTM brand “Beans with a Twist.” Contents: green beans, wax beans, carrots, cranberries, butter, salt, roasted garlic, chives, black pepper. The beans, carrots, and cranberries are mostly cellulose and can’t be broken down (thus they will add some bulk to the diet); sugar contained within the vegetables and fruits can be broken down starting in the mouth and stomach. Butter is fat, and won’t be digested until reaching the small intestine. The seasonings (except the salt) are also plant matter. On the nutrition scale, then, there may not be a lot of “material” goods in my vegetables, but there are lots of vitamins and minerals, making this dish a good part of my dinner. Vitamins could include: thiamine, biotin, and C; minerals could include: chloride, copper, iron, magnesium, and zinc. 126 Chapter Eleven Digestion and Nutrition 119 126 Chapter Eleven Digestion and Nutrition 119 126 Chapter Eleven Digestion and Nutrition 125 126 Chapter Eleven Digestion and Nutrition 125 126 Chapter Eleven Digestion and Nutrition 125