A strong correlation between liking sweet foods and cravings for sweets has been linked to genetic factors (Keskitalo and colleagues, 2007). There are genetic differences with bitter taste perception. This difference has been known for decades, and an individual can be described as being a supertaster, taster, or nontaster (El-Sohemy and colleagues, 2007). A study at the Monell Chemical Senses Center in Philadelphia grouped children and their mothers based on their DNA, aimed at studying the effects of a newly discovered taste gene. As a group, it was found the African American children preferred higher-sugar cereals and were more likely to add sugar to their cereals than white children. However, the genetic differences did not bear out with adults, suggesting an environmental override of the need for sweet cereal (Mennella, Pepino, and Reed, 2005).

Smell is critical to recognizing the flavors in foods, well known by anyone who has had a head cold and could not breathe through his or her nose. There is also evidence that perception of flavor, based on word descriptors, can influence flavor ratings and appears to correlate with specific regions in the brain (Grabenhorst, Rolls, and Bilderbeck, 2008). Different neurons and regions of the brain respond to different sensory properties of food, including flavor, temperature, and mouth feel (Rolls, 2006).

There can also be a physiologic basis for taste preference. Rats, for example, have been found to increase their intake of potassium-based fluids when they are deficient in this mineral (Guenthner and colleagues, 2008). In a study of lambs, their intake of phosphorus-containing foods and water increased or decreased based on their body needs (Villalba and colleagues, 2006). This was later further demonstrated with lambs who were able to discriminate among different-flavored feeds, displaying preferences for calcium and phosphorus ones based on their specific needs (Villalba, Provena, and Hall, 2008).

Digestibility of food refers to the rapidity and ease of digestion and to its completeness. Liquid foods and thoroughly masticated (chewed) solid foods are more rapidly digested than are foods left in large pieces. Well-masticated food begins to leave the stomach 15 to 30 minutes after ingestion. Forms of liquid sugar such as fruit juice leave an empty stomach almost immediately.

Foods that stay in the stomach longer have a higher satiety value. Small meals move out of the stomach faster than do larger ones. Solid foods stay in the stomach longer than liquids. The amount and type of food eaten at one time affects the rapidity of digestion.

Of the three macronutrients, carbohydrates are digested and leave the stomach most rapidly (about 1 hour), proteins are digested and leave less rapidly (about 2 hours), and fats require the longest time for digestion (about 4 hours). Therefore a balanced meal stays in the stomach longer than a meal of only carbohydrate foods. Foods containing a large amount of fiber are digested more slowly than are low-fiber foods. A meal high in both fat and fiber takes the longest to digest and leave the stomach.

Absorption is the passage of soluble digested food materials through the intestinal walls into the blood, either directly or through osmosis by way of the lymphatic system. The primary site of absorption is in the small intestine. Tiny fingerlike projections called villi, which contain small capillaries (tiny blood vessels), line the intestinal wall. The villi are in constant motion and trap the tiny nutrients, which are then taken in by the adjacent cells and transported through the circulatory and lymphatic system to every body cell. Microvilli are even smaller projections on the surface of the villi (Figure 4-2).

FIGURE 4-2 Wall of the small intestine. (From Applegate EJ: *The anatomy and physiology learning system,* ed 3, Philadelphia, 2006, Saunders.)

Simple sugars, amino acids, a few fatty acids, minerals, and water-soluble vitamins reach the general circulation through the capillaries. Undigested carbohydrates that reach the large bowel are fermented to short-chain fatty acids contributing some caloric value. The exact amounts and types of carbohydrate that reach the cecum (upper portion of the large intestine) are unknown but are expected to be higher with ingestion of high-fiber foods. Fiber (nonstarch polysaccharides) clearly affects bowel habits. Water is absorbed from the large intestine. Absorbed materials are carried by the blood to the liver and from there to various organs and tissues to be used as needed. The body is able to digest and absorb about 90% to 98% of an average mixed diet.

Metabolism can be of a constructive nature, resulting in the building up of new substances. This is called anabolism. If metabolism is of a destructive or oxidative nature, resulting in the release of energy, it is called catabolism. Energy metabolism refers to the oxidation of the macronutrients (carbohydrate, protein, and fat) within the body, resulting in the release of heat and energy. Metabolic rate refers to the rate at which food energy is burned, with a high metabolic rate requiring a high amount of calories and a low metabolic rate requiring few calories to sustain life.

All metabolism happens at the cellular level, with the mitochondria acting like small furnaces to burn food energy (catabolism) and allow for anabolism. Food nutrients must enter the body cells in order for the body to metabolize or use these nutrients. It is at the intracellular (inside the cell) level that extremely complex biochemical reactions take place. In fact, through the science of molecular biology it is now recognized that the biochemical processes at the cellular level are so complex, because of multiple pathways of metabolism, that it is highly unlikely they will be totally understood in our lifetime. Good digestion and absorption of food nutrients is, however, essential for proper metabolism at the cellular level. Further, diet quality has an impact. Cellular metabolism is altered in response to the main types of fat: polyunsaturated, saturated, and monounsaturated fatty acids (Khodadadi, Griffen, and Thumser, 2008). A diet high in the fat palmitic acid has been found to lower energy expenditure and fat oxidation compared with a diet high in oleic acid (Børsheim, Kien, and Pearl, 2006). Thus the type of fat may have an effect on weight management (see Chapter 6). Genetic differences also affect the process of metabolism.

Teaching Pearl

If you “fan a fire,” it will burn better. This is somewhat like our own increased intake of air or oxygen from aerobic exercise—our metabolic rate increases because we are “fanning the fire” in the body cells. Exercise also promotes increased muscle mass, allowing for an increased number of mitochondria, or “furnaces” in which to burn kilocalories (kcalories or kcal) more efficiently.

WHAT IS BASAL METABOLISM?

The body needs energy for the internal, involuntary activities of organs and tissues and oxidation within the tissues. Energy is also needed for circulation, respiration, digestion, elimination, and maintenance of muscle tone, heartbeat, and so on. All internal activities continue 24 hours per day, while a person is asleep and awake. The amount of energy required to sustain these processes alone is known as basal metabolism.

The basal metabolic rate is influenced by body composition, body size, and age. The more muscle tissue a person has, the more calories are needed. The basal metabolic rate varies from person to person, but on the average it amounts to approximately 1200 to 1400 kcal daily for women and 1600 to 1800 kcal daily for men. Total energy requirements and weight management are discussed in Chapter 6.

A simple and relatively accurate method of estimating daily basal metabolism is to multiply weight in kilograms by 0.9 for women and 1.0 for men, then by 24 (the number of hours in a day). This estimate is generally accurate enough, except during times of physiologic stress (see Chapter 15). In large institutions, metabolic carts are used to measure a person’s oxygen intake and carbon dioxide output. This technique can precisely measure the basal metabolic kcalorie needs. Various measurements of oxygen intake and carbon dioxide output have been used over the years to determine basal metabolic rate (Figure 4-3).

FIGURE 4-3 A, Metabolic cart with a computer interface. B, A person undergoes procedure using the metabolic cart. C, A sample report with estimated kcalorie needs.

The process of digestion and absorption requires energy. Total food intake alters metabolism. Increased eating raises metabolism and vice versa. This is referred to as the specific dynamic action. This action raises the total energy needs about 10% for a person who eats a mixed diet. It is because of the specific dynamic action that some vegetables may be referred to as having “negative kcalories.” This is because the amount of energy to digest the vegetables is greater than the actual kcalorie content of the vegetables.

HOW ARE THE MACRONUTRIENTS DIGESTED AND ABSORBED?

CARBOHYDRATES

Carbohydrates (except for fiber) are easily digested, and the degree of absorption is high. Digestion of starch starts in the mouth with the enzyme amylase, also called ptyalin, and is completed in the small intestine. Glucose, which is normally formed from carbohydrates eaten in food, is absorbed into the bloodstream through the walls of the small intestine and is metabolized as shown in Figure 4-4.

The monosaccharides glucose and fructose are ready for absorption in the digestive tract because they are single sugar molecules. Double sugars, such as sucrose and lactose, must be changed to simple sugars for absorption, which is a quick process when there is adequate digestive enzyme production. Absorption occurs in the small intestine (Figure 4-5).

FIGURE 4-5 Digestive process of carbohydrate, protein, and fat. (*Cl⁻,* chloride; *CO₂,* carbon dioxide; *HCl,* hydrochloric acid; *H₂O,* water; *K⁺,* potassium; *Na⁺,* sodium; *SO₄⁼,* sulfate.) (Modified from Mahan LK: *Krause’s food, nutrition, and diet therapy,* ed 12, Philadelphia, 2008, Saunders.)

Starch now is known to be digested as quickly as sugars. It was once believed that the glucose links on either end of the polysaccharide chain were disconnected one at a time. It is now recognized that all chemical bonds connecting the chain of glucose molecules break apart simultaneously. It is for this reason that the American Diabetes Association now states that sugar can be included in a diabetes meal plan because the digestion time is no faster than that of starch. Cooking starch does facilitate digestion because it breaks down the cell walls, which makes the action of the digestive enzymes easier.

Dietary fiber is essentially indigestible and passes through the intestinal tract virtually unchanged. Bacteria naturally found in the GI tract do allow for minimal digestion of fiber, which promotes the production of the gases hydrogen and methane, medically referred to as flatus. Reduction in flatus can be achieved by thorough chewing and a slow increase in the regular intake of fiber foods.

There is debate on how to precisely define and therefore quantify carbohydrate content on food labels. Around the world there are three different food energy systems used in food tables and on food labels. One suggested descriptive approach suggests noting the difference between metabolizable energy (ME) and net metabolizable energy (NME). This would take into account the amount of carbohydrate able to be digested and absorbed, versus nondigested but fermentable (Elia and Cummings, 2007). The American Diabetes Association advocates subtracting fiber from the total carbohydrate listed on food labels if it is more than 5 g. This is due to an individual basing insulin injections on carbohydrate consumed to avoid developing hypoglycemia (see Chapter 8). The American Dietetic Association promotes subtracting half of the fiber from the total carbohydrates if it is over 5 g, which is more related to kcalorie utilization and not blood glucose outcomes per se.

Probiotics are now being promoted to help with digestion. These are live microbes found in food supplements aimed at the growth of the beneficial bacteria such as bifidobacteria and lactobacilli within the intestinal tract. Probiotics are naturally found in yogurt and buttermilk. Current studies are focused on their potential to improve immunity and promote health and well-being in a variety of conditions.

Teaching Pearl

The carbon part of carbohydrate is used as energy in animal and human cells, similar to the burning of carbon in coal and wood in actual furnaces. The black substance formed in burnt toast is carbon.

PROTEIN

The proteins in the daily diet must be broken down by digestion into the component parts: peptides and amino acids. The mechanical digestive process begins in the mouth through mastication. The chemical digestive process of proteins begins in the stomach and is completed in the small intestines (see Figure 4-5). Hydrochloric acid prepares protein for enzymatic breakdown. The mechanical and chemical processes of protein digestion in particular require the function of an intact neurologic and hormonal response. These responses allow release of digestive enzymes and other substances that allow full digestion and help propel food through the digestive tract.

Protease is the enzyme that aids protein digestion. Pepsin curdles milk in the process of milk digestion. In fact, cheese was first accidentally made when milk was stored in the stomachs of animals, before the days of glass, metal, or plastic containers. Trypsin is an enzyme released in the small intestine to help complete protein digestion. It specifically hydrolyzes peptides of arginine or lysine (amino acids; see Chapter 2). Once proteins are digested, absorption into the bloodstream from the small intestine can occur. Amino acids are less well absorbed than peptides. Peptides that are not absorbed are fermented by bacteria in the colon.

FAT

Fats, because they are insoluble in water, require special treatment in the GI tract before absorption can take place. No digestion of fats takes place in the mouth, and only finely emulsified fats (such as those found in butter, cream, and egg yolk) can start to be digested in the stomach. For the most part, fats must be emulsified by bile (produced in the liver and stored in the gallbladder for use in fat digestion) and bile salts before they are digested in the small intestine by enzymes from the pancreatic juice. Fats are changed to glycerol and fatty acids during digestion (see Figure 4-4).

Fatty foods are generally digested without difficulty, but they require a longer time for digestion than do carbohydrates. Softer fats are more completely digested and absorbed than are harder fats. Fried foods are not necessarily indigestible, but they are more slowly digested. The presence of carbohydrates in the diet is necessary for the complete oxidation of fats (the chemical step in releasing energy from fat) at the intracellular level (inside body cells); otherwise, ketone bodies accumulate and ketosis results.

HOW ARE MACRONUTRIENTS CONVERTED TO ENERGY?

When the body cells need energy, a series of complex metabolic reactions occur: the Krebs cycle (see Figure 4-4, which shows the central pathways of energy metabolism). Oxygen is necessary for the release of energy by the cells in the body. The process of combining oxygen with a molecule is called oxidation. A person needs hemoglobin to supply oxygen to the cells, and a low level of hemoglobin means oxygen is not available for energy production, which results in a tired feeling. An increased intake of air into the body, such as that achieved with aerobic exercise, tends to raise the body’s rate of metabolism through the process of oxidation (see Chapter 6).

WHAT ROLE DOES EACH PART OF THE DIGESTIVE TRACT PLAY AND WHAT ARE COMMON ASSOCIATED PROBLEMS?

THE MOUTH

The teeth provide the first mechanical function of chewing, with the cutting action of the anterior teeth (incisors) and the grinding action of the posterior teeth (molars). Chewing aids the digestion of food for a simple reason: the digestive enzymes act only on the surface of food particles, and thorough chewing increases the amount of food surface area available to these enzymes.

Another mechanical function is performed by saliva, which moistens food and prepares it for swallowing. A chemical function of the mouth is changing cooked starch to dextrin and then to maltose by the salivary enzyme amylase.

Physical disorders can begin where digestion starts—in the mouth, or oral cavity. One type of birth defect of the oral cavity is cleft palate (an opening or hole in the roof of the mouth sometimes extending to the lip, which may be referred to as have lip). Babies born with cleft palate have difficulty creating a suction seal around the mother’s nipple or a bottle nipple, which leads to inadequate ingestion of breast milk or formula. Severe cases may require surgical correction. Babies with less severe forms of cleft palate, however, may benefit from special bottle nipples that do not require suction or from a slightly larger hole in the bottle nipple. Mothers who are motivated to continue nursing until the problem is resolved should be encouraged to do so with supplemental bottle feedings as needed (see Chapter 11 for more ideas).

Missing teeth, severe dental caries, or ill-fitting dentures can adversely affect food choices. Without adequate nutritional knowledge, omitting food groups may not seem important to a person with dental problems. Alternatives should be discussed, such as eating applesauce in place of fresh apples or eating cooked or soft vegetables in place of raw or hard-to-chew vegetables. Prevention of dental caries is addressed in Chapter 12.

Swallowing problems, referred to as dysphagia, are often related to stroke, head injury, cerebral palsy, and other conditions (Figure 4-6). Inability to swallow correctly may result in aspiration of food into the lungs. Aspiration pneumonia is a frequent complication of dysphagia. Dysphagia requires a review of the swallowing process to determine the best means of feeding. A speech pathologist is trained to help assess swallowing problems. An x-ray examination called videofluoroscopy is used in conjunction with a barium swallow to objectively diagnose dysphagia. The swallowing problem is generally at the pharyngeal area. Liquids are usually the most difficult food to swallow for persons with dysphagia. Liquids that are of a nectar consistency can be used for mild swallowing problems, progressing to the thickness of honey, and as needed the thickness of pudding. Liquids can be thickened with a commercial product or with baby rice cereal. Feeding positions can also help (see Figure 15-4 and Table 15-5). Table 4-1 lists a variety of food consistency considerations. See Chapter 15 for institutional issues and guidelines for provision of altered meal consistencies.

Table 4-1

Food Consistency Considerations

TYPE OF DIET	EXAMPLE	POTENTIAL EFFECT ON ORAL FUNCTION
Thin foods and liquids	Soup broth, juice	More difficult to control within mouth, especially with limited tongue control (i.e., quickly runs to all areas of mouth); often promotes excessive food loss
Thick foods	Pudding, yogurt, applesauce	Improved control within oral cavity because of reduced flow and increased sensory input (i.e., weight and texture)
Pastelike or sticky foods	Peanut butter, thick cheese sauce	May be more difficult to move in oral cavity with limited tongue movement; may stick to the roof of the mouth, especially with a high, narrow palate
Slippery foods	Pasta, Jell-O	Often difficult to control and either triggers reflexive swallow too quickly or runs out of oral cavity before the swallow
Smooth textures	Pudding, pureed foods	Relatively easy to swallow; promotes minimal tongue and jaw movement, especially over time
Coarse textures	Creamed corn, ground foods, Sloppy Joe filling	Increases sensory input to stimulate more jaw and tongue movement; coarseness of food should be carefully graded
Varied textures	Soups with noodles or chunks of vegetables	Difficult to manage in oral cavity, especially with limited tongue movement or decreased oral sensitivity (i.e., liquid is swallowed and solid pieces remain in the mouth)
Scattering textures	Grated carrots, rice, coleslaw, corn bread	Very difficult to manage with limited tongue movement and decreased oral sensitivity
Crisp solids	Carrot sticks, celery sticks	Requires sophisticated biting and chewing to grind pieces into consistency that is safe to swallow
Milk-based substances	Milk, ice cream	Appears to coat mucous membranes in oropharyngeal cavity to interfere with swallowing
Broth	Meat broth, chicken broth	Appears to cut mucus in oropharyngeal cavity and facilitates swallowing
Dry foods	Bread, cake, cookie	May be difficult to chew or swallow with insufficient saliva
Whole, soft foods	Slice of bread	Requires the ability to bite off appropriately sized pieces

Courtesy of the Occupational Therapy Department of the J.N. Adam Developmental Center, Perrysburg, NY.

In general, swallowing can be divided into the following three stages: (1) the voluntary stage, which initiates the swallowing process, or mechanical function; (2) the pharyngeal stage, which is involuntary and involves the passage of food through the pharynx to the esophagus; and (3) the esophageal stage, which involves passage of food through the esophagus to the stomach through peristaltic wave contractions.

Fact & Fallacy

FALLACY

Washing food down with water is a good habit.

FACT

Food must be masticated (chewed) thoroughly so that it can be mixed with saliva, which aids digestion. However, a glass of water at mealtime is beneficial to the digestive process as long as it does not take the place of mastication.

THE ESOPHAGUS

The esophagus transfers food from the oral cavity to the stomach. This process is complicated and can go awry with neurologic or neuromuscular disorders. Respiration is generally only minimally stopped during the act of swallowing. Poorly chewed food, however, increases the risk of obstruction of the airway, especially for persons with an impaired swallowing reflex, in whom oxygen deprivation can occur because breathing and swallowing cannot be done simultaneously.

In achalasia the lower part of the esophagus fails to relax, and swallowing difficulty occurs. The individual senses fullness in the sternal region and may vomit; then there is danger that the contents of the esophagus may be aspirated into the respiratory passages. Weight loss may become a problem that requires nutritional intervention. Dilation of the esophagus or surgical intervention can improve the condition. Including semisolid foods can help a person manage this condition. One case of achalasia was attributed to Wernicke’s encephalopathy (Kennedy and colleagues, 2007).

THE STOMACH

The presence of food in the stomach stimulates functioning of the digestive tract. Food is kept in motion by the muscular walls of the stomach, which bring it into contact with the gastric juice secreted by stomach cells. The fundus of the stomach acts as a temporary storage place for food.

Various gastric juice enzymes work in the stomach to digest the different macronutrients. Complex proteins are partially digested by pepsin (protease); milk protein is coagulated by renin, then is partially digested by pepsin. Emulsified fats are digested to fatty acids and glycerol by lipase. Hydrochloric acid aids these digestive enzymes and increases the solubility of calcium and iron. Mucus protects the lining of the stomach from the hydrochloric acid. Once solid food is reduced to a semiliquid state (chyme), it is passed from the stomach to the small intestine.

Functional disorders of the stomach (reflex disorders) involve a change in body functions without detectable changes in structural tissue. One example is dyspepsia (indigestion). Alterations in the hydrochloric acid content of gastric juice is another functional disorder.

A hiatal hernia is a protrusion of a part of the stomach through the esophageal hiatus (opening) of the diaphragm (Figure 4-7). Persons with this disorder sometimes complain of heartburn because of the reflux of gastric contents into the esophagus. Medical treatment includes ingestion of antacids to neutralize or inhibit gastric secretions and possibly surgery. Small, frequent meals are recommended to reduce symptoms, although dietary modifications cannot eliminate the cause. No food is advised for approximately 3 hours before bedtime, and the person should remain in the upright position after eating. For the obese person, weight loss is indicated to help relieve pressure on the diaphragm. Any source of pressure on the abdomen, such as bandages or clothes that fit too tightly, should be eliminated.

FIGURE 4-7 Sketch of hiatal hernia. (From Damjanov I: *Pathology for the health professions,* ed 3, St. Louis, 2006, Saunders.)

Gastritis (acute or chronic) is an inflammation of the lining of the stomach that results in abdominal pain, nausea, and vomiting. It may be caused by food poisoning, overeating, excessive intake of alcohol, or bacterial and viral infections. A chronic condition may be related to other disease states. It often precedes the development of ulcers or cancer. Acute gastritis, which usually heals within a few days, is often treated first with antibiotics and neutralization of the stomach contents. The stomach is allowed to rest for a while, then the patient drinks clear fluids for the first day or two.

A peptic ulcer is an eroded lesion in the lining (mucosa) of the stomach (gastric ulcer) or duodenum (duodenal ulcer). Excess use of nonsteroidal antiinflammatory drugs (NSAIDs) can contribute to the erosion of the mucosal lining. In recent years Helicobacter pylori infection has been increasingly recognized as being involved in the development of peptic ulcers. Symptoms include burning or gnawing pain in the pit of the stomach. While an ulcer is bleeding, no food is allowed; instead the patient may be given intravenous feedings of dextrose and amino acids. As the condition improves, the patient usually progresses from a full liquid diet to a regular diet with the omission of irritants based on individual tolerances. Common intolerances include caffeine, alcohol, and spicy foods, but some individuals have no adversity with these substances (Table 4-2).

Table 4-2

Possible Dietary Treatment of Peptic Ulcers *

GUIDELINE	RATIONALE
Eat three regular meals or six small meals	Inhibits stomach distention
Avoid caffeine-containing beverages, decaffeinated coffee	Decreases gastric secretions
Avoid alcohol	Reduces damage to stomach lining
Avoid black pepper, chili powder, cloves, nutmeg, curry powder, mustard seed	Reduces irritation to stomach lining
Avoid aspirin	Reduces irritation of stomach lining
Avoid cigarette smoking	Promotes healing of ulcer
Eat in a relaxed atmosphere	Reduces stress

^*Individualization is advised.

The dumping syndrome is a condition related to stomach surgery. Surgery for ulcers is not uncommon. Obesity surgery may also result in the dumping syndrome (see Chapter 6). In the dumping syndrome the food “dumps” into the small intestine rapidly, causing severe cramping and pain. It has been found that the dumping syndrome may occur even years after gastrectomy (removal of the stomach or portions of it). Hypoglycemia can be a consequence. Individuals presenting with hypoglycemia and a history of stomach surgery may have the dumping syndrome. Seizures resulting from hypoglycemia may occur (Harder and colleagues, 2005). Treatment is aimed at drinking beverages separate from solid foods and consuming low-glycemic-load meals.

The condition of gastroesophageal reflux disease (GERD) is the opposite of achalasia, although it may be the cause of the strictures found with achalasia. Esophageal cancer is also related to uncontrolled GERD. In this condition the lower esophageal sphincter is incompetent and allows stomach matter to regurgitate into the esophagus. GERD is considered common in the elderly, but also does occur with young children. It may present various symptoms, such as heartburn, regurgitation, and obstructive sleep apnea (Friedman and colleagues, 2007).

Central obesity along with medications and fluctuation of hormonal level may play a causal role. Lowered esophageal sphincter competency has been associated with use of tobacco and intake of alcohol, chocolate, and high-fat meals. Some individuals have appeared to benefit with avoidance of alcohol, fatty foods, chocolate, coffee, mint, and carbonated beverages (Csendes and Burdiles, 2007). However, successful treatment of GERD, as evidenced by reduced acidity level of the esophagus, has only been shown in the research literature with weight loss, remaining upright after meals, and head elevation during sleep (Kaltenbach, Crockett, and Gerson, 2006

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