DIGESTION, ABSORPTION AND TRANSPORT
The digestive system, responsible for processing foods, is itself dependent on our nutrient intake for its maintenance.
Digestion
The main organs of the digestion system form the gastrointestinal (GI) tract, or alimentary canal, which creates an open tube that runs from the mouth to the anus. Everything we eat is processed through the GI tract
The digestive system prepares ingested nutrients for digestion and absorption and protects against consumed microorganisms and toxic substances.
To achieve these functions, a series of processes occur. These processes of ingestion, digestion, absorption, and elimination depend on the motility or movement of the GI wall and the secretions of digestive juices and enzymes.
Figure 3-1 Digestive system. (From
digestive system
a series of organs that functions to prepare ingested nutrients for digestion and absorption
digestion
the process through which foods are broken down into smaller and smaller units to prepare nutrients for absorption
saliva
the secretions of the salivary glands of the mouth
exocrine glands
glands that secrete chemicals into ducts that release into a cavity or to the surface of the body, such as salivary glands (mouth) and the liver (gallbladder)
chemical digestion the chemical altering effects of digestive secretions, gastric juices, and enzymes on food substance composition
mechanical digestion the crushing and twisting effects of teeth and peristalsis that divide foods into smaller pieces
bolus
a masticated lump or ball of food ready to be swallowed
peristalsis
the rhythmic contractions of muscles causing wavelike motions that move food down the Gl tract
mucosa
the inside Gl muscle tissue layer composed of mucous membrane
submucosa
a layer of connective muscle tissue under the mucosa
muscularis
a thick layer of muscle tissue surrounding the submucosa
serosa
the outermost layer of the Gl wall; made of serous membrane
segmentation
the forward and backward muscular action that assists in controlling food mass movement through the Gl tract
PHYSIOLOGY OF THE DIGESTIVE SYSTEM
The unique physiological processes that take place in the digestive system are digestion, absorption, secretion, motility and excretion.
Digestion is the process whereby large food molecules are broken down to smaller ones. Food is ingested as large pieces of matter containing substances such as protein and starch which are unable to cross the cell membranes of the gut epithelium. Before these complex molecules can be utilised they are degraded to smaller molecules, such as glucose and amino acids, which can be absorbed from the gastrointestinal system into the bloodstream.
The mixture of ingested material and secretions in the gastrointestinal tract contains water, minerals and vitamins as well as fats, carbohydrates and proteins.
The products of digestion, other small dissolved molecules, ions and water are transported across the epithelial cell membranes, mainly in the small intestine. This is the process of absorption. The transported molecules enter the blood or lymph for circulation to the tissues. This process is central to the digestive system, and the other physiological processes of the gastrointestinal tract, such as elimination, subserve it.
Food which is ingested travels along the gastrointestinal tract to the appropriate sites for mixing, digestion and absorption to occur. Most of the gastrointestinal tract is lined by two layers of smooth muscle; contraction of this muscle mixes the contents of the lumen and moves them through the tract. Motility in the digestive system is under neural and hormonal control.
Exocrine glands secrete enzymes, ions, water, mucins and other substances into the digestive tract. The glands are situated within the gastrointestinal tract, in the walls of the stomach, small intestine and large intestine, or outside it in the case of salivary glands, pancreas and liver. Secretion in all regions of the gastrointestinal tract is controlled by nerves and hormones.
Peptides of the gastrointestinal tract
Some of the functions of the gastrointestinal tract are regulated by peptides, derivatives of amino acids and a variety of mediators released from nerves.
These functions include contraction and relaxation of the smooth muscle wall and the sphincters (physiological ‘gatekeepers’ of the gastrointestinal tract); secretion of enzymes for digestion; secretion of fluids and electrolytes; and growth of the tissues of the gastrointestinal tract.
All gastrointestinal hormones are peptides, i.e. small molecules comprising up to 50 amino acids. It is important, however, to realise that not all peptides found in the digestive tract are hormones. The gastrointestinal peptides can be divided into hormones, paracrines and neurocrines, depending on the method by which the peptide is delivered to its target site.
Hormones are peptides released from endocrine cells of the gastrointestinal tract. They are secreted into the portal circulation, pass through the liver and enter the systemic circulation. The systemic circulation then delivers the hormone to target cells with receptors for that specific hormone. These target cells may be in the gastrointestinal tract itself (e.g. gastrin acts upon the parietal cells of the stomach to stimulate acid secretion), or the target cells may be located in another region of the body (e.g. gastric inhibitory peptide acts upon the cells of the pancreas to cause insulin secretion). Several criteria must be met for a substance to qualify as a gastrointestinal hormone:
• The substance must be secreted in response to a physiological stimulus and carried in the bloodstream to a distant site, where it produces a physiological action.
• The hormone’s action must be independent of any neural activity.
• The hormone must have been isolated, purified and chemically identified.
Four gastrointestinal peptides are classified as hormones: gastrin, cholecystokinin (CCK), secretin and gastric inhibitory peptide (GIP).
Most paracrines, with the exception of histamine, are peptides secreted by endocrine cells of the gastrointestinal tract. In contrast to hormones, however, the paracrines act locally within the same tissue that secretes them. Paracrine substances reach their target cells by diffusing short distances through interstitial tissue, or they are carried short distances in capillaries. The only gastrointestinal peptide with a known paracrine function is somatostatin, which has an inhibitory effect throughout the gastrointestinal tract in that it reduces motility and secretion of digestive juices.
Neurocrines are synthesised ieurones of the gastrointestinal tract and are released in response to an action potential. Following release, the neurocrine diffuses across the synapse and acts upon its target cell. There are many neurocrines in the gastrointestinal tract including acetylcholine, noradrenalin and vasoactive intestinal peptide (VIP).
Innervation of the gastrointestinal tract
The gastrointestinal tract is also regulated by the autonomic nervous system, which has an intrinsic component and an extrinsic component. The extrinsic component is the sympathetic and parasympathetic innervation of the gastrointestinal tract. The intrinsic component is called the enteric nervous system.
The enteric system is wholly contained within the wall of the gastrointestinal tract in the submucosal and myenteric plexuses.
Parasympathetic innervation
Parasympathetic nervous innervation is supplied by both the vagus nerve and the pelvic nerve. The pattern of parasympathetic innervation is consistent with its function. The vagus nerve innervates the upper portions of the gastrointestinal tract (upper third of oesophagus, wall of stomach, small intestine and ascending colon), whilst the pelvic nerve innervates the lower portions of the system (striated muscle of external anal canal and walls of the transverse, descending and sigmoid colons).
Postganglionic neurons of the parasympathetic nervous system are classified as either cholinergic or peptidergic. Cholinergic neurons release acetylcholine as the main neurotransmitter and peptidergic neurons release one of several peptides, including VIP.
Sympathetic innervation
Preganglionic cholinergic fibres of the sympathetic nervous system synapse in ganglia outside the gastrointestinal tract. Four sympathetic ganglia serve the gastrointestinal tract: coeliac, superior mesenteric, inferior mesenteric and hypogastric. Postganglionic fibres, which are adrenergic, leave the sympathetic ganglia and synapse on ganglia in the myenteric and submucosal plexus, or they directly innervate smooth muscle, endocrine or secretory cells.
Intrinsic innervation
The intrinsic or enteric nervous system can direct all functions of the gastrointestinal tract, even in the absence of extrinsic innervation. The enteric nervous system is located in the myenteric and submucosal plexus and controls the secretory, contractile and endocrine functions of the gastrointestinal tract.
Mouth
Are you hungry?
Thinking about your favorite food?
Is your mouth watering?
Our mouths really do “water” when we think about or begin to eat foods. However, it is not actually water we sense but a thin mucouslike fluid, called saliva. Saliva is the term for the secretions of the three salivary glands of the mouth.
As exocrine glands, each set of salivary glands produces a different type of secretion that is released into the mouth. The parotid glands create watery saliva that supplies enzymes; the submandibular glands produce mucus and enzyme components; and the sublingual glands, the smallest, create a mucous type of saliva.
A reflex mechanism controls these secretions.
Food in the mouth stimulates chemical and mechanical digestion. Chemical digestion occurs through the action of saliva that not only moistens the foods we chew but also contains amylase, an enzyme that begins the digestive process of starches.
Another digestive process that occurs in the mouth, mechanical digestion, depends on teeth. Teeth rhythmically tear and pulverize food. The enamel that covers teeth is the hardest substance in the body and therefore protects teeth from the harsh effects of chewing. The tongue assists with mechanical digestion by guiding food into chewing positions and then leading the pulverized food into the esophagus.
Another function of the tongue is that of taste. More than 2000 taste buds are responsible for our sensations of sweet, bitter, sour, and salty when tasting foods (Figure 3-2).
As toddlers, we have the highest number of taste buds and a higher degree of taste sensitivity, so bland foods are more appealing. The number of taste buds declines as we grow older, which explains why older adults have diminished taste sensitivity. Older adults may need to be encouraged to avoid the use of too much salt, particularly if they have hypertension or cardiac disorders.
Our sense of smell works along with our taste bud sensations. These two combined senses actually account for the perception (and enjoyment) of the flavors of different foods. Our positive or negative response to specific foods based on our sensory perception affects our food choices.
Portions of the pulverized or masticated food get formed into the shape of a ball called a bolus. The tongue effortlessly forms the bolus, which is then swallowed and passed by the epiglottis into the esophagus within about 5 to 7 seconds. The epiglottis is a flap of tissue that closes over the trachea to prevent the bolus from entering the lungs.
The Esophagus
The esophagus is a muscular tube through which the bolus travels from the mouth to the stomach.
The process begins at the top of the esophagus when peristalsis, the involuntary movements of circular and longitudinal muscles, begins and draws the bolus further into the Gl tract. This mechanical action further breaks down the size of foodstuff and increases exposure to digestive secretions.
Muscular actions depend on the four layers of tissues that form the tube of the Gl tract (Figure 3-3).
The mucosa is composed of mucous membrane and forms the inside layer.
Under the mucosa is the submucosa, which is a layer of connective tissue. Digestion depends on the blood vessels and nerves of the submucosa to regulate digestion.
Surrounding the submucosa is a thick layer of muscle tissue called the muscularis.
The outermost layer of the Gl wall is made of serous membrane called serosa, which is actually the visceral layer of the peritoneum that lines the abdominal pelvic cavity and covers organs.
As a person swallows, food moves from the mouth to the throat, also called the pharynx (1). The upper esophageal sphincter opens (2) so that food can enter the esophagus, where waves of muscular contractions, called peristalsis, propel the food downward (3). The food then passes through the lower esophageal sphincter (4) and moves into the stomach (5).
The coordination of these layers provides the varied movements required for digestion. Essentially, muscular action controls the movement of the food mass through the Gl tract. Churning action within a segment of the Gl tract allows secretions to mix with food mass. Circular muscles surround the Gl tube. Rhythmic contractions of these muscles cause wavelike motions of peristalsis that move food downward. Longitudinal muscles run parallel along the Gl tube. The combined effect of the circular and longitudinal muscles causes segmentation as a forward and backward movement. Sphincter muscles are stronger circular muscles that act as valves to control the movement of the food mass in a forward direction. In effect, sphincter muscles prevent reflux by forming an opening when relaxed and closing completely when contracted.
At the bottom of the esophagus, the cardiac sphincter controls the movement of the bolus from the esophagus into the stomach. It also prevents the acidic contents of the stomach from moving upward back through the esophagus.
The Stomach
The bolus enters the fundus, the upper portion of the stomach that connects with the esophagus. The other divisions of the stomach include the body, or center portion, and the pylorus, the lower portion.
The stomach wall contains gastric mucosa that contains gastric pits.
At the base of the pits are the gastric glands whose chief cells create gastric juice, a mucous fluid that contains digestive enzymes, and parietal cells, which secrete stomach acid called hydrochloric acid.
Gastric secretions occur in three phases: cephalic, gastric, and intestinal.
The cephalic phase is called the “psychic phase” because mental factors can stimulate gastrin, a hormone.
This phase occurs before food enters the stomach and involves preparation of the body for eating and digestion. Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) and G cell (secretes gastrin) activity via D cell secretion of somatostatin.
In the gastric phase, gastrin increases the release of gastric juices when the stomach is distended by food.
This phase takes 3 to 4 hours. It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH. Distention activates long and myenteric reflexes. This activates the release of acetylcholine, which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach. Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete gastric acid. Gastric acid is about 0.5% hydrochloric acid (HCl), which lowers the pH to the desired pH of 1-3. Acid release is also triggered by acetylcholine and histamine
The third phase is the intestinal phase in which the gastric secretions change as chyme passes through to the duodenum.
This phase has 2 parts, the excitatory and the inhibitory. Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.
Gastric secretions are inhibited by exocrine and nervous reflexes of gastric inhibitory peptides, secretin, and cholecystokinin (CCK) (also called pancreozymin), a hormone secreted by intestinal mucosa.
Some gastric juices provide acidity in the stomach to assist the effective function of certain enzymes. As agents of chemical digestion, enzymes are specific in action, working only on individual classes of nutrients and changing substances from one form to a simpler form. Enzymes are “organic catalysts” formed from protein structures; they function at specific pHs and are continually created and destroyed. Specific enzymes are required for energy release and digestion.
Hormones, which regulate the release of gastric juices and enzymes, act as messengers between organs to cause the release of needed secretions. In digestion, hormones affect the secretions from the stomach, intestines, and gallbladder. These secretions may slow or speed digestion and affect the pH levels of gastric juice. Overall, the mechanical and chemical actions work together to complete the process of digestion.
Gastric motility, or movement of food mass through the stomach, requires 2 to 6 hours. The churning and mixing of the food mass with gastric juices creates a semiliquid mixture called chyme. When chyme enters the pylorus section of the stomach, it causes distention and the release of the hormone gastrin. Gastrin sends a message that hydrochloric acid (HCl) is needed to continue the breakdown of chyme. As HCl is released from the stomach lining, thick mucus is also secreted to protect the stomach walls from the harsh HCl.
Every 20 seconds, chyme is released into the duodenum, the upper portion of the small intestine; this action is controlled by the hormonal and nervous system mechanism of enterogastric reflex. This consists of duodenal receptors in the mucosa that are sensitive to the presence of acid and distention. The impulses over sensory and motor fiber in the vagus nerve cause a reflex restriction of gastric peristalsis. For example, the gastric inhibitory peptide released in response to fats in the duodenum decreases peristalsis of stomach muscles and slows chyme passage. This results in decreased motility, and the stomach empties more slowly when a person eats a high-fat diet.
The combined action of mechanical digestion (the strong muscular movements of peristalsis) and chemical digestion (the effects of the gastric juices) works to prepare nutrients for the process. Chyme is kept in the stomach by the actions of the pyloric sphincter, which slowly releases it into the duodenum.
The Small Intestine
The small intestine is a convoluted tube, with two concentric layers of smooth muscle, that extends from the pyloric sphincter to its junction with the large intestine at the ileo-caecal valve. It is approximately
The small intestines consist of three sections:
• duodenum
• jejunum
• ileum
The first
After the duodenum, the proximal two-fifths of the small bowel (
The surface of the duodenum is folded. These folds are known as plicae circulares (circular folds). However, there is a gradual decease in the diameter, thickness of the wall and number of folds, with distance from the duodenum.
The folds are virtually absent in the terminal portion of the ileum. The mucosa of the small intestine is covered with tiny projections, known as villi. These villi become less numerous, smaller and more finger-like with distance from the duodenum. Numerous lymphatic nodes, called Peyer’s patches, are present in the mucosa and submucosa in the ileum. These are circular, aggregated lymph nodes that participate in the body’s immune response and synthesise antibodies.
The terminal junction of the ileum joins the large intestine at the ileo-caecal valve. It is approximately
The wall of the small intestine is composed of the following layers:
• Serosa: an outer layer composed of peritoneum and connective tissue.
• Muscularis: containing outer longitudinal and inner circular layers of muscle, separated by the myenteric plexus nerve network.
• Submucosa: connective tissue, which contains blood vessels, lymphatic tissue and a submucosal nerve plexus.
• Mucosa: inner mucosa layer.
The surface of the small intestine forms a series of circular folds which increase the surface area available for absorption of nutrients. The surface has a velvety appearance owing to the presence of fine hair-like projections called villi, each containing a lymph vessel (lacteal) and blood vessels. Each villus is lined with simple columnar epithelial cells. Below the epithelium the lamina propria separates the mucosa from the submucosa. A brush border consisting of multiple microvilli covers the surface of each columnar cell. The mucosa is supplied with simple, tubular-type glands which secrete intestinal juice.
The villus is regarded as a unit of absorption. Its length can vary between 0.5 and
The mucosa of the small intestine is simple columnar epithelium. Four major cell types are present:
• Absorptive cells, which produce digestive enzymes and absorb nutrients from digested food.
• Goblet cells, which produce protective mucus that lubricates the surface and protects it from mechanical damage.
• Granular cells, which protect the intestinal epithelium from bacteria.
• Endocrine cells, which produce peptide hormones that regulate secretion and motility in the gastrointestinal tract, liver and pancreas.
Two types of gland are present in the duodenum.
At the base of the villi are tubular invaginations that reach almost to the muscularis layer; these are known as the intestinal crypts or crypts of Lieberkuhn. The submucosa of the duodenum contains coiled compound tubular mucous glands, known as the glands of Brunner, which secrete an alkaline fluid rich in mucus. These are more numerous in the proximal region of the duodenum.
Blood supply to the small intestine
The duodenum receives arterial blood from the hepatic artery, whereas the rest of the small intestine blood is derived from the superior mesenteric artery.
Numerous arterial branches form an extensive network in the submucosal layer, which supplies the wall of the small intestine. Venous blood from the entire small bowel drains through the superior mesenteric vein.
Nerves, hormones and local paracrine factors control the intestinal circulation.
Stimulation of sympathetic nerves causes vasoconstriction and reduced blood flow, enabling a redistribution of blood away from the small intestine.
In the blood vessels of the villi, this vasoconstriction effect is short-lived owing to the presence of vasodilator metabolites, such as adenosine, which accumulate during the vasoconstrictor response. Blood flow to the small intestine increases by 50–300% during ingestion of food and this is termed functional hyperaemia. Distension of the small intestine walls and substances present in chyme also stimulate blood flow.
Absorption in the small intestine
The primary function of the small intestine is the absorption of nutrients from chyme. It receives up to eight litres of chyme per day and passes only 500–1000 ml to the large intestine. The remaining fluid is absorbed by the columnar cells of the villous epithelium. Most substances are absorbed in the proximal small intestine. The duodenum is the primary site of iron and calcium absorption, and the jejunum is the site where absorption of fats, carbohydrates and proteins takes place. Finally, only a few substances such as vitamin B12 and bile salts are actively absorbed in the ileum. There are a number of barriers to transport from the intestinal lumen to the blood: the luminal plasma membrane, the cell’s interior, the intercellular space, the basement membrane of the capillary and the cell membranes of the endothelial cell of the capillary or lymphatic vessels.
There are five basic mechanisms for absorption to take place effectively in the small intestine: hydrolysis, non-ionic movement, passive diffusion, facilitated diffusion and active transport.
Absorption of water in the small intestine
Water transport in the gastrointestinal tract is largely a function of the small intestine. It is an example of passive diffusion across the wall of the small intestine. The stomach is almost impermeable to water but the small intestine is highly permeable; therefore the transport of water in the small intestine can occur both from the lumen to the blood, or from the blood to the lumen. Net transport of water is achieved by the osmotic gradient and it will occur in whatever direction the osmotic forces dictate. Water will be secreted into the lumen if the chyme is hypertonic to the plasma, and absorbed into the blood if it is hypotonic. The chyme entering the duodenum from the pylorus of the stomach is normally hypertonic iature.
Absorption of electrolytes in the small intestine
Sodium is absorbed along the length of the small bowel but mainly in the jejunum via active transport. Magnesium, phosphate and potassium are absorbed throughout the small intestine.
Absorption of most water-soluble vitamins takes place by diffusion. The exception is vitamin B12, which combines with intrinsic factor (produced by the parietal cells in the stomach) for active transport and is mainly absorbed in the terminal ileum.
Carbohydrates are broken down by digestive enzymes in the intestine into simple sugars (glucose, galactose and fructose), which are then absorbed into the bloodstream via the intestinal mucosa, using either active transport or facilitated diffusion. Proteins are hydrolysed by proteolytic digestive enzymes into amino acids, which are absorbed by active transport.
Fats are emulsified and then broken down into glycerol, fatty acids and glycerides, primarily by the enzyme pancreatic lipase.
Control of absorption in the small intestine
Various factors are involved in the control of water and electrolyte absorption by the cells near the tip of the villi. These include endocrine, paracrine and nervous influences. Glucocorticoids stimulate electrolyte and water absorption in both the small and large intestines. Somatostatin stimulates electrolyte and water absorption in the ileum. Absorption can be inhibited by inflammatory mediators such as histamine and prostaglandins, which are released from cells of the gastrointestinal immune system.
Intestinal secretion
In addition to the absorptive functions of the small intestine, the cells of the small intestine secrete digestive juices, mucus and a variety of hormones. This alkaline intestinal juice contains electrolytes, mucus and water and is secreted throughout the length of the small intestine. The small intestine also receives a variety of secretions from the pancreas and the liver.
The microvilli on the brush border contain the peptidases and disaccharides that are required for digestion of proteins and carbohydrates. Brunner’s glands, which are located in the proximal duodenum, secrete a clear, alkaline (pH 8.2– 9.0), viscous fluid that acts as a protective layer from gastric acid secretions in the duodenal mucosa. Goblet cells located on and between the villi on the mucosal lining secrete a protective mucus.
Between two and three litres of watery fluid are produced by the crypts of Lieberkuhn each day. This fluid contains a carrier substance for the absorption of nutrients when chyme comes in contact with the villi. There are a variety of endocrine cells located in these crypts, which produce a number of peptides and hormones, including cholecystokinin, secretin, gastrin inhibitory peptide, somatastatin, vasoactive intestinal peptide and serotonin.
Control of intestinal secretions
Secretion in the small intestine can be controlled by hormones, paracrine factors and nervous activity. Hormones and paracrine factors such as gastrin, serotonin and prostaglandins stimulate the epithelial cells directly. The cells are innervated by secretomotor neurones, mainly the ganglia in the submucosal plexus, but also via ganglia in the myenteric plexus. The submucosal neurones release acetylcholine, vasoactive inhibitory peptide and serotonin, to stimulate secretion.
Parasympathetic nerves innervate nerves in the enteric nerve plexi. They enhance secretion by releasing acetylcholine onto the neurones in the plexi.
Parasympathetic tone contributes to the basal secretion in the small intestine.
Reflexes triggered by distension of the small intestinal lumen, and the presence of various substances (i.e. glucose, bile salts, acid, alcohol) in the intestinal chyme, stimulate secretion.
There are two ways that noradrenalin has an inhibitory effect upon intestinal secretion. First, it acts directly upon the epithelial cells, and second it acts upon the neurones in the submucosal ganglia to inhibit secretory nerves that stimulate the epithelial cells.
Motility in the small intestine
The smooth muscle lining the small intestine performs two functions. First, it is responsible for a thorough mixing of digestive juices arriving from the pancreas and liver via the common bile duct with the chyme received from the stomach. Second, it is responsible for moving the contents, usually slowly but occasionally rapidly, along the
Within the small bowel, three types of movement contribute to the mixing of chyme:
• Concentric, segmenting contractions. Segmentation helps to mix the secretions of the small intestine with the chyme particles.
• Peristaltic waves or short, propulsive contractions. These slowly push the chyme in the direction of the ileo-caecal valve. Peristaltic waves are strongest in the proximal portion of the small bowel.
• The continuous shortening and lengthening of the villi constantly stirs the intestinal contents.
As chyme approaches the large bowel, contractions in the ileum increase.
Control of motility in the small intestine
Motility in the small intestine is under physiological control via several factors, including stretch, extrinsic autonomic nerves, intrinsic nerves of the intramural plexi paracrine factors and circulating hormones.
Neural control of motility in the small intestine
Activation of the intrinsic nerves in the intramural plexi can control segmentation and short peristaltic waves by influencing the basal electrical rhythm, in the absence of hormones or extrinsic nerves. Segmentation and peristalsis are increased by activation of parasympathetic nerves, and inhibited by stimulation of sympathetic nerves. Activation of the sympathetic nervous system, in response to stress for example, results in the release of adrenalin into the circulation, which inhibits intestinal motility. Sympathetic activation also results in vasoconstriction of the blood vessels in the small intestine.
There are many other transmitters, in addition to acetylcholine and catecholamines (adrenalin and noradrenalin), which can influence motility in the small bowel. These include peptides, amines and nucleotides. The peptides include vasoactive intestinal peptide and somatostatin.
Hormonal control of motility in the small intestine
Gastrin, which is released into the blood in response to the presence of peptides in the stomach, and secretin and cholecystokinin, released into the blood in response to the presence of fats and acids in intestinal chyme, all increase intestinal motility. Motilin, a peptide released from the walls of the small intestine into the blood when the intestinal chyme becomes alkaline, increases intestinal motility. Another peptide which is released in the presence of chime in the small intestine is enteroglucagon. It is released in response to particles of glucose and fat in the chyme. This hormone inhibits peristalsis, and its role allows additional time for absorption of glucose and fat before the chime reaches the ileo-caecal valve.
Reflex control of motility in the small intestine
Activation of pressure receptors by the distension of the intestinal walls is involved in the reflex control of intestinal motility. A bolus of food placed in the small intestine will cause smooth muscle behind it to contract, and in front of it to relax. When food is present in the stomach, motility increases in the ileum and the ileo-caecal sphincter relaxes. This is known as the gastro-ileal reflex. This reflex appears to be mainly under the control of external nerves to the mucosa of the intestine; however, gastrin released into the blood in response to chyme in the stomach may augment this reflex.
The chyme entering the duodenum soon moves through to the jejunum and ileum of the small intestine. During intestinal motility, because of peristalsis and segmentation, it takes about 5 hours for chyme to pass through the small intestine. Segmentation in the duodenum and upper jejunum mixes chyme with digestive juices from the pancreas, liver, and intestinal mucosa. Peristalsis is controlled by intrinsic stretch reflexes and initiated by CCK, the hormone secreted by intestinal mucosa.
In the small intestine, the nutrients in chyme are completely prepared for absorption. The small intestine is the major organ of digestion, and the final stages of the digestive process occur here. Because it is also the site of almost all of the absorption of nutrients, the intestinal lining must be able to accommodate the actions of both digestion and absorption. The intestinal walls are covered with a thin layer of mucus that protects the walls from digestive juices. The walls are also adapted to enhance the absorption process. Fingerlike projections called villi greatly increase the amount of mucosal layer available for the absorption of nutrients.
On the villi are hairlike projections called microvilli. These also enhance absorption by their structure and movements.
As chyme enters the small intestine, hormones begin sending messages that regulate the release of digestive juices to continue the process of chyme digestion. Some hormones are provided by the small intestine; several are released by other organs into the small intestine. These secretions include enzymes from the small intestines, bile produced in the liver, and digestive juices from the pancreas.
One of the first hormones released by the small intestine is secretin. This hormone causes the pancreas to send bicarbonate to the small intestine to reduce the acidic content of the chyme. As the acidic level decreases, other pancreatic juices enter and begin their work. Another hormone secreted by the small intestine is cholecystokinin (CCK), or pancreozymin; it functions to initiate pancreatic ex-ocrine secretions; act against gastrin by inhibiting gastric HC1 secretion; and activate the gallbladder to contract, causing bile to be released into the duodenum.
Bile, secreted by the liver and stored in the gallbladder, is released to emulsify fats, which aids in the digestion of lipids. The emulsification creates more surface area that allows lipid enzymes to digest fats to their component parts. The liver always secretes bile. CCK and secretin spur the gallbladder to release bile for the digestion of fats. In addition, the small intestine produces enzymes to assist in the digestive process. Although much of the chyme is absorbed, the rest—which usually consists of fiber, minerals, and water—passes through the next sphincter (ileocecal valve) and into the large intestine (ascending colon).
The Large Intestine
The large intestine consists of the cecum, colon, and rectum. The cecum is a blind pocket; therefore the mass bypasses it and enters the ascending colon, which leads into the transverse colon that runs across the abdomen over the small intestine to the descending colon. The descending colon extends down the left of the abdomen into the sigmoid colon and leads into the descending colon, on to the rectum, and into the anal canal. Finally, any remaining mass passes out through the anus. The journey through the large intestine takes about 9 to 16 hours.
In the large intestine or colon, final absorption of any available nutrients, usually water and some minerals, occurs. Bacteria residing in the large intestine produce several vitamins, which are then absorbed. Water is withdrawn from the fibrous mass, forming solidified feces. Mucous glands in the intestinal wall create mucus that lubricates and covers feces as it forms. Again, peristalsis continues to move substances through the GI tract, resulting in the excretion of feces from the colon through the anus, the last sphincter muscle of the GI tract.
The movement of the food mass through the GI tract is controlled to enhance digestion and absorption.
During passage through the GI tract, more than 95% of the carbohydrates, fats, and proteins ingested are absorbed. Some minerals, vitamins, and trace elements may be less absorbed.1
Table 3-1 summarizes the primary mechanisms of the digestive system.
Absorption
Although the food mass has possibly spent several hours in the tube of the GI tract, it is not yet actually inside the body until its nutrient components are absorbed. Absorption is the process by which substances pass through the intestinal mucosa into the blood or lymph. Transport processes provide the means for nutrients to actually pass through the wall of the small intestine. These include passive diffusion and osmosis, facilitated diffusion, energy-dependent active transport, and engulfing pinocytosis (Figure 3-5). Passive diffusion occurs when pressure is greater on one side of the membrane and the substance then moves from the area of supplied by the cell and a “pumping” mechanism, which are assisted by a special membrane protein carrier.
Engulfing pinocytosis takes place when a substance, either fluid or nutrient, contacts the villi membrane, which then surrounds the substance and creates a vacuole that encompasses the substance. Passing through the cell cytoplasm, the substance is then released into the circulatory system.
The amounts of vitamins and minerals absorbed depend on the body’s storage levels and immediate need for these nutrients. Nutrients such as fats, carbohydrates, and protein are easily absorbed regardless of the level of need. The structure of the small intestine, the site of almost all nutrient absorption, allows for efficient absorption to occur. The microvilli are sensitive to the exact nutrient needs of the body. Their wavelike motions, caused by peristalsis, result in the most exposure of the nutrient-laden chyme to the absorbing cells. This exposure allows needed nutrients to leave the GI tract and pass through the microvilli cells. At this point, the nutrients are truly “inside” the body.
Figure 3-5 Methods of absorption. Passive diffusion, the movement of molecules from a region of high concentration to low concentration; facilitated diffusion, the movement of molecules by a carrier protein across the cell membrane from a region of high to low concentration; active transport, the movement of molecules and ions by means of a carrier protein against fluid pressures that require expenditure of cellular energy. (From Mahan LK, Escott-Stump S: Krause’s food, nutrition, and diet therapy, ed 10, Philadelphia, 2000, WB Saunders.)
Various factors may affect absorption of nutrients. Combinations of naturally occurring substances such as fiber or binders may move nutrients through the GI tract too quickly for optimum absorption to occur. Individual nutrient absorption and other issues of bioavailability are addressed in other chapters. The relationship between food and drug absorption is also an important issue of medical treatment. Ingesting medications with food may decrease the absorption rate of the medication and may also interfere with the absorption of other nutrients contained in the food consumed.
Once “inside” the body, the nutrients enter the circulatory systems of the bloodstream or lymphatic system. The general circulatory or blood system receives absorbed protein, carbohydrates, small parts of broken down fats, and most vitamins and minerals. This system transports these nutrients throughout the body. The lymphatic system, a secondary circulatory system, receives large lipids and fat-soluble vitamins. The nutrients traveling in the lymphatic system are deposited into the bloodstream near the heart. All nutrients then circulate throughout the body in the blood, providing for the nutrient requirements of cells.
Soon after entering the bloodstream, nutrients pass by the liver. This allows the liver to have “first choice” of the available nutrients. The liver is a powerhouse organ that provides a wide variety of services and substances; thus its nutrient needs are a priority. From there, the bloodstream’s journey of nutrients continues to the heart to also give it a prime nutrient selection. The journey then continues through the circulatory system to all cells. Some nutrients end up iutrient storage sites of the body. These sites include the bones, liver, and kidneys. Other nutrients, if not discarded or used by cells, are filtered out of the blood by the kidneys to be reabsorbed or excreted in urine.
Elimination
The expulsion of feces or body waste products is called defecation. When the rectum is distended because of waste accumulation, the reflex to defecate occurs. The residue may include substances, such as cellulose and other dietary fibers and connective tissue from meat collagen, that are unable to be digested by human enzymes. Undigested fats may combine with dietary minerals, such as calcium and magnesium, and form residue. Additional residue may include water, bacteria, pigments, and mucus.
Figure 3-6 Summary of digestive organ functions. (From Rolin Graphics.)
Digestive Process across the Life Span
Over the course of the life span, the main and accessory organs of digestion develop and change. The immature GI tract, particularly the intestinal mucosa of young infants, may allow intact proteins to be absorbed without complete digestion occurring. This incomplete digestion may result in an allergic response by the immune system and is part of the reason to delay the introduction of solid foods (e.g., cereals) until the GI tract has matured sufficiently. Another age-related condition is lactose intolerance in which the body ceases to produce lactase, the enzyme that breaks down the milk carbohydrate of lactose.
For some people, this occurs once the primary growth need for nutrients contained in milk is met. For others, this may not occur until adulthood or not at all. Older adults sometimes experience lactose intolerance as the secretion of enzymes, such as lactase, decreases as part of the aging process. Conditions of the middle years include gallbladder disease and peptic ulcers (sores that may occur on the epithelial surfaces of the stomach or small intestine). Older years may be marked by problems of constipation and diverticulosis. These conditions may be associated with age-related reduced peristalsis and decreased physical activity, and may be worsened by a lifelong history of chronic low dietary fiber consumption.
Metabolism
It is hard to imagine that a lunch consisting of tuna on rye bread will actually end up being part of the cells of the body. Fortunately, the human body is able to transform the nutrients of the sandwich into substances usable by cells. Metabolism is a set of processes through which absorbed nutrients are used by the body for energy and to form and maintain body structures and functions. The two main processes of metabolism involve catabolism and anabolism. Catabolism is the breakdown of food components into smaller molecular particles, which causes the release of energy as heat and chemical energy. Anabolism is the process of synthesis from which substances are formed, such as new bone or muscle tissue. Both processes happen within cells at the same time.
Wheutrients finally reach individual cells, they may be chemically changed through anabolism to help form new cell structures or to create new substances such as hormones and enzymes. Some vitamins and minerals assist in the use of other nutrients within the cell. They act as catalysts or coenzymes to initiate and support the transformation and use of carbohydrates, proteins, and lipids. Other nutrients may be used as energy to continue life-supporting processes. These processes include the energy needed to support deoxyribonucleic acid (DNA) reproduction and create proteins and other molecules, nerve impulses, and muscle contractions. Some energy is stored in a ready-to-use state.
Waste products from metabolism are discarded by the cells and wind up circulating in the blood. They are then excreted through the lungs, kidneys, or large intestine. The lungs release excess water and carbon dioxide. The kidneys filter and excrete metabolic waste and excess vitamins and minerals but reabsorb nutrients that the body needs to retain. Waste products may also be discarded through the large intestine in feces.
Fortunately, we do not have to consciously control these processes. Our responsibility is to provide an adequate selection of nutrients through the foods we choose to eat and to eat those foods in a way that enhances the functioning of the GI tract.
Metabolism across the Life Span
Metabolic changes are most noticeable later in life as the amount of food energy required decreases in relation to lowered metabolic rates. Nutrient needs, however, remain constant. Our challenge as we (and our clients) enter the middle years and beyond is to meet nutrient needs while maintaining or reducing our kcaloric needs to equal actual metabolic use. Recognition of this change can forestall the unexpected weight gain that appears to accompany aging in the
Breakdown of Food and Fat
The digestive process breaks down food by chemical and mechanical action into substances that can pass into the bloodstream and be processed by body cells.
Certaiutrients, such as salts and minerals, can be absorbed directly into the circulation. Fat, complex carbohydrates, and proteins are broken down into smaller molecules before being absorbed.
Fat is split into glycerol and fatty acids; carbohydrates are split into monosaccharide sugars; and proteins are split into linked amino acids called peptides, and then into individual amino acids.
Mouth and oesophagus
Food is chewed with the teeth and mixed with saliva. The enzyme amylase, present in saliva, begins the breakdown of starch into sugar. Each lump of soft food, called a bolus, is swallowed and propelled by contractions down the oesophagus into the stomach.
Stomach
Pepsin is an enzyme produced when pepsinogen, a substance secreted by the stomach lining, is modified by hydrochloric acid (also produced by the stomach lining).
Pepsin breaks proteins down into smaller units, called polypeptides and peptides. Lipase is a stomach enzyme that breaks down fat into glycerol and fatty acids. The acid produced by the stomach also kills bacteria.
Duodenum
Lipase, a pancreatic enzyme, breaks down fat into glycerol and fatty acids. Amylase, another enzyme produced by the pancreas, breaks down starch into maltose, a disaccharide sugar. Trypsin and chymotrypsin are powerful pancreatic enzymes that split proteins into polypeptides and peptides.
Small Intestine
Maltase, sucrase, and lactase are enzymes produced by the lining of the small intestine. They convert disaccharide sugars into monosaccharide sugars. Peptidase, another enzyme produced in the intestine, splits large peptides into smaller peptides and then into amino acids.
Large Intestine
Undigested food enters the large intestine, where water and salt are absorbed by the intestinal lining. The residue, together with waste pigments, dead cells , and bacteria, is pressed into faeces and stored for excretion.
Besides water, the diet must provide metabolic fuels (mainly carbohydrates and lipids), protein (for growth and turnover of tissue proteins), fiber (for roughage), minerals (elements with specific metabolic functions), and vitamins and essential fatty acids (organic compounds needed in small amounts for essential metabolic and physiologic functions). The polysaccharides, triacylglycerols, and proteins that make up the bulk of the diet must be hydrolyzed to their constituent monosaccharides, fatty acids, and amino acids, respectively, before absorption and utilization. Minerals and vitamins must be released from the complex matrix of food before they can be absorbed and utilized.
Globally, undernutrition is widespread, leading to impaired growth, defective immune systems, and reduced work capacity. By contrast, in developed countries, there is often excessive food consumption (especially of fat), leading to obesity and to the development of cardiovascular disease and some forms of cancer. Deficiencies of vitamin A, iron, and iodine pose major health concerns in many countries, and deficiencies of other vitamins and minerals are a major cause of ill health. In developed countries, nutrient deficiency is rare, though there are vulnerable sections of the population at risk. Intakes of minerals and vitamins that are adequate to prevent deficiency may be inadequate to promote optimum health and longevity.
Excessive secretion of gastric acid, associated with Helicobacter pylori infection, can result in the development of gastric and duodenal ulcers; small changes in the composition of bile can result in crystallization of cholesterol as gallstones; failure of exocrine pancreatic secretion (as in cystic fibrosis) leads to undernutrition and steatorrhea. Lactose intolerance is due to lactase deficiency leading to diarrhea and intestinal discomfort. Absorption of intact peptides that stimulate antibody responses causes allergic reactions, and celiac disease is an allergic reaction to wheat gluten.
DIGESTION & ABSORPTION OF CARBOHYDRATES
The digestion of complex carbohydrates is by hydrolysis to liberate oligosaccharides, then free mono- and disaccharides. The increase in blood glucose after a test dose of a carbohydrate compared with that after an equivalent amount of glucose is known as the glycemic index. Glucose and galactose have an index of 1, as do lactose, maltose, isomaltose, and trehalose, which give rise to these monosaccharides on hydrolysis. Fructose and the sugar alcohols are absorbed less rapidly and have a lower glycemic index, as does sucrose. The glycemic index of starch varies between near 1 to near zero due to variable rates of hydrolysis, and that of nonstarch polysaccharides is zero. Foods that have a low glycemic index are considered to be more beneficial since they cause less fluctuation in insulin secretion.
Amylases Catalyze the Hydrolysis of Starch
The hydrolysis of starch by salivary and pancreatic amylases catalyze random hydrolysis of (1→4) glycoside bonds, yielding dextrins, then a mixture of glucose, maltose, and isomaltose (from the branch points in amylopectin).
Disaccharidases Are Brush
Border Enzymes
The disaccharidases—maltase, sucrase-isomaltase (a bifunctional enzyme catalyzing hydrolysis of sucrose and isomaltose), lactase, and trehalase—are located on the brush border of the intestinal mucosal cells where the resultant monosaccharides and others arising from the diet are absorbed. In most people, apart from those of northern European genetic origin, lactase is gradually lost through adolescence, leading to lactose intolerance. Lactose remains in the intestinal lumen, where it is a substrate for bacterial fermentation to lactate, resulting in discomfort and diarrhea.
There Are Two Separate Mechanisms for the Absorption of Monosaccharides in the Small Intestine
Glucose and galactose are absorbed by a sodium-dependent process. They are carried by the same transport protein (SGLT 1) and compete with each other for intestinal absorption . Other monosaccharides are absorbed by carrier-mediated diffusion. Because they are not actively transported, fructose and sugar alcohols are only absorbed down their concentration gradient, and after a moderately high intake some may remain in the intestinal lumen, acting as a substrate for bacterial fermentation.
DIGESTION & ABSORPTION OF LIPIDS
The major lipids in the diet are triacylglycerols and, to a lesser extent, phospholipids. These are hydrophobic molecules and must be hydrolyzed and emulsified to very small droplets (micelles) before they can be absorbed. The fat-soluble vitamins—A, D, E, and K— and a variety of other lipids (including cholesterol) are absorbed dissolved in the lipid micelles. Absorption of the fat-soluble vitamins is impaired on a very low fat diet. Hydrolysis of triacylglycerols is initiated by lingual and gastric lipases that attack the sn-3 ester bond, forming 1,2-diacylglycerols and free fatty acids, aiding emulsification.
Pancreatic lipase is secreted into the small intestine and requires a further pancreatic protein, colipase, for activity. It is specific for the primary ester links—ie, positions 1 and
Monoacylglycerols are hydrolyzed with difficulty to glycerol and free fatty acids, so that less than 25% of ingested triacylglycerol is completely hydrolyzed to glycerol and fatty acids.
Bile salts, formed in the liver and secreted in the bile, enable emulsification of the products of lipid digestion into micelles and liposomes together with phospholipids and cholesterol from the bile. Because the micelles are soluble, they allow the products of digestion, including the fatsoluble vitamins, to be transported through the aqueous environment of the intestinal lumen and permit close contact with the brush border of the mucosal cells, allowing uptake into the epithelium, mainly of the jejunum. The bile salts pass on to the ileum, where most are absorbed into the enterohepatic circulation.
Within the intestinal epithelium, 1-monoacylglycerols are hydrolyzed to fatty acids and glycerol and 2-monoacylglycerols are re-acylated to triacylglycerols via the monoacylglycerol pathway. Glycerol released in the intestinal lumen is not reutilized but passes into the portal vein; glycerol released within the epithelium is reutilized for triacylglycerol synthesis via the normal phosphatidic acid pathway . All long-chain fatty acids absorbed are converted to triacylglycerol in the mucosal cells and, together with the other products of lipid digestion, secreted as chylomicrons into the lymphatics, entering the blood stream vit the thoracic duct .
DIGESTION & ABSORPTION OF PROTEINS
Few peptide bonds that are hydrolyzed by proteolytic enzymes are accessible without prior denaturation of dietary proteins (by heat in cooking and by the action of gastric acid).
Several Groups of Enzymes Catalyze the Digestion of Proteins
There are two main classes of proteolytic digestive enzymes (proteases), with different specificities for the amino acids forming the peptide bond to be hydrolyzed.
Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments, eg, pepsin in the gastric juice and trypsin, chymotrypsin, and elastase secreted into the small intestine by the pancreas.
Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, from the ends of polypeptides.
Carboxypeptidases, secreted in the pancreatic juice, release amino acids from the free carboxyl terminal, and aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal. Dipeptides, which are not substrates for exopeptidases, are hydrolyzed in the brush border of intestinal mucosal cells by dipeptidases. The proteases are secreted as inactive zymogens; the active site of the enzyme is masked by a small region of its peptide chain, which is removed by hydrolysis of a specific peptide bond.
Pepsinogen is activated to pepsin by gastric acid and by activated pepsin (autocatalysis). In the small intestine, trypsinogen, the precursor of trypsin, is activated by enteropeptidase, which is secretedby the duodenal epithelial cells; trypsin can then activate chymotrypsinogen to chymotrypsin, proelastase to elastase, procarboxypeptidase to carboxypeptidase, and proaminopeptidase to aminopeptidase.
Free Amino Acids & Small Peptides Are Absorbed by Different Mechanisms
The end product of the action of endopeptidases and exopeptidases is a mixture of free amino acids, di- and tripeptides, and oligopeptides, all of which are absorbed. Free amino acids are absorbed across the intestinal mucosa by sodium-dependent active transport. There are several different amino acid transporters, with specificity for the nature of the amino acid side chain (large or small; neutral, acidic, or basic). The various amino acids carried by any one transporter compete with each other for absorption and tissue uptake. Dipeptides and tripeptides enter the brush border of the intestinal mucosal cells, where they are hydrolyzed to free amino acids, which are then transported into the hepatic portal vein. Relatively large peptides may be absorbed intact, either by uptake into mucosal epithelial cells (transcellular) or by passing between epithelial cells (paracellular). Many such peptides are large enough to stimulate antibody formation— this is the basis of allergic reactions to foods.
DIGESTION & ABSORPTION OF VITAMINS & MINERALS
Vitamins and minerals are released from food during digestion—though this is not complete—and the availability of vitamins and minerals depends on the type of food and, especially for minerals, the presence of chelating compounds. The fat-soluble vitamins are absorbed in the lipid micelles that result from fat digestion; water-soluble vitamins and most mineral salts are absorbed from the small intestine either by active transport or by carrier-mediated diffusion followed by binding to intracellular binding proteins to achieve concentration upon uptake. Vitamin B12 absorption requires a specific transport protein, intrinsic factor; calcium absorption is dependent on vitamin D; zinc absorption probably requires a zinc-binding ligand secreted by the exocrine pancreas; and the absorption of iron is limited.
Calcium Absorption Is Dependent on Vitamin D
In addition to its role in regulating calcium homeostasis, vitamin D is required for the intestinal absorption of calcium. Synthesis of the intracellular calciumbinding protein, calbindin, required for calcium absorption, is induced by vitamin D, which also affects the permeability of the mucosal cells to calcium, an effect that is rapid and independent of protein synthesis. Phytic acid (inositol hexaphosphate) in cereals binds calcium in the intestinal lumen, preventing its absorption. Other minerals, including zinc, are also chelated by phytate. This is mainly a problem among people who consume large amounts of unleavened whole wheat products; yeast contains an enzyme, phytase, which dephosphorylates phytate, so rendering it inactive. High concentrations of fatty acids in the intestinal lumen—as a result of impaired fat absorption—can also reduce calcium absorption by forming insoluble calcium salts; a high intake of oxalate can sometimes cause deficiency, since calcium oxalate is insoluble.
ENERGY BALANCE:
OVER- & UNDERNUTRITION
After the provision of water, the body’s first requirement is for metabolic fuels—fats, carbohydrates, and amino acids from proteins (and ethanol) .
Food intake in excess of energy expenditure leads to obesity, while intake less than expenditure leads to emaciation and wasting, as in marasmus and kwashiorkor. Both obesity and severe undernutrition are associated with increased mortality.
The body mass index, defined as weight in kilograms divided by height in meters squared, is commonly used as a way of expressing relative obesity to height. A desirable range is between 20 and 25.
|
BMI Category |
||
|
below 18.5 |
– |
Underweight |
|
18.5-24.9 |
– |
Healthy Weight |
|
25-29.9 |
– |
Overweight |
|
over 30 |
– |
Obese |
Energy Requirements Are Estimated by Measurement of Energy Expenditure
Energy expenditure can be determined directly by measuring heat output from the body but is normally estimated indirectly from the consumption of oxygen. There is an energy expenditure of 20 kJ/L of oxygen consumed regardless of whether the fuel being metabolized is carbohydrate, fat, or protein. Measurement of the ratio of the volume of carbon dioxide produced to volume of oxygen consumed (respiratory quotient; RQ) is an indication of the mixture of metabolic fuels being oxidized . A more recent technique permits estimation of total energy expenditure over a period of 1–2 weeks using dual isotopically labeled water, 2H2 18O. 2H is lost from the body only in water, while 18O is lost in both water and carbon dioxide; the differencein the rate of loss of the two labels permits estimation of total carbon dioxide production and thus oxygen consumption and energy expenditure.
Basal metabolic rate (BMR) is the energy expenditure by the body when at rest—but not asleep—under controlled conditions of thermal neutrality, measured at about 12 hours after the last meal, and depends on weight, age, and gender. Total energy expenditure depends on the basal metabolic rate, the energy required for physical activity, and the energy cost of synthesizing reserves in the fed state. It is therefore possible to calculate an individual’s energy requirement from body weight, age, gender, and level of physical activity. Body weight affects BMR because there is a greater amount of active tissue in a larger body. The decrease in BMR with increasing age, even when body weight remains constant, is due to muscle tissue replacement by adipose tissue, which is metabolically much less active. Similarly, women have a significantly lower BMR than do men of the same body weight because women’s bodies have proportionately more adipose tissue than men.
Energy Requirements Increase With Activity
The most useful way of expressing the energy cost of physical activities is as a multiple of BMR. Sedentary activities use only about 1.1–1.2 ??BMR. By contrast, vigorous exertion, such as climbing stairs, cross-country skiing, walking uphill, etc, may use 6–8 ??BMR.
Ten Percent of the Energy Yield of a Meal May Be Expended in Forming Reserves
There is a considerable increase in metabolic rate after a meal, a phenomenon known as diet-induced thermogenesis. A small part of this is the energy cost of secreting digestive enzymes and of active transport of the products of digestion; the major part is due to synthesizing reserves of glycogen, triacylglycerol, and protein.
There Are Two Extreme Forms of Undernutrition
Marasmus can occur in both adults and children and occurs in vulnerable groups of all populations.
Kwashiorkor only affects children and has only been reported in developing countries. The distinguishing feature of kwashiorkor is that there is fluid retention, leading to edema. Marasmus is a state of extreme emaciation; it is the outcome of prolonged negative energy balance. Not only have the body’s fat reserves been exhausted, but there is wastage of muscle as well, and as the condition progresses there is loss of protein from the heart, liver, and kidneys. The amino acids released by the catabolism of tissue proteins are used as a source of metabolic fuel and as substrates for gluconeogenesis to maintain a supply of glucose for the brain and red blood cells. As a result of the reduced synthesis of proteins, there is impaired immune response and more risk from infections. Impairment of cell proliferation in the intestinal mucosa occurs, resulting in reduction in surface area of the intestinal mucosa and reduction in absorption of such nutrients as are available.
PROTEIN & AMINO ACID REQUIREMENTS
Protein Requirements Can Be Determined by Measuring Nitrogen Balance
The state of proteiutrition can be determined by measuring the dietary intake and output of nitrogenous compounds from the body. Although nucleic acids also contaiitrogen, protein is the major dietary source of nitrogen and measurement of total nitrogen intake gives a good estim. The output of nitrogen from the body is mainly in urea and smaller quantities of other compounds in urine and undigested protein in feces, and significant amounts may also be lost in sweat and shed skin.
The difference between intake and output of nitrogenous compounds is known as nitrogen balance.
Three states can be defined: In a healthy adult, nitrogen balance is in equilibrium when intake equals output, and there is no change in the total body content of protein.
In a growing child, a pregnant woman, or in recovery from protein loss, the excretion of nitrogenous compounds is less than the dietary intake and there is net retention of nitrogen in the body as protein, ie, positive nitrogen balance.
In response to trauma or infection— or if the intake of protein is inadequate to meet requirements— there is net loss of proteiitrogen from the body, ie, negative nitrogen balance.
The continual catabolism of tissue proteins creates the requirement for dietary protein even in an adult who is not growing, though some of the amino acids released can be reutilized. Nitrogen balance studies show that the average daily requirement is
There Is a Loss of Body Protein in Response to Trauma & Infection
One of the metabolic reactions to major trauma, such as a burn, a broken limb, or surgery, is an increase in the net catabolism of tissue proteins. As much as 6–7% of the total body protein may be lost over 10 days. Prolonged bed rest results in considerable loss of protein because of atrophy of muscles. Protein is catabolized as normal, but without the stimulus of exercise it is not completely replaced. Lost protein is replaced during convalescence, when there is positive nitrogen balance. A normal diet is adequate to permit this replacement.
The Requirement Is Not for Protein Itself but for Specific Amino Acids
Not all proteins are nutritionally equivalent. More of some than of others is needed to maintain nitrogen balance because different proteins contain different amounts of the various amino acids. The body’s requirement is for specific amino acids in the correct proportions to replace the body proteins.
The amino acids can be divided into two groups: essential and nonessential. There are nine essential or indispensable amino acids, which cannot be synthesized in the body: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. If one of these is lacking or inadequate, then—regardless of the total intake of protein—it will not be possible to maintain nitrogen balance since there will not be enough of that amino acid for protein synthesis. Two amino acids—cysteine and tyrosine—can be synthesized in the body, but only from essential amino acid precursors (cysteine from methionine and tyrosine from phenylalanine).
The dietary intakes of cysteine and tyrosine thus affect the requirements for methionine and phenylalanine. The remaining 11 amino acids in proteins are considered to be nonessential or dispensable, since they can be synthesized as long as there is enough total protein in the diet—ie, if one of these amino acids is omitted from the diet, nitrogen balance can still be maintained. However, only three amino acids—alanine, aspartate, and glutamate—can be considered to be truly dispensable; they are synthesized from common metabolic intermediates (pyruvate, oxaloacetate,and λ-ketoglutarate, respectively).
The remaining amino acids are considered as nonessential, but under some circumstances the requirement for them may outstrip the organism’s capacity for synthesis.
OVERCOMING BARRIERS
Some of our lifestyle behaviors affect the functioning and health of our GI tracts and therefore influence our nutritional status (see the Social Issue box, “Hunger Vs. Appetite Vs. Time”). Some common GI tract health problems are caused by the everyday decisions that we make but that can be changed. Prevention suggestions and treatment strategies for some common GI tract health problems follow.
Heartburn
Heartburn, fortunately, has nothing to do with the health of the heart. Instead, it is a burning sensation felt in the esophagus when food that has already been passed to the stomach refluxes or passes back up through the cardiac sphincter into the esophagus. The esophagus is not lined with acid-resistant mucus, as is the stomach, so the acidic mixture of food burns the walls of the esophagus and causes pain. Heartburn, or gastroesophageal reflux (GER), is a common experience. Depending on the frequency and severity of heartburn, including symptoms such as asthma, chronic cough, and other ear, nose, and throat ailments, a diagnosis of either gastroesophageal reflux disease (GERD) or laryngopharyngeal reflux (LPR) (in which reflux affects the larynx or pharynx) may occur.
Prevention and treatment strategies attempt to reduce the amount of pressure in the stomach so that the cardiac sphincter is not opened by excess pressure from stomach contents. A primary approach is to avoid overeating, so that the stomach can easily accommodate its contents. Other strategies include avoiding the following5-6:
• Constipation. Straining to defecate affects the contents of the stomach by creating additional pressure.
• Lying down shortly after eating. Resting or sleeping with a full stomach may push contents against the cardiac sphincter. Wait several hours after a meal before lying flat or keep head and shoulders elevated when reclining.
• High-fat meals. Slow emptying of the stomach from eating high-fat food increases the chance of reflux.
• Tight clothing. Wearing restrictive clothing around the waist and midriff affects the functioning of the stomach and may increase stomach pressure.
• “Eating on the run.” Eating meals while under stress or trying to do other activities at the same time may cause food to not be chewed enough. Big clumps of foods in the stomach force the stomach muscles to react strongly, which may cause reflux (see the Health Debate box, “Are Advertisers Leading Us Astray?”).
• Certain foods and drinks. Consuming chocolate, alcohol, peppermints, spearmints, or liqueurs and possibly caffeine, tomatoes, and citrus fruits and juices may irritate and cause heartburn.
• Some medications. Taking certain medications regularly may initiate heartburn.
If heartburn often occurs when taking birth control pills, antihistamines, tranquilizers (e.g., diazepam [Valium]), or any drug taken often, check with the primary care provider. Heartburn could be caused by these medications.
If these strategies do not help and heartburn remains, consult a primary care provider. Chronic heartburn or GER may result in esophagitis or may be caused by hiatal hernia, which requires medical intervention.
Vomiting
Although vomiting is not usually related to lifestyle behaviors, it is a common digestive disorder worthy of review. Vomiting is reverse peristalsis. Instead of food moving down the GI tract, the peristalsis muscles move the contents of the stomach back through the esophagus and forcefully out the mouth. It is an involuntary muscular action that we cannot easily control. Often it is painful; the contents of the stomach already consist of a mixture of food and acidic gastric juices that burns the unprotected esophagus.
Vomiting is a way of the body protecting itself. Perhaps an intruding virus or toxin has entered the GI tract; vomiting removes the offending substance. Mixed messages regarding the body’s sense of equilibrium during air or sea travel can result in motion sickness, of which vomiting may be a symptom. Dehydration is a concern when vomiting is continuous. Vomiting causes a loss of fluid and elecgastroesophageal trolytes, such as magnesium, potassium, and sodium, which stresses the functioning of the body. Infants are at particular risk for dehydration because their bodies consist mostly of fluids.
A primary healthcare provider should be consulted to determine the cause of vomiting and to recommend treatment.
Also at medical risk are individuals who vomit as a way to control their weight and suffer from eating disorders such as anorexia nervosa and bulimia.
Repetitive self-induced vomiting can injure the esophagus and wear away the enamel of teeth. Anyone practicing this self-destructive behavior should consult a primary care provider or mental health professional as soon as possible.
Intestinal Gas
Annoying, embarrassing, and offensive are all terms that come to mind when inflatus testinal gas, or flatus, is the subject. Actually, everyone’s body produces and reintestinal gas leases gas from the lower intestinal tract. Most gas leaves the GI tract without our awareness because it is odorless. Sometimes if the gas passes through too quickly, it is quite noticeable!
Bacteria in the large intestine may cause gas formation when specific indigestible carbohydrates ferment. These may include some of the carbohydrates found in legumes (dried beans) such as soybeans and black beans. Another cause may be lactose intolerance, the inability to break down lactose, the carbohydrate in milk.
The lactose then begins to ferment, causing gas buildup, bloating, and diarrhea (see Chapter 4). The longer any undigested substances linger in the large intestine, the more likely it is that fermentation will occur, leading to gas formation. This may result from constipation that slows the passage of chyme through the GI tract.
Another factor contributing to flatulence may be eating so quickly that food is swallowed in large clumps, which thereby requires more time to sufficiently process the chyme before it is excreted.
Generally, however, intestinal gas can probably be decreased through some simple changes of food-related behaviors. Here are some suggestions:
• If making dietary changes to increase fiber intake, gradually add more fibrous foods such as legumes to allow the system to adjust.
• Notice the effects of drinking milk. Drink fluid milk in small quantities over several weeks, working up to an 8-oz glass. Note at what level gas may develop. If a problem occurs, consider eating other milk-related products such as yogurt, cheese, or lactose-reduced milk.
• Increase fluid intake and consume sufficient amounts of fiber to prevent constipation.
• Take the time to consider which foods may be problematic. Each person’s cause of flatulence may be different.
• Eat slower and chew foods more thoroughly.
Constipation
There is no clear definition of constipation. It is usually considered as difficulty and discomfort associated with defecation. Individuals may interpret these terms differently and may vary in their natural urge to defecate. Not everyone needs to pass a bowel movement daily. Normal functioning ranges from once a day to every 3 days. Generally, constipation is recognized as straining to pass hard, dry stools.5’6-8
The causes of constipation are usually related to lifestyle behaviors that can easily be changed. The following strategies address these behaviors:
• Choose foods that are high in fiber, particularly insoluble fiber such as wheat bran. Whole grain breads, fruits, and vegetables are important foods to consume.
Fiber provides bulk that softens the stool and makes elimination easier.
• Listen to body signals and follow a schedule that allows time for a bowel movement to occur. Ignoring the natural urge to defecate causes feces to remain in the colon longer. This allows more water to be withdrawn, resulting in harder, drier feces.
• Exercise regularly. Lack of exercise can lead to a loss of tone in the muscles of the lower GI tract.
• Drink enough liquids. Fluid intake should be approximately 8 to 10 cups a day. Most of us need to consciously remember to drink water or other liquids to fulfill this need.
• Relax. Stress tightens muscles throughout the body and may inhibit proper bowel functioning.
• Consume regular meals. The body works best with an intake of nutrients and fiber throughout the day.
Constipation caused by lifestyle behaviors should respond to these strategies. If using these strategies does not relieve constipation, consult a primary care provider to rule out more serious disorders.
SUMMARY
• Digestion involves hydrolyzing food molecules into smaller molecules for absorption through the gastrointestinal epithelium. Polysaccharides are absorbed as monosaccharides; triacylglycerols as 2-monoacylglycerols, fatty acids, and glycerol; and proteins as amino acids.
• Digestive disorders arise as a result of (1) enzyme deficiency, eg, lactase and sucrase; (2) malabsorption, eg, of glucose and galactose due to defects in the Na+-glucose cotransporter (SGLT 1); (3) absorption of unhydrolyzed polypeptides, leading to immunologic responses, eg, as in celiac disease; and (4) precipitation of cholesterol from bile as gallstones.
• Besides water, the diet must provide metabolic fuels (carbohydrate and fat) for bodily growth and activity; protein for synthesis of tissue proteins; fiber for roughage; minerals for specific metabolic functions; certain polyunsaturated fatty acids of the n-3 and n-6 families for eicosanoid synthesis and other functions; and vitamins, organic compounds needed in small amounts for many varied essential functions.
• Twenty different amino acids are required for protein synthesis, of which nine are essential in the human diet. The quantity of protein required is affected by protein quality, energy intake, and physical activity.
• Undernutrition occurs in two extreme forms: marasmus in adults and children and kwashiorkor in children. Overnutrition from excess energy intake is associated with diseases such as obesity, type 2 diabetes mellitus, atherosclerosis, cancer, and hypertension.
