Nutrient Intake, Digestion, Absorption, and Excretion Flashcards
Which cells in the GI tract secrete more than 30 GI hormone peptides?
Neuroendocrine cells
What are the major inputs influencing appetite regulation?
Short term signals related to meal ingestion that are transmitted by the “gut-brain” axis
Signals associated with energy stores that are mediated by leptin
Signals deriving from lean body mass
Circadian rhythm
What is the major orexigenic (appetite stimulating) gut hormone?
Ghrelin
Name anorexigenic (appetite suppressing) gut hormones
Glucagon-like peptide-1 and -2 (GLP-1, GLP-2)
Oxyntomodulin (OXM)
Peptide tyrosine-tyrosine (PYY)
Pancreatic polypeptide (PP)
Cholecystokinin (CCK)
How are gut- and fat-derived hormones (ghrelin, leptin, insulin, and PYY) involved in feedback regulation of feeding?
Through signals affecting hunger, satiety, and energy needs
How does ghrelin increase food intake?
By stimulating the ARC of the hypothalamus
Where in the brain are neural and hormonal signals that influence food intake located?
The arcuate nucleus (ARC) of the hypothalamus and the brainstem
Name the site of secretion, stimulating factors, mechanism of action, and effect of GLP-1 (glucagon-like peptide-1)
Distal gut
Stimulated by food intake proportional to energy intake
Works by binding GLP-1 receptors in pancreatic islet cells, heart, lungs, and brain (ARC and PVC)
Reduces appetite and energy intake; delays gastric emptying; enhances postprandial insulin release
Name the site of secretion, stimulating factors, mechanism of action, and effect of OXM (oxyntomodulin)
L cells of the distal gut
Stimulated by food intake
Works as agonist at glucagon receptor; has undefined neural effects
Reduces food intake; increases energy expenditure
Name the site of secretion, stimulating factors, mechanism of action, and effect of PYY (peptide-tyrosine-tyrosine)
L cells of the distal gut
Stimulated by food intake (released in proportion to energy, fat, and protein intake)
Mechanism of action: Y receptors found throughout the CNS and on vagal afferents, NPY (neuropeptide Y) inhibition, POMC (proopiomelanocortin) activation, associated with increased activity of OFC (orbitofrontal cortex)
Reduces food intake
Name the site of secretion, stimulating factors, mechanism of action, and effect of PP (pancreatic polypeptide)
Pancreatic polypeptide cells of the islets of Langerhans in the pancreas
Stimulated by food intake and vagal stimulation
Enters CNS via diffusion in the brain stem and ARC
Reduces food intake
Name the site of secretion, stimulating factors, mechanism of action, and effect of CCK (cholecystokinin)
L cells of the gut, nerves in distal ileum and colon, neurons in the brain
Stimulated by dietary protein and fat, gastric acid
Reduces hypothalamic NPY (neuropeptide Y)
Inhibits gastric emptying and reduces food intake
Name the site of secretion, stimulating factors, mechanism of action, and effect of Leptin
Large amounts from the gastric mucosa; white adipose tissue
Food deprivation is associated with low levels
Low levels influence ARC; possibly decreases gene expression of NPY and increases activity of POMC-secreting neurons
Low levels increase energy intake and decrease energy expenditure
Name the site of secretion, stimulating factors, mechanism of action, and effect of Ghrelin
Stomach
Food intake decreases levels; fasting increases levels
Stimulates ARC via receptors; stimulated GH secretion
Increases food intake
Describe the role of fiber (especially resistant starch R2) on the feeding response
It has satiety value with associated decreases in food intake
True or false: stomach size can influence the amount of food eaten
True. Its size is related to the amount of food habitually eaten.
How is aging associated with decreased appetite and food intake
Likely because of decreased basal hunger rather than increased meal satiety
Name some adverse effects that have been associated with megestrol
Nausea, vomiting, gas, diarrhea
GI muscle fibers are depolarized (contraction of the muscle) in response to?
Stretching of the muscle fiber
Acetylcholine released by parasympathetic neurons
Gut hormones
Inhibition of GI muscle contraction is associated with a hyperpolarized state which is caused by?
Norepinephrine or epinephrine
Sympathetic nerves that secrete norepinephrine
How is the enteric nervous system connected to the central nervous system?
By parasympathetic fibers (stimulates motility) and sympathetic fibers (modulates activity of ENS with inhibitory signals)
Factors that regulate gastric emptying
Volume of food
Gastrin
Enteric gastric nervous reflexes from the duodenum
Ghrelin
Hormonal feedback from the duodenum
What are the neurotransmitters of the ENS?
Acetylcholine
Norepinephrine and serotonin
y-aminobutyrate
Adenosine triphosphate (ATP)
Nitric oxide (NO) and carbon monoxide (CO)
Dopamine
CCK
Substance P
Vasoactive intestinal peptide (VIP)
Somatostatin
Leu-enkephalin
Met-enkaphalin
What are some sensory stimuli in the ENS that are involved in the neural control of the gut?
Irritation of the mucosa, excessive distention, or chemical stimuli
Describe the stimuli for secretion, site of secretion and action of gastrin for GI motility
Stimuli are protein, GI distention, gastric-releasing peptide
Secreted by G cells of the antrum, duodenum, and jejunum
Stimulates gastric acid secretion and mucosal growth; promotes gastric emptying
Describe the stimuli for secretion, site of secretion, and action of cholecystokinin for GI motility
Stimuli are protein, fat, and acid
Secreted by I cells of the duodenum, jejunum, and ileum
Stimulates pancreatic enzyme secretion, gallbladder contraction, and growth of exocrine pancreas; inhibits gastric emptying
Describe the stimuli for secretion, site of secretion, and action of secretin for GI motility
Stimulus is acid
Secreted by S cells of the duodenum, jejunum, and ileum
Stimulates pepsin, pancreatic and biliary bicarbonate secretion, and growth of exocrine pancreas; inhibits gastric emptying and gastric acid secretion
Describe the stimuli for secretion, site of secretion, and action of gastric inhibitory peptide (GIP) for GI motility
Stimuli are protein, fat, carbohydrate
Secreted by K cells of the duodenum and jejunum
Stimulates insulin release and secretion; inhibits gastric acid secretion and emptying
Describe the stimuli for secretion, site of secretion, and action of motilin for GI motility
Stimuli are fat, acid, gastric distention, bile acids, serotonin, and low pH in duodenum
Secreted by M cells of the duodenum and jejunum, stomach, colon
Stimulates gastric motility and intestinal motility
Describe the stimuli for secretion, site of secretion, and action of vasoactive intestinal peptide (VIP) for GI motility
Stimulus is GI distention
Secreted by nerves of the GI tract
Stimulates secretion of electrolytes and water secretion; inhibits gastric acid
Describe the stimuli for secretion, site of secretion, and action of somatostatin for GI motility
Stimulus is acid
Secreted by pancreas, GI mucosa, hypothalamus
Inhibits secretion of gastrin, VIP, GIP, secretin, motilin, exocrine pancreatic secretion, gallbladder contraction, gastric acid secretion, and gastric motility
Describe the stimuli for secretion, site of secretion, and action of serotonin (5-HT) for GI motility
Stimuli are luminal contents including glucose and SCFAs, GI distention
Secreted by nerve fibers of the enteric nervous system
Increases intestinal motility
Describe the stimuli for secretion, site of secretion, and action of peptide-tyrosine-tyrosine (PYY) for GI motility
Stimulus is fat
Secreted by jejunum
Inhibits gastric acid secretion and gastric motility
What motor functions of the stomach contribute to digestion?
Storing food, mixing food with gastric secretions, and emptying the semifluid mixture (chyme) into the duodenum
What factors decrease gastric tone?
Duodenal distention, colonic distention, ileal perfusion with glucose
The inhibitory reflexes of the duodenum are stimulated by what 5 factors?
- duodenal distention
- irritation of the duodenal mucosa
- pH less than 3.5 or 4.0
- high osmolality
- the presence of breakdown products of protein and fat digestion
Which macronutrient is the strongest stimulus for hormones of the duodenum and jejunum?
Dietary fat
The tone of which portions of the stomach influence liquid and solid emptying?
Proximal stomach influences liquid emptying
Distal stomach is involved in solid emptying
What is the average half-emptying time of water or isotonic saline?
12 minutes
Example: if one drinks 300 ml of water, 150 ml will enter the duodenum in about 12 minutes
What factors regulate liquid emptying?
Duodenal osmoreceptors, secretin, and VIP
What is the half-emptying time of solids?
45-110 minutes
Intragastric pressure ____ in response to the swallowing reflex and in response to gastric distention by the presence of food
Falls
What is the volume that a relaxed stomach can hold?
0.8 to 1.5 L
What factors influence the rate of gastric emptying time of solids?
Meal particle size, energy content, and fat content
When the swallowing mechanism is bypassed, such as in NG feeding, the rate of gastric emptying is ______.
Faster
Why would gastric emptying time decrease if feeding tubes are placed in the jejunum, bypassing the duodenum?
The duodenal mucosa possesses sensory receptors that are associated with neurohormonal reflexes that influence gastric emptying time
What factors determine the amount of chyme that enters the intestine from the stomach?
Duodenal distention, acidity, osmolar changes, and the presence of products of carbohydrate, protein, and fat digestion
When are segmentation contractions (mixing contractions) of the small intestine elicited?
When the small intestine is distended by chyme
What is the average time it takes for chyme to travel from the pylorus to the ileocecal valve?
3-5 hours. The rate of mixing contractions is 2-3 per minute or maximally 12 per minute. Propulsive movements of the small intestine (peristalsis) move in the direction of the anus at a rate of 0.5-2.0 cm/second.
When is neural control partly initiated during intestinal peristalsis?
When chyme stretches the intestinal wall
What are the stimulatory hormones of intestinal peristalsis?
Gastrin, CCK, insulin, motilin, and serotonin
What is the volume of chyme that empties into the cecum?
1500-2000 ml/day
What is the purpose of the ileocecal valve?
To prevent backflow of fecal content into the small intestine
How long does it take for chyme to move from the ileocecal valve through the colon?
8-15 hours
On average, after abdominal surgery, the small intestine regains peristalsis within ____ hours (or sooner), and motility of the stomach and colon returns between ____ and ____ hours
24, 48, 72
In a previously well-nourished patient, parenteral nutrition should only be considered after how many days of NPO?
7 days. PN has been associated with higher infection morbidity in previously well-nourished patients compared with standard care
Aside from abdominal surgery, what other conditions/situations can cause ileus?
Infection, inflammation, electrolyte imbalance, the use of certain drugs such as sedatives, opioid analgesics, a2-adrenergic receptor agonists, catecholamine vasopressors
Name potential etiologies or contributing factors of ileus?
Opiate use, sympathetic hyperactivity, altered spinal-intestinal neural reflexes, changes in hormone expression and secretion, hypomagnesemia, hypokalemia, and local and systemic inflammation
What is chronic intestinal pseudo-obstruction (CIP)?
A motility disorder of peristalsis that most often affects the small bowel but can occur at any point in the GI tract
Name the secondary causes of CIP
Collagen vascular and endocrine diseases, neurological diseases, medication-related effects
Describe the treatment plan for CIP
Palliation of symptoms including nausea and vomiting with antiemetics and/or prokinetics, provision of IV hydration or PN, nocturnal cyclic EN when possible, and, when necessary, withholding of oral intake of food
What is the preferred mode of nutrition for CIP?
Oral
Describe the MNT for CIP patients who can consume an oral diet
Recommend small, frequent meals low in fat and fiber, with an emphasis on liquid forms of energy and protein. Daily multivitamin and mineral supplement is recommended, and if the patient has small intestinal bacterial overgrowth (SIBO), emphasis should be placed on fat-soluble and B12 vitamins.
List serious consequences of diarrhea
Severe fluid and electrolyte abnormalities; fluid loss, dehydration, hypovolemia; hypovolemic shock and cardiovascular collapse; increased risk of pressure ulcers, compromised nutrition status, increase hospital LOS
What percent of cases of antibiotic-associated diarrhea (AAD) is from C. difficile infection?
20%
What is the standard recipe for oral rehydration solution published by the World Health Organization?
2.6 gm NaCl, 13.5 gm anhydrous glucose, 1.5 gm potassium chloride, 2.9 gm trisodium citrate, and 1 L water
What MNT should be offered to a patient who has reobstruction of the small intestine after surgery (lysis of adhesions) for small bowel obstruction?
If the patient has a normal nutrition status on admission, the best nutrition intervention is no initial intervention. Would keep patient NPO on bowel rest and evaluate for PN on hospital day 7
What enteral MNT would you offer to a patient with severe diarrhea related to C.diff pseudomembranous colitis (PMC)?
Evidence-based diet recommendations for PMC diarrhea have not been established, however the goal is to initiate nutrition through the GI tract as early as possible to limit gut mucosa atrophy. Would keep patient NPO until diarrhea improves of antibiotic therapy, then initiate on caffeine-free clear liquids, and then advance to low-lactose, low-fat, low-fiber diet. Supplemental fiber use in C.diff diarrhea is not supported by ASPEN. IV fluids if oral intake does not keep up with losses. PN should not be offered unless patient presents severely malnourished.
How do SCFAs help control diarrhea?
By stimulating the uptake of water and electrolytes by colonocytes
Name some causes of gastroparesis
Diabetes mellitus (at least 25% of cases), complications of surgery, renal disease, collagen disease, drugs, malignancy, hypothyroidism, and possible idiopathic reasons
What surgical intervention can be offered for a patient who fails medical treatment for gastroparesis?
A near-total gastrectomy and Roux-en-Y gastrojejunostomy.
What are the dietary treatments for gastroparesis?
Small, frequent meals, drinking fluids with meals, limiting dietary fat and fiber, maintaining good glucose control, and, when necessary, feeding past the pylorus
What nutrition intervention is recommended for a patient with gastroparesis who has failed to respond to dietary intervention?
Jejunostomy-tube placement for long-term intestinal feedings and oral diet of small amounts of low-fat, low-fiber foods with liquids as tolerated (to address the psychosocial need to eat). If patient is receiving nighttime cyclic feedings, may also need to change to a longer-acting insulin to control glucose levels during nighttime feeding. Jejunal feedings bypass the stomach and obviate the potential of formula intolerance associated with delayed gastric emptying.
Name conditions associated with constipation
Diabetes mellitus, hypothyroidism, hypercalcemia, dehydration, neurologic disorders, anorectal disease, and collagen vascular/muscular diseases
What are the first-line medication treatments for constipation when nonpharmacological approaches fail? Second-line?
First-line: Hydrophilic colloids (psyllium, bran methylcellulose, polycarbophil), stool softeners, osmotic laxatives (polyethylene glycol, lactulose, sorbitol, and magnesium salts).
Second-line: Stimulants (senna/docusate, bisacodyl/docusate, or casanthranol/docusate) and lubricants (mineral oil).
What is the disadvantage of using mineral oil for constipation?
It binds fat-soluble vitamins and, if aspiration occurs, could cause a lipoid pneumonia.
What is dumping syndrome?
A group of symptoms that develop after gastric surgery and are related to increased gastric emptying following vagotomy and bypass or destruction of the pylorus. Can also occur with too rapid infusion of EN through a small-bore feeding tube. It is accompanied by both GI symptoms and vasomotor symptoms.
What are the GI and vasomotor symptoms of dumping syndrome?
GI symptoms: feeling of fullness after eating, crampy abdominal pain, nausea, vomiting, and explosive diarrhea
Vasomotor symptoms: diaphoresis, weakness, dizziness, flushing, and palpitations
What are the time frames for early vs late dumping?
Early: within 30-60 minutes of eating
Late: occurs 2-3 hours after eating and is limited to vasomotor symptoms
What is the cause of the GI and vasomotor symptoms of dumping syndrome?
GI: related to the rapid emptying of hyperosmolar chyme into the small intestine from the stomach, eliciting an osmotic diuresis into the intestinal lumen and subsequent distention
Vasomotor: rapid delivery of glucose to the upper small intestine, which results in peripheral and splanchnic vasodilation. This rapid delivery precipitates and increased serum insulin release followed by hypoglycemia and vasomotor symptoms
What is the dietary treatment for dumping syndrome?
Involves slow introduction of solid food, elimination of simple sugars, frequent small meals, and no liquids with meals. Patients are advised to lie down after eating and consider adding functional fibers to delay gastric emptying. The initial post-op period should not include simple sugars. Dairy products should be restricted initially because temporary lactose intolerance may occur. Liquid multivitamin/mineral supplements and vitamin B12 injections may be warranted in patients who have undergone a gastric resection.
What are the 2 primary functions of secretions in the GI tract?
To supply digestive enzymes and to supply lubricating mucus
What populations are at risk for xerostomia?
Obese patients, older adults, patient’s with Sjogren’s syndrome, those undergoing radiotherapy or chemotherapy for cancer, those with hormone disorders and infections
What are some consequences of hyposalivation?
Alterations in taste perception, chewing and swallowing problems, intolerance of spicy foods, can encourage dietary choices that compromise nutrition status and increase the risk for dental plaque and periodontal disease
How does hyposalivation occur with obesity?
Via increased leptin and decreased ghrelin
What are the contents of saliva?
Mucin (for lubrication), ptyalin (alpha-amylase; enzyme for starch digestion), immunoglobulin A (IgA), lysozyme (for protection against oral bacteria), components that play a role in innate immunity
How much saliva do humans secrete each day?
Approximately 1000-1500 ml/day
Where do gastric secretions derive from?
Mucus-secreting cells, oxyntic glands (gastric glands), and pyloric glands
What 3 cells make up oxyntic glands?
Mucus-producing cells, peptic (chief) cells, and parietal cells
Peptic cells secrete ___ and ___; parietal cells secrete ___ and ___.
Pepsinogen and gastric lipase
Hydrogen chloride (HCl) and intrinsic factor
What is the pH of the gastric acid?
Approximately 0.8
What is a possible consequence of the destruction of parietal cells, such as with chronic gastritis?
Achlorhydria (the absence of acid) develops in addition to pernicious anemia and vitamin B12 deficiency caused by loss of intrinsic factor. Intrinsic factor is required for the absorption of vitamin B12 and is secreted by the same cells that secrete HCl
What are the biochemical stimulants for acid secretion?
Gastrin, histamine via H2 receptors, and acetylcholine released by parasympathetic stimulation
Gastrin is secreted in response to the presence of ___
Luminal oligopeptides (peptide products of protein digestion), gastric distention, and GRP (bombesin)
The presence of carbohydrate, protein, or fat in the duodenum ____ gastric acid and pepsin secretion as well as gastric motility
Inhibits
Caffeine and alcohol ____ gastrin and acid production
Stimulate
Peptic ulcers develop when ____ is compromised
The gastric lining’s protective barrier against irritation and autodigestion
What is the treatment for peptic ulcers?
Suppression of gastric acid production, ie. Proton-pump inhibitors (exert their effects on acid suppression by interfering with hydrogen potassium ATP activity), H2-receptor antagonist drugs (block histamine stimulation of acid production)
What are the 3 phases in which gastric juice is secreted?
- Cephalic phase
- Gastric phase
- Intestinal phase
Describe the cephalic phase of gastric juice secretion
Acid is secreted via vagal stimulation in response to the sight, smell, and/or taste of food. Vagus nerve releases acetylcholine (stimulates ECL cells and parietal cells). Accounts for 30% of the volume of acid secretion.
Describe the gastric phase of gastric juice secretion
Begins when food arrives in the stomach. Amino acids and peptides stimulate the G cells in the antrum to produce gastrin (enters the general circulation and stimulates parietal cells as an endocrine hormone). Gastric distention also leads to acid secretion via a reflex involving the vagus nerve. Accounts for 60% of the volume of acid secretion.
Describe the intestinal phase of gastric juice secretion
Food in the duodenum continues to stimulate small amounts of gastric secretions (likely related to the small amount of gastrin that is secreted by the duodenal mucosa). Accounts for approximately 10% of gastric acid secretion.
Describe the multiple ways in which gastric secretions are inhibited by signals from the intestines
Food in the small intestine stimulate gastrin release and initiates a reverse enterogastric reflex that inhibits gastric secretions. The presence of acid, fat, products of protein digestion, hyper- or hypotonic fluids, or irritants in the proximal small bowel stimulate the secretions of gastric secretion-inhibiting hormones (secretin, GIP, VIP, somatostatin). The presence of carb, protein, or fat in the duodenum inhibits gastric acid and pepsin secretion and gastric motility.
What are the components of bile?
Water, bile salts, bile pigments, cholesterol, lecithin, fatty acids, and electrolytes
What are the two principal bile acids?
Cholic acid and chenodeoxycholic acid
What are the secondary bile acids that are converted by colonic bacteria?
Cholic acid converts to deoxycholic acid
Chenodeoxycholic acid converts to lithocholic acid
What are the bile pigments?
Bilirubin and biliverdin
Describe jaundice and how it can be detected
Jaundice occurs in pathological conditions when bilirubin accumulates in the blood, skin, sclera, and mucous membranes. It imparts a yellow color and can be detected when total plasma bilirubin exceeds 2 mg/dL
Bile production is stimulated by ___ and ___
The vagus nerve and secretin
What is the sphincter of Oddi?
The exit of the common bile duct into the duodenum. It remains closed between meals, directing bile to the gallbladder. It relaxes (opens) after a meal (especially if it is high in fat) when the gallbladder contracts. Relaxation of the sphincter is mediated by the action of CCK
How much bile is secreted per day?
Approximately 500 mL
What is enterohepatic circulation?
Reabsorbed bile salts (90-95% of which are reabsorbed in the terminal ileum) and bile pigments are transported to the liver via the portal vein and then are reexcreted in the bile
What is the role of bile salts in the digestion of fat?
To act as emulsifiers along with phospholipids and monoglycerides
In the fasting state without any EN, gastric secretions can amount to ___ to ___ mL/day, and biliary and pancreatic secretions can be up to ____ to ____ mL/day.
500-1000 mL/day
1000-2000 mL/day
When colonic pressure increases, the ileocecal valve ____. When ileal pressure increases or when food exits the stomach, the ileocecal valve ___.
Shuts
Opens
Brunner glands secrete ___
Mucus, to protect the duodenum from gastric acid
Pancreatic secretions, intestinal secretions, and bile neutralize gastric acid to raise the pH of duodenal contents to ___
6-7
Name the pancreatic enzymes found in pancreatic juice
Carbohydrate - pancreatic amylase
Protein - pancreatic proteases (trypsin, chymotripsin, carboxypolypeptidase, proelastase, collagenase)
Fat - pancreatic lipase, cholesterol esterase, phospholipase
Why do oral rehydration solutions used to treat Na+ and water losses from diarrhea contain NaCl and glucose?
Na+ facilitates the absorption of glucose, some amino acids, and bile acids
What percent of dietary iron is absorbed under conditions of iron deficiency?
3-6%
Iron absorption is inhibited by:
Phytates in cereal, phosphates, and oxalates. These compounds for insoluble compounds with iron
Does the passage of feces continue during prolonged bowel rest when patients are restricted from consuming food?
Yes, fecal contents include material other than food residue. Feces contains bacteria (comprises 30% of the dry weight of fecal matter), inorganic material, fiber, water, sloughed off undifferentiated stem cells
How are medium-chain triglycerides absorbed?
MCTs are water soluble, they do not require the formation of micelles or the action of bile salts. MCTs are hydrolyzed and pass through enterocytes directly into the portal circulation. MCTs may be used as an energy supplement in patients who maldigest or malabsorb fat
What percent of cardiac output is delivered to the splanchnic circulation at rest?
25%
What percent of total body oxygen is consumed at rest by the splanchnic organs?
30%
During feeding, splanchnic blood flow increases __% to __% and oxygen demand in the splanchnic-supplied organs increases up to __%
40% to 60%
30%
Where does all blood in the GI tract, including the spleen and pancreas, flow to?
To the liver via the portal vein
What is the reticuloendothelial system?
A system of macrophages that clears the blood of bacteria and other particulate matter to prevent systemic infection before blood leaves the liver. In the liver, blood encounters RED cells in the lining of sinusoids (liver blood cells).
Name the arteries that supply blood to the small intestine. The stomach?
Superior and inferior mesenteric arteries (branch off the aorta) for the small intestine. Celiac artery for the stomach.
When does mucosal atrophy occur?
During starvation, stress, PN, and bowel rest
True or false: the bowel mucosa requires luminal nutrients to supply its nutrient needs
True. After 1 week of a protein-deficient diet, the microvilli shorten
What is the principal metabolic fuel for intestinal cells?
Glutamine. Its absence may directly contribute to mucosal atrophy that accompanies bowel rest.
When should EN be withheld in patients with hypoperfused gut?
When patients are being initiated on catecholamines, when catecholamine doses are increasing, or when patients require a high level of hemodynamic support including high-dose catecholamines (norepinephrine, epinephrine, dopamine) to maintain cellular perfusion.
