Digestion And Absorption

Question 1

What is lactose intolerance?

Answer 1

Failure in ability to digest dairy carbs
Brush border lactase enzyme activity is deficient or absent resulting in undigested and unabsorbed lactose
Lactose in the SI is converted to SCFAs and H gas causing it to remain in the lumen holding H2O and leading to osmotic diarrhea
Ferments into methane and H gas

Question 2

Describe carb digestion in the mouth

Answer 2

Begins breakdown of some starch

Contains salivary amylase which breaks starch down into maltose and 3-9 polymers of glucose

Question 3

Describe carb digestion in the SI

Answer 3

Most starch breakdown occurs here
Pancreatic amylase is present which continues to breakdown starch
Many disaccharides reach SI intact

Question 4

Describe the structure of intestinal mucosa

Answer 4

Ideal for absorption of large amounts of nutrients
Lumen surface of SI arranged in longitudinal folds (folds of Kerckring)
Villi and microvilli increase SA for absorption
Diseases that disrupt the mucosa will influence absorption of nutrients

Question 5

Describe absorption of carbs

Answer 5

Co-transport mechanisms and facilitated diffusion
On the luminal side Na/K ATPase creates concentration gradient
Secondary active transport of glucose and galactose occurs through SLGT1
Facilitated diffusion of fructose through GLUT5
GLUT2 then is used for transport of all sugars into blood

Question 6

What is one way to test for a CHO assimilation disorder?

Answer 6

After an overnight fast 25g D-xylose ingested and urine collected for next 5 hours
Key is that it will be absorbed not utilized
Absorbed by both active Na cotransport and by passive diffusion (at dose used will generally be able to absorb via passive diffusion)
Measures absorptive capacity of proximal SI specific to sugars

Question 7

What are other tests that are used for CHO assimilation disorders?

Answer 7

Breath tests such as methane, isotopically labeled 13-CO2 from different sugars, lactose/sucrose breath test

Question 8

What diseases are caused by a deficiency of pancreatic enzymes or defect in transporters of intestinal epithelial cells that lead to protein assimilation disorders?

Answer 8

Chronic pancreatitis, congenital trypsin absence, Cystinuria, hartnup disease, cystic fibrosis

Question 9

What is chronic pancreatitis?

Answer 9

Deficiency of pancreatic enzymes
Lack of proteases (i.e. trypsinogen)
Inability to neutralize stomach acid

Question 10

What is congenital trypsin absence?

Answer 10

Absence of trypsin
All pancreatic enzymes are gone
Cannot activate proteases

Question 11

What is cystinuria?

Answer 11

Defect in transport or absence of di-basic amino acid transporter thus cysteine, arginine, lysine and ornithine are not reabsorbed at the proximal tubule
Due to intestinal deficiency - amino acids secreted in feces/urine

Question 12

What is Hartnup disease?

Answer 12

Cannot absorb neutral amino acids
Sx resemble pellagra (niacin deficiency)
Autosomal recessive genetic disorder, Na dependent neutral amino acid transporter
Sx: diarrhea, mood changes, neuro problems (e.g. uncoordinated movements, abnormal muscle tone), red scaly skin rash, photosensitivity, short stature, urine sample displays high excretion of neutral amino acids (tryptophan) and by-products (serotonin)

Question 13

What is cystic fibrosis?

Answer 13

CFTR regulated Cl channel on apical membrane of duct cells
Pancreas problems persist and present early
Some CFTR mutations associated with loss of HCO3 secretion
-cannot move enzymes from ducts
-can lead to acute and chronic pancreatitis
Cannot neutralize stomach acid

Question 14

Describe protein digestion in the stomach

Answer 14

Pepsin secreted as pepsinogen and activated at low pH in the stomach (2-3)
Not essential for protein digestion (10-20%)

Question 15

Describe the pancreas’s contribution to protein digestion

Answer 15

Enzymes: trypsin, chymotrypsin, carboxypeptidase, elastase
Mostly breaks proteins down into di and tripeptides, some amino acids
Trypsin and chymotrypsin -> small polypeptides
Carboxypeptidase -> cleaves aa at carbonyl ends

Question 16

Describe protein digestion in the SI

Answer 16

Brush border enzymes: aminopolypeptidase and dipeptidases (some others)
Amino acids, di and trip peptides are absorbed into enterocytes

Question 17

Pancreatic enzymes are secreted as what?

Answer 17

Zymogens and activated by either enterokinase (trypsinogen) or trypsin itself

Question 18

Describe protein absorption in the SI

Answer 18

Na/K ATPase establishes Na gradient (blood side)
Separate cotransporters for each amino acid type (Na amino acid cotransporters on luminal side)
Separate facilitated diffusion mechanisms for each aa type on blood side
Diffusion of di and tri peptides into circulation

Question 19

What is celiac sprue?

Answer 19

Autoimmune disorder with hereditary component
Abs develop against gluten components which leads to destruction of SI villi (atrophy) and hyperplasia of intestinal crypts
Malabsorption deficiencies in folate, iron, Ca and vitamins A, B12 and D

Question 20

What are the GI sx of celiac sprue?

Answer 20

Abdominal pain, constipation, diarrhea, unexplained weight loss, vomiting nausea, steatorrhea

Question 21

What are the non-GI sx of celiac sprue?

Answer 21

Related to malabsorption
Tingling or numbness of extremities (hands and feet), itchy skin with a rash, fatigue, seizures, easy bruising, bone fractures

Question 22

What is tropical sprue?

Answer 22

Loss or decreased number of intestinal epithelial cells
Cause unknown
Reduced microvilli SA, lipid absorption impaired due to loss (steatorrhea)
Nutritional deficiencies occur (folate and vitamin B12)
Major sx: diarrhea (as well as cramps, nausea, weight loss, gas, indigestion)
Tx with tetracycline and folate for 6 months

Question 23

Describe lipid digestion

Answer 23

Complicated by insolubility

Bile salts and lecithin emulsify lipid products into micelles

Question 24

What enzyme is present in the mouth that is responsible for lipid digestion?

Answer 24

Lingual lipase

Question 25

Describe lipid digestion in the stomach

Answer 25

Gastric lipase acts on TAGs Dietary proteins perform emulsification here CCK: time is required so CCK inhibits gastric emptying for more mixing along with activating gallbladder emptying

Question 26

Describe lipid digestion in the SI

Answer 26

Most digestion of lipids occurs in the SI Bile salts emulsify lipids Pancreatic enzymes secreted into small intestine complete digestion (pancreatic lipase, cholesterol ester hydrolase, phospholipase A2)

Question 27

What is pancreatic lipase?

Answer 27

Secreted as active enzyme | Inactivated by bile salts w/out colipase

Question 28

What is colipase?

Answer 28

Activated by trypsin | Once activated it displaces bile salts by binding pancreatic lipase

Question 29

What is cholesterol ester hydrolase?

Answer 29

Secreted as active enzyme Catalyzes production of free cholesterol Hydrolyzes triglycerides to glycerol

Question 30

What is phospholipase A2?

Answer 30

Secretes as proenzyme activated by trypsin

Question 31

What are the steps of lipid absorption?

Answer 31

Solubilization by micelles Diffusion of micelles content across apical membrane Re-esterification Chylomicron formation Exocytosis of chylomicrons into lymphatics

Question 32

What are common problems that are associated with lipid digestion and absorption?

Answer 32

Pancreatitis, pancreatic insufficiency and Zollinger Ellison syndrome

Question 33

What is pancreatitis?

Answer 33

``` Pancreatic enzymes (trypsin) become activated and self digest the pancreas Impaired HCO3 and enzyme secretions ```

Question 34

What is pancreatic insufficiency?

Answer 34

Failure to secrete proper amounts of pancreatic enzymes

Question 35

What is Zollinger-Ellison syndrome?

Answer 35

Gastrin secreting tumor of pancreas Increased H secretion by gastric parietal cells Overload of acid into duodenum

Question 36

What factors cause deficits in bile salts?

Answer 36

Ileal restriction (total bile salt pool is reduced) due to loss fo enterohepatic recirculation of bile salts Liver disease Gallbladder disease Interferes with formation of micelles in SI

Question 37

What is small intestine bacterial overgrowth (SIBO)?

Answer 37

Two main causes: too little gastric secretion and SI dysmotility Inappropriate growth of bacteria in SI causes pain, bloating, gas and diarrhea/constipation Can result in malabsorption issues, steatorrhea, vitamin/mineral deficiency Changes in pH result in improper activity of lipase enzymes in SI Breath test detects methane and H Bacteria deconjugate bile salts, impairing micelle formation Severe bacterial overgrowth damages the intestinal mucosa

Question 38

Describe the general principles of vitamin absorption

Answer 38

Vitamins must be acquired often from diet and absorbed along the GI tract Fat soluble vitamins (ADEK) have same absorption mechanisms as lipids Water soluble vitamins (B,C) are absorbed via Na/K dependent co-transport except for vitamin B12

Question 39

Which proteins does vitamin B12 form complexes with to be absorbed?

Answer 39

``` R proteins (secreted in salivary juices) Intrinsic factor (IF) Transcobalamin II ```

Question 40

What is pernicious anemia?

Answer 40

Caused by failure of RBC maturation when vitamin B12 is deficient Other hematological disorders due to vitamin B12 deficiency are macrocytic and megaloblastic anemia’s

Question 41

What are common causes for pernicious anemia?

Answer 41

``` Atrophic gastritis (chronic inflammation of the stomach mucosa that leads to los of parietal cells) Autoimmune metaplastic atrophic gastritis (immune system attaches IF protein or gastric parietal cells) ```

Question 42

What are surgical implications of vitamin B12 absorption?

Answer 42

Gastrectomy (loss of parietal cells) | and gastric bypass (exclusion of the stomach, duodenum, and proximal jejunum alters absorption of vitamin B12)

Question 43

Vitamin D deficiency results in what?

Answer 43

Inadequate Ca absorption | Rickets in children and osteomalacia in adults

Question 44

Describe Ca absorption

Answer 44

Depends on presence of vitamin D Absorbed across enterocytes but also reabsorbed in kidney In SI, vitamin D dependent Ca binding protein (calbindin D-28K) Calictrol and PTH also govern

Question 45

Describe iron absorption

Answer 45

Liver secretes apotransferrin into bile and enters the duodenum Apotransferrin binds with free iron and with hemoglobin forming transferrin Transferrin binds to receptors on membranes of intestinal epithelial cells absorbed into cells via pinocytosis and later released

Question 46

Describe electrolyte absorption

Answer 46

The SI and colon not only absorb large quantities of electrolytes (Na, Cl, HCO3 and K) and water but the epithelial cells lining the crypts of the SI also secrete fluid and electrolytes The 1st step in this process is the absorption of solute followed by the absorption of water Absorbate is always isosmotic meaning that solute and water absorption occur in proportion to each other

Question 47

Describe electrolyte absorption in the SI

Answer 47

Jejunum absorbs lots of Na | Ileum absorbs Na and secretes HCO3

Question 48

Describe electrolyte absorption in the colon

Answer 48

Absorbs Na and secretes K | Aldosterone modulates

Question 49

Describe cholera

Answer 49

Cholera increases cAMP resulting in increased Cl secretion in the colon The resulting Cl secretion is accompanied by secretion of Na and H2O Volume of fluid secreted into the intestinal lumen overwhelms the absorptive mechanisms of the SI and colon leading to massive secretory diarrhea