Term
Digestion & absorption of CHO
- average American diet
- monosaccharides
- disaccharides
- polysaccharides (3)
Absorption |
|
Definition
300-500 g CHO (40-45% caloric intake), 50% starch (amylose), 30% sucrose, 6% lactose, 2% maltose
- glucose, galactose, fructose; breakdown products of complex CHO which are absorbed by SI
- 2 mono linked together by glycosidic bond; must by hydrolyzed to constituent monomers by brush border enzymes in SI
- lactose = glucose + galactose (lactase)
- sucrose = glucose + fructose (sucrase)
- maltose = glucose + glucose (maltase)
1. Starch (plant storage form of glucose)
- alpha-amylose: glucose linked in straight chains
- amylopectin: branched glucose chains
- amylase hydrolyzes alpha-1,4-glycosidic bonds
2. cellulose (in plant cell walls)
- unbranched, linear chains of glucose monomers linked by beta-1,4-glycosidic bonds
- dietary fiber; requires cellulase (bacteria)
3. glycogen (animal storage form of glucose)
- glucose monomers linked by alpha-1,4 glycosidic bonds
- glucose & galactose are absorbed via Na+-co transport mechanism (what else requires a co-transporter? bile!!)
- fructose is absorbed via facilitated diffusion, doesn't require energy
- all three are moved from epithelial cell of SI into blood by GLUT2 |
|
|
Term
Digestion & absorption of proteins
- average American diet
- activation of pancreatic enzymes
- absorption of di-, tri-peptides & AA across brush border |
|
Definition
70-100 g daily
Trypsinogen acted on by enterokinase (ONLY in the brushborder -> trypsin, activates pancreatic proenzymes
- trypsinogen kept inactive in pancreas by trypsin inhibitor
meat & plant proteins are absorbed into epi cells as di-, tri-peptides and free AA -> digestion by epithelial peptidases
*active Na+-cotransporter mechanism*
- bile salts, glucose, AA all absorbed by Na+ co-transporter
5 different AA and peptide transport proteins in luminal membrane
- lots of transporters, lots of redundancies - lacking a single transporter doesn't mean you'll have a deficiency in a specific AA |
|
|
Term
Digestion & absorption of lipids
- average American diet
- what happens to lipids in the stomach?
- role of bile salts!
- chylomicrons |
|
Definition
120-150 g daily - long chain TG, phospholipids, cholesterol, fat soluble vitamins
emulsified by acid, pepsin in the stomach (minimal digestion by lingual or gastric lipase)
- emulsification in SI
- digestion by pancreatic lipases
- bile salt micelles transport fats to surface of epithelial cells
- fats diffuse into epithelial cells (bile salt stays in lumen)
fats recombined to form new TG in ER -> aggregate into globules w/ cholesterol, phospholipids in Golgi -> coated by apoprotein -> exit via exocytosis to lymph, enter blood circulation
Small chain fats can diffuse through epithelial cells into blood (40% of dietary lipids!)
absorption of bile salts - via Na+-cotransporter in terminal ileum |
|
|
Term
| The role of bile salts in the uptake of lipid digestion products (monoacylglycerol and FA) by the SI |
|
Definition
|
|
Term
| Chylomicrons, VLDL made by the SI |
|
Definition
|
|
Term
Fat-soluble vitamins
- mechanism of absorption by GI tract |
|
Definition
A
D
E
K
require bile salts for absorption |
|
|
Term
Water-soluble vitamins
- mechanism of absorption by GI tract |
|
Definition
C
B1
B2
niacin
B6
biotin
folic acid
B12 (req's intrinsic factor; terminal ileum) |
|
|
Term
Mechanism of absorption of:
Na+
- mechanisms at luminal membrane
- at basolateral membrane
Absorption of chloride ions
Absorption of bicarbonate |
|
Definition
25-35 g/day
Luminal membrane:
- diffusion through Na+ channel
- coupled Na+-Cl- co-transport
- Na+-H+ co-transport
- Na+-solute co-transport (glucose, galactose, amino acids, bile salts)
Basolateral membrane:
- active transport into intercellular spaces
Cl- follows electrochemical gradient created by Na+ absorption
active absorption in distal ileum and LI in exchange for HCO3- ions (for buffering acid products formed by bacteria)
HCO3- absorbed in jejunum with Na+
active secretion in distal ileum and LI in exchange for Cl- ions |
|
|
Term
Mechanism of absorption of:
Ca++ |
|
Definition
25-80% absorbed
active (Ca++/Na+ exchange, ATP, or secreted in a vesicle)
finely regulated according to body's need by parathyroid hormone and vitamin D
d/c plasma [Ca++] -> PTH -> conversion of Vit D to 25-OH cholcalciferol in liver -> 1,25-OH cholcalciferol in kidney -> SI epithelial cell -> release of Ca++ into blood
Vit D deficiency -> less Ca++ absorbed |
|
|
Term
Mechanism of absorption of:
Fe |
|
Definition
ingest 15-25 g; 0.75-1.5 g absorbed
- meat (Fe+3), plants (Fe+2)
- gastric acid, absorbic acid converts Fe+3 -> Fe+2
active
- carrier = gastroferrin (parietal cell), keeps iron from forming insoluble salts
absorbed relative to body's need
- absorbed into enterocyte, binds to ferritin -> stored as apoferritin (if body doesn't need iron, apoferritin is sloughed off with epithelial cell)
- transferrin = iron binding protein in blood. If no iron is bound to transferrin, ferritin releases iron , moves into blood, binds to transferrin -> apotransferrin
- transported to liver, RBCs, bone, RE cells
Liver, spleen, bone marrow: iron binds to ferritin, stored as apoferritin |
|
|
Term
How water is absorbed in the GI tract
- where is most of the water absorbed?
- how is it absorbed? |
|
Definition
*most of the water in the GI tract is absorbed in the SMALL intestine!* (although colon has a larger absorptive capacity, in a healthy person the SI does most of the water absorption - 400 ml/day compared to 8500 ml)
diffusion, mainly through pores b/t epithelial cells (85-95% in upper SI)
follows osmotic gradient created by absorption of nutrients, electrolytes
- EXCEPT: entry of hyperosmotic chyme into duodenum -> osmosis of water into intestinal lumen (SI cells secrete to dilute chyme to allow for proper digestion) |
|
|
Term
malabsorption syndromes of the SI
- lactose intolerance
causes of malabsorption in general
- postsurgery
- intestinal mucosal lesion
- pancreatic insufficiency
- bile salt deficiency
- stomach
- liver
- pancreas
- SI |
|
Definition
lactase deficiency, secondary to injury in intestinal mucosa (Kwashiorkor, colitis, gastroenteritis, celiac & tropical sprue, excessive EtOH)
--> osmotic effect -> i/c peristalstis -> malabsorption of fats/proteins/drugs, watery diarrhea
postsurgery:
- gastric resection,
- intestinal bypass/resection,
- chronic gastritis,
- distal ileal resection/bypass (B12)
intestinal mucosal lesions:
- tropical sprue
- gluten-sensitive enteropathy (celiac sprue)
- ischemic enteropathy
- radiation enteropathy
- drug therapy
- enzyme deficiency (e.g. lactase)
- infection
- motility disorders
- bacterial overgrowth -> brush border damage (disaccharidases, pancreatic proteases...)
pancreatic insufficiency
- pancreatitis
- cystic fibrosis
- Zollinger-Ellison -> inactivation of pancreatic enzymes by excess gastric acid secretion
bile salt deficiency:
- liver disease
- ileal dysfunction/resection
- obstruction, hepatic dysfunction |
|
|
Term
CHO digestion - summary
- role of alpha-amylases
- disaccharidases |
|
Definition
|
|
Term
Dietary fiber
- soluble
- insoluble
- effects? |
|
Definition
Soluble: pectins, mucilages, gums
Insoluble: cellulose, hemicellulose, ligins
CHO not digested by GI tract
- bind water
- d/c transit time
- *adsorb minerals Ca, Fe, Mg, Zn*
- *adsorb organic material (bile salts, lipids, cholesterol)*
- colonic bacteria digest -> short-chain FA, lactate, H2, CO2, CH4
- failure to absorb -> osmotic diarrhea (mechanism of action of many laxatives) |
|
|
Term
| Protein digestion - starting at the mouth... |
|
Definition
|
|
Term
Lipid digestion
- where do most lipases, cholesterol esterases come from? |
|
Definition
|
|
Term
Transport of:
Na+, K+, Cl-, HCO3-
in the:
jejunum, ileum, colon
(table) |
|
Definition
|
|
Term
Mechanism of absorption of:
K+ |
|
Definition
| passive, through tight junctions and lateral spaces |
|
|
Term
Colon stuff
- absorption in LI
- bacterial action
- feces |
|
Definition
1500 ml enters colon, most of H2O & electrolytes absorbed, 50-200 ml excreted in feces
- proximal colon: active absorption of Na+, Cl- (via electrochemical gradient), water (osmosis)
- distal colon: active secretion of HCO3- in exchange for Cl- (buffers acid)
bacteria forms: vit K, B12, thiamin, riboflavin, various gases
95% water
solids:
- 30% dead bacteria
- 10-20% fat
- 10-20% inorganic matter
- 2-3% protein
- undigested fiber
- bile pigments
- sloughed epithelial cells
- color, odor (gross) |
|
|
Term
| Summary of major sites of nutrient absorption |
|
Definition
|
|
Term
Pancreatitis
- common causes
- consequences of enzymes dammed |
|
Definition
(Inflammation of the pancreas)
duct obstruction
- ampullary obstruction
- chronic alcoholism
=> interstitial edema -> impaired blood flow -> ischemia -> acinar cell injury
acinar cell injury (alcohol, drugs, trauma, ischemia, viruses)
=> release of intracellular proenzymes and lysosomal hydrolases -> activation of enzymes -> acinar cell injury
defective intracellular transport (alcohol, duct obstruction)
=> delivery of proenzymes to lysosomal compartment -> intracellular activation of enzymes -> acinar cell injury
blockage of papilla of Vater with gallstone so enzymes and HCO3- don't reach SI
Enzymes dammed up in ducts and acini:
- overcome trypsin inhibitor
- trypsin activates peptidases -> rapid autodigestion of pancreas
- death can result |
|
|
Term
Malabsorption from SI
- 3 diseases
- what is it/what does it do? |
|
Definition
Iliac sprue
Celiac disease
Gluten enteropathy
- both celiac sprue and lactase deficiency result in defective lactose hydrolysis
due to toxic effects of gluten
destroys villi via immunological rxn
early on: fat absorption impaired -> steatorrhea
severe: absorption of protein, CHO, other nutrients impaired |
|
|
Term
Summary: CHO
- products of digestion
- site of absorption
- mechanism |
|
Definition
- glucose, galactose, fructose
- SI
- Na+-glucose cotransport
Na+-galactose cotransport
facilitated diffusion (fructose) |
|
|
Term
Summary: Proteins
- products of digestion
- site of absorption
- mechanism |
|
Definition
- amino acids, dipeptides, tripeptides
- SI
- Na+-amino acid cotransport
H+-dipeptide cotransport
H+-tripeptide cotransport |
|
|
Term
Summary: Lipids
- products of digestion
- site of absorption
- mechanism |
|
Definition
- FA, monoglycerides, cholesterol
- SI
- bile salts form micelles in SI
diffusion of FA, monoglycerides, and cholesterol into intestinal cells
reesterification in the cell to TG and phospholipids
chylomicrons form in the cell (requiring apoprotein) and are transferred to lymph |
|
|
Term
Summary: Fe2+
- products of digestion
- site of absorption
- mechanism |
|
Definition
- Fe3+ reduced to Fe2+
- SI
- binds to apoferritin in the intestinal cell
binds to transferrin in blood |
|
|
Term
Summary: Ca2+
- site of absorption
- mechanism |
|
Definition
SI
Vit D-dependent Ca2+-binding protein |
|
|
Term
Summary: fat-soluble vitamins
- site of absorption
- mechanism |
|
Definition
SI
micelles form with bile salts and products of lipid digestion
diffusion into the intestinal cell |
|
|
Term
Summary: water-soluble vitamins
- site of absorption
- mechanism |
|
Definition
SI
Na+-dependent cotransport |
|
|
Term
Summary: Vit B12
- site of absorption
- mechanism |
|
Definition
|
|
Term
Summary: bile salts
- site of absorption
- mechanism |
|
Definition
ileum
Na+-salt acid cotransport |
|
|
