Download Digestion and Absorption Part 2

GI Lecture #42
Monday, Mar. 3, 9:00 am
Dr. Gwirtz
Brian Anderson
Page 1 of 4
Digestion and Absorption Part 2
This is a continuation of the 8 am lecture that I call Part 1.
I. Mechanisms of Iron Absorption
a. Iron along with calcium are absorbed as needed by the body
b. Two forms of iron are absorbed in the small intestine (at the brush border)
i. Ferric= Fe3+ (Heme form usually from meat sources)
ii. Ferrous= Fe 2+ --usually much easier to absorb
c. Mechanisms to convert Fe 3+  Fe 2+
i. Gastroferrin- secreted by parietal cell
ii. Vitamin C
iii. Stomach acid
d. Fe 2+ is bound in the cell by an apoferritin called mobil ferrin
e. In the basilar membrane there is a receptor for iron in the blood, transferrin.
When blood iron is low the transferrin is considered free.
f. Free ferritin causes movement away from the apoferritin, into the blood
g. Amount of iron absorbed depends on the amount of free ferritin. Low blood
iron increases free ferritin increasing absorption.
II. Vitamin Absorption
a. Water soluble- easily absorbed by various transport mechanisms
i. C, B1, B2, niacin, B6, biotin, folic acid, B12
b. * Vitamin B12 requires intrinsic factor (IF); terminal ileum (only
mechanism to worry about)
1. binds to IF in lumen which aids in absorption into epithelium
2. taken up by mitochondria then released to be taken into the portal blood
c. Fat soluble
i. A, D, E, K
ii. vitamins travel with lipids, packaged in chylomicrons and go into the
lacteals where they enter the lymphatic system and then systemic circulation
III. Absorption in Large Intestine
a. 1500 ml enter colon, most of water and electrolytes absorbed, 50-200 ml fluid
excreted in feces (depending on dietary factors such as fiber)
b. Proximal colon: active absorption of Na+, Cl- (via electrochemical gradient),
and water (via osmosis)
c. Distal colon: active secretion of HCO3 - in exchange for Cl- (buffers acid
produced by intestinal bacterial)
IV. Summary of Major Sites of Nutrient Absorption
a. Stomach- some lipid soluble (drugs, alcohol, aspirin)
b. Duodenum- carbohydrates, protein, lipids, (calcium, folate, and iron in 1st
part)
c. Jejunum- carbohydrates, protein, lipids
GI Lecture #42
Monday, Mar. 3, 9:00 am
Dr. Gwirtz
Brian Anderson
Page 2 of 4
d. Ileum- bile acids, vitamin b12
e. Colon- electrolytes and water
f. Need to know sites of absorption b/c diff. consequences when removing
i. removing stomach will not cause malabsorption except intrinsic factor
ii. duodenum removal leads to nutritional deficiency (CHO, AA, lipid,
Vit)
iii. remaining regions can begin to replace the lost function (except for
ileum, such as in Crohn’s Disease or ulcerative colitis, Vit B12 must be
replaced
V. Bacterial Action in Colon
a. Numerous bacteria, especially colon bacilli
b. Substances formed by bacteria: able to be absorbed in colon (unknown mech.)
i. vitamin K (important for blood coagulation)
ii. vitamin B12
iii. thiamin
iv. riboflavin
v. various gases- to be expelled
VI. Composition of Feces
a. Water
95%
b. Composition of Solids:
c. Dead bacteria
30%
d. Fat
10-20%
e. Inorganic matter
10-20%
f. Protein
2-3%
g. Undigested fiber
h. Bile Pigments – small percentage
i. Sloughed epithelial cells – from esophagus, stomach, intestines, and colon
j. Color due to stercobilin and urobilin- bile pigments
k. Odor due to indole, skatole, mercaptans and hydrogen sulfide – made by
bacteria
VII. Common Causes for Malabsorption
a. Enzyme deficiency (often inherited)
a. Lactase deficiency
b. Bile salt deficiency (liver disease)- poor lipid soluble vitamin absorption
c. Drug therapy- can increase gut motility
d. Infection- bacterial or viral affect secretions and motility
e. Mucosal disease of the small bowel
a. celiac disease – immune d/o pt becomes allergic to gluten
f. Stomach: postgastrectomy,
a. achlorhydria (lack of acid/enzyme prod),
b. Zollinger-Ellison Syndrome (acid/enzyme over production by parietal
cells inactivating pancreatic enzymes causing malnutrition)
GI Lecture #42
Monday, Mar. 3, 9:00 am
Dr. Gwirtz
Brian Anderson
Page 3 of 4
g. Liver: chronic bile salt deficiency: cirrhosis,
a. depletion of bile salt pool due to ileal disease or resection, unconjugated
bile salts due to bacterial overgrowth
b. leads to malnutrition and poor digestion of lipids and lipid soluble vit.
h. Pancreas:
a. pancreatic exocrine insufficiency -lack of enzymes for digestion and
bicarb for neutralization of acid
b. chronic pancreatitis, cystic fibrosis, neoplasm
i. plug up secretions preventing there release into the duodenum;
(could be a gall stone blocking the duct)
i. Small Intestine:
a. Celiac disease colon
b. Crohn’s disease-inflammatory disease of terminal ileum
c. Whipple’s disease –bacterial infection deactivating brush border enzymes
d. lactase deficiency
e. Tropical sprue – bacterial infection affecting brush border
f. short bowel syndrome – following removal of intestinal region following
ulcer or cancer
g. bacterial overgrowth – growth of resistant strains following antibiotics
h. AIDS
i. diabetes mellitus
j. motility disorders
k. infectious causes
j. Bulimics do not make nutrients available for absorption
k. Laxative overuse causes material to pass too quickly for effective absorption
VIII. Pancreatitis
a. Inflammation of the pancreas
b. Common causes
a. alcohol abuse
b. blockage of papilla of Vater with gallstone so enzymes and HCO3- do not
reach small intestine
c. Enzymes dammed up in ducts and acini
a. overcome trypsin inhibitor which keeps proteases inactive
b. trypsin activated, activates peptidases
c. rapid digestion of pancreas and ductal cells
d. death can result when proteases in blood stream digest various organs
IX. Malabsorption from the Small Intestine
a. Idiopathic sprue (a.k.a. celiac disease or gluten enteropathy)
b. due to toxic effects of gluten which is present in grains such as wheat and rye
c. destroys villi via immunological reaction causing flattened epithelium
decreasing surface area for absorption >malnutrition
d. fat digestion first impaired leading to steatorrhea due to undigested fat in stool
(will come in for fatty stool)
GI Lecture #42
Monday, Mar. 3, 9:00 am
Dr. Gwirtz
Brian Anderson
Page 4 of 4
e. severe: absorption of protein, CHO, and other nutrients impaired
X. Lactase deficiency
d. no digestion or absorption of lactose b/c lactase is missing from the brush
border
e. lactose is fermented by the bacteria in the colon
f. osmotic diarrhea with lots of gas – water is pulled into the colon
*Last slide is a chart from the textbook describing the nutritional deficiency due to
various diseases.
Major GI diseases and associated nutritional deficiencies
Celiac disease – mainly affects the duodenum and jejunum impairing fat absorption and
lactose hydrolysis
Chronic Pancreatitis – first enzymes affected are the lipases impairing fat digestion
Resection of Ileum/ Crohn’s disease of Ileum – impaired vitamin B12, and bile acid
absorption causing a decrease in the bile salt pool leading to poor lipid digestion
Lactase deficiency – will affect mainly the small intestine