Digestion and Absorption Flashcards
What is lactose intolerance?
Lactase enzyme deficiency; lactose cannot be broken down so remains undigested and unabsorbed
Describe the sequence of events for lactose intolerance
- Lactose remains in lumen and brings in H2O
- Passes to the small intestine
- Converted to SCFAs and hydrogen gas
- Produces osmotic diarrhea
What are the carbohydrate digestion locations and which is the main one?
Mouth and Small intestine
- Small intestine = most breakdown done here
What enzyme in the mouth begins to break down starch?
Salivary amylase
What enzyme in the small intestine continues to break down starch?
Pancreatic amylase
What are some primary sugars?
Starch, lactose and sucrose
Where does most starch (carbohydrate) digestion and absorption occur?
Small intestines - duodenum
What is the site of activity for most digestive enzymes in the small intestine where large amounts of nutrients are absorbed?
Brush border - microvillar surface
Starch is broken down into?
Maltose - > Glucose + Glucose
Trehalose is broken down into?
Glucose + Glucose
Lactose is broken down into?
Glucose + Galactose
Sucrose is broken down into?
Glucose + Fructose
The brush border is important for what?
Digestion and absorption of many carbohydrates such as starch, lactose and sucrose
How does glucose or galactose get absorbed into the duodenal epithelial cell?
Secondary active transport using SGLT1
How does fructose get absorbed into the duodenal epithelial cell?
Facilitated diffusion using GLUT5
How does glucose, fructose, galactose reach the blood after being absorbed by duodenal epithelial cells?
Facilitated transport using GLUT2
What would need to be inhibited in order to inhibit glucose absorption?
SGLT1 transporter in the duodenal epithelial cells
What are 2 ways to test for carbohydrate assimilation disorders (malabsorption)?
- D-xylose test
2. Lactose/Sucrose/Methane breath tests
Describe the D-xylose test
Testing for abnormal carbohydrate assimilation;
- Ingest D-xylose (pentose monosaccharide)
- Measure urine excretion of D-xylose
- Abnormal and LOW amounts of D-xylose in the urine = malabsorption of carbohydrates
What are the protein assimilation disorders?
- Chronic pancreatitis
- Congenital trypsin absence
- Cystinuria
- Hartnup’s disease
- Cystic Fibrosis
What are the main locations for protein digestion?
Stomach
Pancreas
Small intestine
What enzyme in the stomach can begin protein digestion?
Pepsin
What organs’ enzymes primarily break down proteins into di- and tripeptides and some amino acids?
Pancreatic enzymes
Where are most of the protein broken down pieces (di and tripeptides and amino acids) absorbed?
Small intestine
What 2 things can activate the pancreatic enzymes for protein digestion?
Enterokinase at the brush border in small intestine OR
Trypsin itself
Describe how to activate pancreatic enzymes for protein digestion
In the small intestine:
- Trypsinogen is converted to Trypsin by ENTEROKINASE at brush border
- Trypsin then activates ALL of the rest of the pancreatic enzymes
What are the main pancreatic enzymes that are activated by Trypsin?
Trypsin
Chymotrypsin
Elastase
Carboxypeptidase A and B
Deficiency in pancreatic enzymes or defect in transporters of intestinal epithelial cells will result in?
Protein assimilation disorders
Pancreatic enzymes are released as pro-enzymes, meaning what?
They are secreted into the duodenum (SI) as their inactive form; they will be activated by enterokinase at the brush border or by Trypsin itself
Chronic pancreatitis
Deficiency in pancreatic enzymes = lack of proteases
- Cannot neutralize stomach acid as well
Congenital Trypsin absence
Absence of Trypsin = ALL pancreatic enzymes are GONE and cannot be activated!
Why is Congenital Trypsin absence so detrimental?
Trypsin activates ALL pancreatic enzymes for protein digestion - without trypsin they will not be activated for digestion
Cystinuria
Defect in transport/uptake of basic AA
- Kidney will be affected bc cannot absorb AA either
- AA will be secreted in urine and feces
Hartnup’s Disease
Defect in transport/uptake of neutral AA
- Kidney will be affected bc cannot absorb AA either
- AA will be secreted in urine and feces
Cystic Fibrosis
CFTR Cl- channel mutation = impaired bicarbonate (HCO3-) secretion from the pancreatic ducts
- inability to neutralize stomach acid and incorrect functioning of proteases
What are 2 lipid assimilation disorders?
Celiac sprue (disease) Tropical sprue
What is celiac sprue?
Antibodies develop against gluten
- leads to destruction of intestinal villi and malabsorption of nutrients
What are the symptoms of celiac sprue?
Abdominal pain, diarrhea, steatorrhea, weight loss, tingling in extremities
Celiac sprue patients need to cut what out of their diet?
Gluten
What is tropical sprue?
Infection causes loss of intestinal microvilli
- impaired lipid absorption
Tropical sprue main symptom?
Diarrhea
Where does most of lipid digestion occur?
Small intestine
Enzyme found in the mouth for lipid digestion?
Lingual lipase
CCK function for lipid digestion in the stomach?
Delays gastric emptying and increases gallbladder emptying
What occurs in the small intestine for lipid digestion?
Bile salt emulsification with the help of pancreatic lipases
What does pancreatic lipase need to displace bile salts in order to digest lipids?
Colipase
Describe the order of events in the small intestine for lipid digestion
- Bile salts form a micelle to solubilize TAGs
- Micelle contents diffuse across SI cell membrane
- Micelle contents are re-esterified
- Formation of a chylomicron
- Chylomicron is released in lymphatics
Problems anywhere in the process of lipid digestion result in?
Steatorrhea
Lipid digestion problems can arise if what 2 major players are dysfunctional?
Pancreatic enzyme secretions OR
Bile salt and micelle formation
How does ileal resection affect lipid digestion?
Bile acids are recycle in the distal ileum for reuse. If there is no distal ileum, there will be deficient bile salts to create micelles for lipid digestion
How does SIBO affect lipid digestion?
Small intestine bacteria overgrowth;
Bacteria deconjugates bile salts and will cause an impaired micelle formation if too much bacteria
What are the fat soluble vitamins?
ADEK
What are the water soluble vitamins?
B and C
How are ADEK vitamins absorbed?
Same process as lipids
How are B and C vitamins absorbed (besides B12)?
Na+ dependent transport
What is vitamin B12 important for?
Metabolic functions
Nerve myelination
RBC maturation
What is necessary for vitamin B12 absorption and where does it occur?
Intrinsic factor
Distal ileum
Pernicious anemia
Failure of RBC to mature due to vitamin B12 deficiency due to failure of parietal cells in the stomach to release intrinsic factor
What are 2 causes for pernicious anemia?
- Gastritis - inflammation of stomach kills parietal cells
2. Autoimmune - body attacks parietal cells or IF
What 2 surgeries affect vitamin B12 absorption?
Gastrectomy
Gastric bypass
Vitamin D is needed for?
Calcium absorption
Vitamin C is needed for?
Iron absorption
Jejunum absorbs what electrolyte?
Sodium
Ileum absorbs and secretes which electrolytes?
Absorbs sodium
Secretes bicarbonate
Colon absorbs and secretes which electrolytes?
Absorbs sodium
Secretes potassium
Cholera toxin
Causes an increase in Cl- secretion in colon
- Na+ and H2O follow
= massive secretory diarrhea
