Lecture 11: Malabsorption Flashcards
What are the two main types of malabsorption?
Global and Partial/Isolated Malabsorption
What are the features of global malabsorption?
- Diffuse mucosal involvement due to a disease process
- Decreased absorptive surface
- Decreased absorption of almost all nutrients
What are the symptoms of global malabsorption?
- No symptoms
- Mild ex. anorexia
- More severe ex. diarrhea, weight loss despite eating
- Symptoms specific to the underlying disease process like cystic fibrosis, Zollinger-Ellison Syndrome, carcinoid syndrome, and celiac disease
What are features of partial/isolated malabsorption?
Diseases that interfere with absorption of specific nutrients like pernicious anemia (deficient Vit B12 absorption)
What is the general approach to malabsorption? Give two examples of each.
- Basic laboratory assessment
ex. Albumin, Vit B12, Fe, Vit D - Specific lab investigations
ex. Celiac testing, Fat malabsorption testing - Imaging
Abdominal ultrasound/CT, endoscopy
What are the three different types of malabsorptions for macronutrients?
Fat, Protein and Carbohydrates
What are the causes of fat malabsorption? Give two examples of each.
- Lipase deficiency
ex. cystic fibrosis, chronic pancreatitis - Bile salt deficiency
ex. liver failure, chronic cholestasis - Lack of an absorptive surface
ex. celiac disease, inflammatory bowel disease
What are the clinical syndromes of fat malabsorption?
- Diarrhea w/o flatulence
- Steatorrhea - pale, floating, foul-smelling
- Deficiency of fat soluble vitamins (A, E, D, and K)
What can we do to test for fat malabsorption? Explain the results.
- 72 hr fecal fat determination
Bad = > 7g/day - Acid steatocrit
Increased fat to total volume ratio - Microscopic fecal fat analysis
Red droplets under microscope, more and big is bad - TG and Vit D ➔ decreased
- PT ➔ prolonged
What are the limitations for 72 hr fecal fat determination?
- Patient compliance
- Must have diarrhea
- Fecal fat can be moderately elevated in diarrheal diseases w/o fat malabsorption
What are the limitations for acid steatocrit?
Good qualitative but not quantitative
Only can detect steatorrhea when >11g/day
How does microscopic fecal fat analysis work?
- Stool treated with Oil Red O which stains neutral fats only and makes red droplets under light microscopy; presence of TG means a deficiency in pancreatic lipase a.k.a fat digestion problem
- Sample is acidified + heated + Oil Red O again which neutralizes split fats (FAs) which can now take up Oil Red O; means deficiency in absorption
tl;dr Step 1 ➔ neutral, Step 2 (acid + heat) ➔ split
What are the causes of carbohydrate malabsorption? Give two example of each.
- Enzyme deficiency
ex. Congenital amylase deficiency, Sucrase deficiency - Lack of an absorptive surface
ex. Celiac disease, Inflammatory bowel disease
What is the clinical presentation of carbohydrate malabsorption?
- Abdominal distension, bloating
- Watery diarrhea
- Onset of symptoms usually within 90 mins of ingestion
What are the tests for carbohydrate malabsorption and what are the results suggestive for malabsorption?
- Stool pH
pH < 5.6; screening test - Fecal osmotic gap
Increased stool osmotic gap - Stool Reducing Substances
Presence of stool reducing substances like maltose and lactose - Lactose tolerance test
+ve - Lactose hydrogen breath test
Increased breath H₂
Explain how the fecal osmotic gap test works.
a. If osmotic gap > 125 mOsm/kg:
This means that there are unabsorbed materials in the gut which lead to water retention and hence decreases the amount of Na and K in the stool which leads to an increase in the fecal osmotic gap
b. If osmotic gap < 50 mOsm/kg:
This means that there is a decrease in the absorption of electrolytes from gut lumen or an increase in the secretion of electrolytes into gut lumen hence increasing the amount of electrolytes passing through the GI tract, which leads to a decrease in the fecal osmotic gap
To calculate the fecal omsotic gap:
Fecal osmotic gap = 290 - 2 x (fecal Na + fecal K)
Explain how the stool reducing substances test works.
This test is based on Fehling’s reaction
Reducing sugar (lactose or maltose) oxidizes Cu²⁺ which is blue in colour, to Cu⁺ which is red in colour, under alkaline conditions
This doesn’t work on sucrose because it does not have a hemiacetal nor a hemiketal group
Explain how the lactose tolerance test works.
Used for diagnosis of lactose intolerance due to decreased levels of intestinal lactase
1. Give 50 g of oral lactose
2. Blood samplesssss over 2 hr period
3. Glucose concentration
Increase in blood glucose by < 1.1 mmol/L + development of symptoms is diagnostic of lactose intolerance
What are the limitations of the lactose tolerance test?
- Inaccurate if you have diabetes, bacterial overgrowth or abnormal gastric empyting
- Replaced by lactose breath hydrogen test
Explain how the lactose hydrogen breath test works.
- Give 25 g oral lactose (must fast before)
- Breath measured at baseline and every 30 mins for 3 hrs
- H₂ detected by gas chromatography or electrochemical detection
Results:
< 10 ppm = Normal
10 - 20 ppm = Indeterminate unless + symptoms
> 20 ppm = lactose malabsorption
What are the pros and cons of the lactose hydrogen breath test?
Pros:
1. Simple, non-invasive
2. Better sensitivity and specificity than lactose tolerance test
Cons:
1. False +ve = inadequate fasting before test or smoking
2. False -ve = recent antibiotics, delayed gastric emptying or gut bacteria doesn’t produce H₂
What are the causes of protein malabsorption? Give 2 examples.
- Enzyme deficiency
a. Cystic Fibrosis
b. Chronic pancreatitis - Lack of an absorptive surface
a. Muscle atrophy
b. Edema of the lower extremities
What are lab findings suggestive of protein malabsorption?
- Hypoalbuminemia
- Hypoproteinemia
What is a test used for protein malabsorption?
Fecal elastase
How does fecal elastase work? How can it tell that protein malabsorption is caused by enzyme deficiency or from lack of an absorptive surface?
Elastase is secreted from the pancreas and catalyzes the hydrolysis of certain peptide bonds. Small amounts of elastase transits the entire bowel and is present in the stool.
An abnormal low amount of fecal elastase suggest pancreatic insufficiency. Lack of an absorptive surface will show normal levels of elastase in the stool.
