2. Malabsorption of carbohydrate
A 21-year-old student at a engineering school. During the past 6 months, she experienced several bouts of severe abdominal bloating and cramps, followed by diarrhea. At first, she though these episodes were caused by the stress of her demanding academic program. However, she noticed that the symptoms occurred approximately 1 hour after she drank milk or ate ice cream. On a visit home, she mentioned the symptoms to her mother, who exclaimed, “Don’t you know that your father and I have never been able to drink milk?” She was examined by her primary care physician, who found her to be excellent health. Because her symptoms
were temporally related to ingestion of dairy products, the physician ordered a lactose-H2 breast test, which confirmed that she has lactose intolerance. Her fecal osmolar gap was measured and was elevated. As further confirmation of the diagnosis, she abstained from dairy products for 1 week and had no episodes of bloating, cramping, or diarrhea.
1. How are dietary carbohydrates digested in the gastrointestinal tract? What are the roles of salivary, pancreatic, and intestinal mucosal brush border enzymes in carbohydrate digestion? What three monosaccharides are the final products of these digestive steps?
2. How are dietary carbohydrates absorbed from the lumen of the gastrointestinal tract into the blood? Draw a small intestinal epithelial cell that shows the appropriate transporters in the apical and basolateral membranes.
3. Describe the steps involved in the digestion and absorption of lactose.
4. Propose a mechanism for her lactose intolerance.
5. Why did her lactose intolerance cause diarrhea?
6. Her lactose-H2 breath test was positive. Why?
7. What is the fecal osmolar gap? Why was her fecal somolar gap elevated?
Her lactose-H2 breath test was positive because undigested lactose in the lumen of the gestrointestinal tract was fermented by colonic bacteria. H2 gas, a byproduct of this fermentation, was absorbed into the bloodstream, expired by the lungs, and then detected in the test.