(8/19/10) - Malabsorption

1. Build on information and concepts delivered in the session titled “Introduction to Digestion”.

2. Understand the mechanism for normal gastrointestinal absorption of nutrients, including:

A. Carbohydrates.

Amylase, glucoamylase, isomaltase, sucrase, and lactase hydrolyze starches, maltase, maltotriose, sucrose, and lactose into glucose, fructose, and galactose for use by the body. GLUT5 (for fructose) and SGLT1 (for glucose and galactose) move these monosaccharides across the brush border membrane.

B. Protein.

Amino acids, and di/tri-peptides are transported across intestinal epithelial cells by active transporters. Amino acids are transported by Na+-dependant transporters. Di/Tri-peptides are transported by the H+-coupled transporter, PepT1.

C. Fat (and fat-soluble vitamins, (A, D, E, K)

Bile emulsifies fats (lipids) into digestible micelles. Monoglycerides, cholesterol, and fat-soluble vitamins in micelles are absorbed by diffusion through villi plasma membranes. Medium-chain and short-chain fatty acids (less than 10 carbon atoms) transport across intestinal epithelial cell basolateral membrane into capillaries and then portal circulation. Long-chain fatty acids-are transported across microvillus membrane, bound to fatty-acid binding proteins and used to construct triglycerides.

D. Vitamins.

E. Minerals.

Calcium absorption is dependant on vitamin D.

Iron absorption is enhanced by vitamin C and alcohol and inhibited by calcium.

3. Understand the clinical presentations of malabsorption due to defects in these normal mechanisms.

see malabsorption handout

4. Be aware of symptoms, signs and diagnostic strategies used in identifying disease processes of malabsorption, particularly related to:

A. Lactose Intolerance.

Bloating, gas, and diarrhea after ingestion of milk or dairy products; diagnosis made by empirical treatment with lactose-free diet or by hydrogen breath test after oral administration of lactose

B. Celiac Disease

Inflammation of small intensetine precipitated by ingestion of wheat, rye, barley (gluten) in individuals with certain genetic predispositions. Presents watery diarrhea, weight loss, and the appearance of vitamin/mineral deficiencies. All nutrients are malabsorbed. Diagnosed with a small intestin biopsy that checks for characteristic changes (see handout), improvement with gluten-free diet, serological tests (testing for antibodies to certain things in blood).