The term “malabsorption” denotes disorders in which there is a disruption of digestion and nutrient absorption. The clinical and laboratory manifestations of malabsorption are summarized in Table 15–11.
++ Table Graphic Jump Location Table 15–11.Clinical manifestations and laboratory findings in malabsorption of various nutrients. ||Download (.pdf) Table 15–11. Clinical manifestations and laboratory findings in malabsorption of various nutrients.
|Manifestations ||Laboratory Findings ||Malabsorbed Nutrients |
|Steatorrhea (bulky, light-colored stools) ||Increased fecal fat; decreased serum cholesterol; decreased serum carotene, vitamin A, vitamin D ||Triglycerides, fatty acids, phospholipids, cholesterol. Fat-soluble vitamins: A, D, E, K |
|Diarrhea (increased fecal water) ||Increased stool volume and weight; increased fecal fat; increased stool osmolality gap ||Fats, carbohydrates |
|Weight loss; muscle wasting ||Increased fecal fat; decreased carbohydrate (D-xylose) absorption ||Fat, protein, carbohydrates |
|Microcytic anemia ||Low serum iron ||Iron |
|Macrocytic anemia ||Decreased serum vitamin B12 or red blood cell folate ||Vitamin B12 or folic acid |
|Paresthesia; tetany; positive Trousseau and Chvostek signs ||Decreased serum calcium or magnesium ||Calcium, vitamin D, magnesium |
|Bone pain; pathologic fractures; skeletal deformities ||Osteopenia on radiograph; osteoporosis (adults); osteomalacia (children) ||Calcium, vitamin D |
|Bleeding tendency (ecchymoses, epistaxis) ||Prolonged prothrombin time or INR ||Vitamin K |
|Edema ||Decreased serum total protein and albumin; increased fecal loss of alpha-1-antitrypsin ||Protein |
|Milk intolerance (cramps, bloating, diarrhea) ||Abnormal lactose tolerance test ||Lactose |
Normal digestion and absorption may be divided into three phases: intraluminal, mucosal, and absorptive.
A. Intraluminal Phase
Dietary fats, proteins, and carbohydrates are hydrolyzed and solubilized by pancreatic and biliary secretions. Fats are broken down by pancreatic lipase to monoglycerides and fatty acids that form micelles with bile salts. Micelles are important for the solubilization and absorption of fat-soluble vitamins (A, D, E, K). Proteins are hydrolyzed by pancreatic proteases to di- and tripeptides and amino acids. Impaired intraluminal digestion may be caused by insufficient intraluminal concentrations of pancreatic enzymes or bile salts. These conditions will not be covered in detail here (see Chapter 16).
Pancreatic insufficiency may be caused by chronic pancreatitis, cystic fibrosis, or pancreatic cancer. Pancreatic enzymes may also be inactivated within the intestinal lumen by acid hypersecretion (Zollinger-Ellison syndrome). Significant pancreatic enzyme insufficiency generally results in significant steatorrhea (due to malabsorption of triglycerides)—often more than 20–40 g/24 h—resulting in weight loss, gaseous distention and flatulence, and large, greasy, foul-smelling stools. The digestion of proteins and carbohydrates is affected to a far lesser degree and is generally not clinically significant. Because micellar function and intestinal absorption are normal, signs of other nutrient or vitamin deficiencies are rare.
Decreased bile salt concentrations may be due to biliary obstruction or cholestatic liver diseases. Because bile salts are resorbed in the terminal ileum, resection or disease of this area (eg, Crohn disease) can lead to insufficient intraluminal bile salts. Finally, destruction or loss of bile salts may be caused by ...