Malabsorption syndromes are a heterogenous group of conditions that can cause distressing gastrointestinal symptoms. Celiac disease is most common and is triggered by exposure to gluten. Tissue transglutaminase immunoglobulin A is the diagnostic test of choice; management is gluten avoidance. Lactose intolerance is caused by absence or declining levels of the enzyme lactase. Diagnosis typically is clinical, but breath tests can be helpful if diagnosis is uncertain. Management is lactose avoidance. Bile acid malabsorption results in unabsorbed bile acids in the colon, leading to diarrhea. The ⁷⁵selenium homotaurocholic acid test is most accurate but is not widely available. Therefore, a trial of bile acid sequestrants (typically cholestyramine) is a reasonable alternative when the diagnosis is suspected. Exocrine pancreatic insufficiency is caused by decreased production of pancreatic enzymes, typically occurring in patients with preexisting pancreatic damage from alcohol, surgery, radiation, diabetes, or cystic fibrosis. Diagnosis involves fecal fat or fecal elastase-1 tests. Management is pancreatic enzyme replacement. Small intestinal bacterial overgrowth is caused by pathologic overgrowth of the small bowel microbiome. Diagnosis is by jejunal biopsy or, more commonly, breath tests. Antibiotics (typically rifaximin) are the initial management. Other options include dietary changes, probiotics, and prokinetic drugs.

Case 4. RB is a 32-year-old woman who comes to your office for abdominal pain and bloating that has been going on for years, but which has worsened in recent months. RB describes foul-smelling, watery, nonbloody stools and reports that many foods seems to worsen the symptoms. On questioning, she tells you she has noticed that foods containing lactose and gluten particularly worsen symptoms. However, many foods still exacerbate the symptoms even when she is consuming a very restrictive elimination diet.