Malabsorption

Malabsorption is the impaired absorption of nutrients in the gastrointestinal (GI) tract, caused by congenital or acquired defects in the epithelial absorptive surface or failure of secretion of digestive enzymes by the pancreas. In this section, we review the presentation and diagnosis of malabsorption syndromes and some common causes, including small intestinal bacterial overgrowth (SIBO) and celiac disease. Chronic pancreatitis is covered separately in the section on Pancreatitis in this rotation guide.

After food is ingested, it is broken down further in the stomach by strong contractions and gastric juices containing hydrochloric acid and pepsin, a proteolytic enzyme. Stomach contents pass into the small intestine, where food is further digested in the lumen and absorbed through the mucosa. Pancreatic enzymes digest protein into amino acids, starch into long and short chains of sugars, and fat into fatty acids. Bile salts in the lumen of the duodenum help to absorb fat by solubilizing fatty acids into micelles that facilitate uptake by the villi.

Clinical Features

Malabsorption can be isolated (impaired absorption of single nutrients) or global (impaired absorption of almost all nutrients), and it is important to differentiate between the two based on clinical workup and evaluation.

Clinical symptoms of malabsorption include:

• chronic diarrhea

• unexplained weight loss

• unexplained nutrient deficiencies and related symptoms

Evaluation

• Fecal fat testing: A 72-hour quantitative fecal fat excretion assay is the historic gold standard but no longer used as frequently. A qualitative stool-fat assay that uses lipid staining to examine for fat droplets can be performed on a single stool specimen and is simple, fast, and convenient as a screening test for steatorrhea. (See Chronic Pancreatitis for more on steatorrhea and fat malabsorption.)

• Blood tests do not directly diagnose malabsorption but can screen for nutritional deficiencies. Tests include:

  • complete blood count

  • iron studies

  • vitamins A, D, E, K

  • coagulation studies (indirect marker of vitamin K)

  • triglyceride

  • cholesterol

  • carotene concentration

  • copper, calcium, magnesium, and zinc level

  • vitamin B₁₂/folate

  • protein and albumin

• Imaging findings in malabsorption are nonspecific but can include:

  • CT or MR enterography or barium follow-through to assess the gross morphology of the small intestine for small diverticula or abnormalities associated with bacterial overgrowth

  • CT of the abdomen for signs of chronic pancreatitis, diseases of the bowel, or surgical removal of small bowel

• Specific investigations for potential underlying causes:

  • celiac serology

  • hydrogen breath test for carbohydrate malabsorption

  • stool microscopy and culture, particularly for parasitic infections

  • endoscopy and small-bowel biopsy

  • Fecal alpha-1 antitrypsin

Small Intestinal Bacterial Overgrowth

Small intestinal bacterial overgrowth (SIBO) is defined as a clinical disorder in which signs and symptoms or laboratory abnormalities are attributed to changes in bacterial composition or number of bacteria in the small intestine. The following table summarizes conditions that predispose to SIBO:

Evaluation

• The signs and symptoms of SIBO can include nausea, bloating, increased flatulence, abdominal distension, abdominal pain/cramping, diarrhea, constipation, and symptoms related to malabsorption of nutrients.

• Laboratory tests may demonstrate anemia, low vitamin B₁₂ level, and other signs of malnutrition (e.g., lymphopenia, low serum albumin, and transferrin). Serum folate and vitamin K levels are often elevated, as these are produced by bacteria.

• The diagnosis of SIBO is made with breath tests. Specific tests for SIBO include carbohydrate breath test (carbon-14 D-xylose) and the hydrogen breath test.

  • Hydrogen breath tests are readily available and more widely used. In SIBO, fermentation of carbohydrates in the small bowel produces a large amount of hydrogen gas. Following glucose ingestion, a significant rise in hydrogen level (characterized as a double peak or baseline level >20 parts per million [ppm] or rise >12 ppm from baseline in 90 minutes) is considered a positive test.

• Small bowel aspirate and culture via endoscopy and bacterial counts can be used for diagnosis but is not used often in practice because it is invasive, can be contaminated by oropharyngeal flora, and has limited sensitivity.

Management

• Treatment of SIBO consists of treating the underlying disease, eradicating the overgrowth, and addressing nutritional deficiencies.

• Antimicrobials are the mainstay of treatment, and options include rifaximin, amoxicillin-clavulanic acid, ciprofloxacin, doxycycline, metronidazole, neomycin, tetracycline, and trimethoprim-sulfamethoxazole.

• Data are limited regarding the efficacy of probiotics and dietary modification.

Celiac Disease

Celiac disease is a gluten-induced, immune-mediated small intestinal enteropathy associated with frequent systemic manifestations and nutritional deficiencies (including B₁₂, folate, zinc, iron, and vitamin D). The point prevalence is around 1%.

Evaluation

• Diagnosis of celiac disease involves both serologic and histologic testing from duodenal biopsies on endoscopy.

• Testing should be performed in patients with symptoms suggestive of celiac disease, such as diarrhea, steatorrhea, weight loss, bloating, and abdominal pain.

• First-degree relatives of patients with celiac disease are at increased risk of developing the disease and should be considered for testing even if asymptomatic. Therefore, assessment of family history is important.

• Serum tests for diagnosis of celiac disease are outlined in the following table:

  

  (Source: Celiac Disease. N Engl J Med 2012.)

• Definitive diagnosis of celiac disease requires a duodenal biopsy showing:

  • increased intraepithelial lymphocytes (>25 per 100 enterocytes)

  • crypt hyperplasia

  • partial-to-total villous atrophy

Management

• Most patients with celiac disease can be cured by lifelong adherence to a strict gluten-free diet to eliminate diarrhea and allow healing of the mucosal lesion.

• Assessment for diet adherence by a dietitian is considered standard of care.

• Untreated or partially treated celiac disease is also associated with extraintestinal manifestations including joint pain, dermatitis herpetiformis, urticaria, poor growth/short stature (in children), and increased risk of malignancy and osteoporosis.

• Patients should be followed to monitor and facilitate dietary adherence, for active recognition and treatment of comorbidities and nutritional deficiencies and to provide preventive care.

• Celiac-specific autoantibodies should be measured at least every 6-12 months after initiation of a gluten-free diet.

• Mucosal healing is suggested as an endpoint for therapy and may be considered two years after starting gluten free diet.