at: inbox

Gastroenterology - Malabsorption - Fast Facts | NEJM Resident 360

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 Pancreatitis section.

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

Additional Clinical Features Associated with Malabsorption of Specific Nutrients

Macronutrient	Symptoms
Fat	Weight loss
Steatorrhea
Protein	Edema
Muscle atrophy
Amenorrhea
Carbohydrates	Watery diarrhea
Increased flatulence
Milk intolerance

Evaluation

• Stool-test fat malabsorption: A 72-hour quantitative stool assay is the historic gold standard but no longer used as frequently. A qualitative stool-fat assay that examines for fat droplets by lipid staining 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

• • iron deficiency:

  • vitamin B₁₂/folate

  • fat-soluble vitamins

  • protein and albumin

  • fat malabsorption:

• Imaging tests can include:

• • 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

Small Intestinal Bacterial Overgrowth

Small intestinal bacterial overgrowth (SIBO) is defined as excess colonic bacteria in the small intestine (>10⁵ CFU/mL). The following table summarizes conditions that predispose to SIBO:

Conditions That Predispose to SIBO

| Structural
Abnormalities | Motility
Abnormalities | Biochemical
Abnormalities | Immune Deficiency | Other | | --- | --- | --- | --- | --- | | Small-bowel
diverticula

Small-bowel
strictures or fistula
(Crohn disease)

Blind intestinal loops

Incompetent
ileocecal valve

Adhesions | Primary dysmotility
(gastroparesis)

Scleroderma

Hypothyroidism

Type 2 diabetes

Medications
(opiates,
anticholinergics)

Irritable bowel
syndrome | Hypochlorhydria
(atrophic gastritis,
PPI use)

Chronic pancreatitis | IgA deficiency

HIV

Combined variable
immunodeficiency | Liver
cirrhosis |

Abbreviation: PPI, proton pump inhibitor

Evaluation

Diagnosis of SIBO is based on laboratory tests demonstrating 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.

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) is considered a positive test.

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, ciprofloxacin, doxycycline, metronidazole, and neomycin. Data are limited regarding the efficacy of probiotics and dietary modification.

Celiac Disease

Celiac disease is a gluten-induced immune-mediated enteropathy associated with frequent systemic manifestations and nutritional deficiencies (including B₁₂, folate, zinc, iron, and vitamin D).

Evaluation

First-degree relatives of patients with celiac disease are at increased risk of developing the disease. Therefore, assessment of family history is important. Investigation for suspected celiac disease begins with screening for serum antibodies, as 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)

• elongation of the crypts

• 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. Untreated or partially treated celiac disease is also associated with extraintestinal manifestations including joint pain, dermatitis herpetiformis, urticaria, poor growth, short stature, and increased risk of malignancy and osteoporosis. Patients should be followed every 12 months to monitor dietary adherence, weight gain, and correction of nutritional deficiencies. Celiac-specific autoantibodies should be measured at least every 6–12 months after initiation of a gluten-free diet. Once a patient has normalized, it is presumed that dietary adherence has resulted in mucosal recovery. ACG guidelines recommend follow-up endoscopy and duodenal biopsy to confirm mucosal recovery.

inbox