Malabsorption

Despite the distinct underlying pathological mechanisms of malabsorption and maldigestion, the term malabsorption is used to refer to both disorders in clinical practice. Malabsorption (e.g., celiac disease, lactose intolerance) describes a malfunction of the intestinal wall, resulting in the insufficient absorption of breakdown products. Maldigestion (e.g., exocrine pancreas insufficiency or cholestasis) is caused by an intraluminal disorder (insufficient secretion of pancreatic enzymes or bile), which prevents the adequate breakdown of food in the intestinal lumen. Both disorders can lead to malnourishment, which, in children, may manifest with delayed and inadequate physical development. Adults mainly present with chronic diarrhea, weight loss, and signs of malnutrition (e.g., iron deficiency anemia). Diagnostic tests assess the digestion of individual food components. Management focuses on the underlying disease; e.g., patients with celiac disease should adhere to a gluten-free diet.

Malabsorption disorders can be caused by either the insufficient absorption or digestion of nutrients. The condition can be further defined as global or partial.

• Global malabsorption: in diseases causing diffuse mucosal damage or a reduction of the absorptive surface (e.g., celiac disease)
• Partial malabsorption: caused by a localized absorption impairment, resulting in deficiencies of specific nutrients (e.g., vitamin B₁₂ deficiency in patients with diseases affecting the terminal ileum)

Maldigestion

Impaired breakdown of food in the intestinal lumen

• Exocrine pancreatic insufficiency; → lack of enzymes → reduced hydrolysis of carbohydrates, proteins, and lipids
  • Cystic fibrosis
  • Shwachman-Diamond syndrome: rare autosomal recessive condition and second most common cause of exocrine pancreatic insufficiency in children (after cystic fibrosis)
• Following gastric resection → lack of gastric acid → impaired breakdown of nutrients
• Bile acid deficiency (e.g., cholestasis, bile acid malabsorption) → incomplete emulsification of fats
• Medications: Orlistat (reversibly inhibits gastric and pancreatic lipase)

Malabsorption

Impaired absorption of digested food caused by alterations of the intestinal mucosa

• Inflammatory bowel disease; (e.g., Crohn's disease)
• Celiac disease (gluten-sensitive enteropathy)
• Infections (e.g.,tropical sprue; , giardiasis, traveler's diarrhea, Whipple's disease)
• Lactose intolerance
• Small intestine resection (short bowel syndrome)
  • Definition: a condition in which the small intestine is unable to adequately absorb nutrients, water, and electrolytes
  • Etiologies: surgical resection (e.g., for Crohn disease or trauma), congenital abnormalities
  • Complications: intestinal failure that requires intravenous supplementation
• Small intestinal bacterial overgrowth (SIBO; blind loop syndrome)
• HIV enteropathy

References:^{[1][2][3][4][5][6]}

General symptoms

• Global malabsorption
  • Diarrhea (most common), steatorrhea, abdominal distension, flatulence
  • Weight loss, fatigue
  • Additional symptoms of specific deficiencies
• Partial (isolated) malabsorption: only symptoms specific to individual nutrient deficiencies (e.g., impaired cobalamin absorption with megaloblastic anemia)

Deficiencies

• Deficiency of fat-soluble vitamins (e.g., night blindness and xeroderma due to vitamin A deficiency; osteomalacia and rickets due to vitamin D deficiency)
• Deficiency of water-soluble vitamins (vitamin B complex, vitamin C)
• Zinc deficiency
  • Etiology
    • Inadequate dietary intake
    • Acrodermatitis enteropathica (congenital deficiency)
    • Crohn disease, liver and renal disease
  • Clinical findings: immune dysfunction, impaired wound healing, hypogonadism, diarrhea, dermatitis, alopecia, abnormal taste and smell
  • Diagnosis: measurement of plasma zinc levels
  • Treatment: oral zinc supplementation
• Other deficiencies
  • Protein: edema, cachexia
  • Iron deficiency anemia or microcytic anemia
  • Electrolytes: hypokalemia or hypocalcemia

References:^[1][2][7][8]

• Blood tests: macrocytic and/or microcytic anemia; ↓ electrolytes, ↓ total protein, vitamin deficiencies
• Stool tests
  • Analysis of fecal fat over 72 hours
  • Detection of pathogens
• D-xylose absorption test: assesses the absorptive function of the upper small intestine
  • Overnight fast → collection of urine for 5 hours following oral administration of 25 g xylose → if urine xylose levels < 4 g/5 h, diagnostic for malabsorptive disorders with damage to the intestinal mucosa (e.g., celiac disease, Whipple's disease)
  • Collection of a venous blood sample after one hour → levels < 20 mg/dL of serum D-xylose are pathological
• Hydrogen breath test: assess the intestinal absorption of individual carbohydrates
  • Lactulose breath test: determination of orocecal transit time (small intestine transit time)
  • Glucose hydrogen breath test: if abnormal bacterial colonization of the small intestine is suspected
• Further testing for underlying diseases: e.g., celiac disease or lactose intolerance

References:^[1][2][9]

Protein-losing enteropathy

• Etiology
  • Massive loss of protein caused by various gastrointestinal disorders
  • Erosive or ulcerative (e.g., Crohn's disease, clostridium difficile infection)
  • Increased intestinal permeability to proteins (e.g., celiac disease, bacterial overgrowth)
  • Impaired lymphatic circulation (e.g., intestinal lymphangiectasia, Whipple's disease)
• Clinical features: hypoalbuminemia with peripheral edema
  • In cases of systemic diseases, symptoms may be those of the underlying condition.
  • Further gastrointestinal symptoms may occur depending on the underlying disorder.
• Treatment
  • Treatment of the underlying disease

The differential diagnoses listed here are not exhaustive.