malassimilation

Malassimilation is a medical condition characterized by impaired digestion and absorption of nutrients by the small intestine, leading to malnutrition and nutrient deficiencies despite adequate dietary intake. It reflects abnormalities in luminal digestion, mucosal absorption, or both, and may occur as part of a broader malabsorption syndrome or in isolation.

Common causes include pancreatic insufficiency (such as chronic pancreatitis or cystic fibrosis), reduced bile acid availability, and diseases that damage the intestinal mucosa (for example celiac disease, Crohn's disease, tropical sprue). Postoperative states (short bowel syndrome) and small-intestinal bacterial overgrowth can also contribute, as can congenital or acquired defects in brush-border enzymes or nutrient transporters.

Pathophysiology involves failure of normal luminal digestion and/or absorption, so nutrients remain in the intestinal lumen and are lost in stools. Lipid malassimilation typically produces steatorrhea and fat-soluble vitamin deficiencies; carbohydrate and protein absorption may also be impaired, with systemic weight loss and weakness.

Clinical features include chronic diarrhea, abdominal discomfort, bloating, weight loss, and signs of nutrient deficiencies such as bone or muscle weakness, anemia, or dermatitis. In children, growth retardation and poor weight gain may occur.

Diagnosis relies on clinical suspicion supported by tests such as fecal fat quantification, fecal elastase to assess pancreatic function, and tests of mucosal absorption (eg, D-xylose). Endoscopy with biopsy and serologic testing help identify mucosal diseases. Treatment targets the underlying cause, with pancreatic enzyme replacement, vitamin supplementation, dietary adjustments, and, when needed, antibiotics for bacterial overgrowth or parenteral nutrition in severe cases.