Cause:

Failures of digestion:

• Blind loops, strictures, diverticula, and inadequate mixing syndromes, eg, postsurgery or associated with disorders of motility with secondary bacterial overgrowth causing bile salt breakdown, poor lipase stimulation, and poor absorption esp of fats (Nejm 1967;276:1393)
• Zollinger-Ellison syndrome, high duodenal pH inactivates pancreatic enzymes
• Pancreatic, either disease destroying >90% of the organ (Nejm 1972;287:813) like cystic fibrosis, alcoholic pancreatitis, or postgastrectomy lack of cholecystokinin stimulation of pancreas

Inadequate absorption length:

• Gross lesions, eg, >50% small bowel resection, gastric acid often overwhelms the residual bowel
• Specific lesions, eg, ileal and/or proximal colon resection causing loss of enterohepatic circulation and hence of bile salts, which causes diarrhea
• Relative, ie, rapid transport syndromes
• Mesenteric arterial insufficiency
• Lymphatic obstruction impairing fatty acid and triglyceride absorption as in tumor or Whipple's disease

Enteropathies:

• Inflammatory, like sprue, regional enteritis, tbc, amyloid, sarcoid, milk allergy (Nejm 1967;276:761), giardia, coccidiosis (Nejm 1970;283:1306)
• Biochemical and/or genetic disorders like hypoparathyroidism and defects in carbohydrate, fat, and protein absorption