Chronic diarrhea is defined as a decrease in stool consistency (loose BM) for more than 4 weeks (Am Fam Physician, 2011).

It can be divided into 3 categories:

- watery
- fatty (malabsorption)
- inflammatory

Watery diarrhea

Watery diarrhea may be subdivided into:

- osmotic
- secretory
- functional, e.g. IBS

Watery diarrhea includes irritable bowel syndrome (IBS), which is the most common cause of functional diarrhea. Another example of watery diarrhea is microscopic colitis, which is a secretory diarrhea affecting older persons.

Laxative-induced diarrhea is often osmotic.

Malabsorptive diarrhea

Malabsorptive diarrhea is characterized by excess gas, steatorrhea, or weight loss. Giardiasis is a classic infectious example of malabsorptive diarrhea.

Celiac disease (gluten-sensitive enteropathy) is also malabsorptive, and typically results in weight loss and iron deficiency anemia.

Inflammatory diarrhea

Inflammatory diarrhea, such as ulcerative colitis (UC) or Crohn disease, is characterized by blood and pus in the stool and an elevated fecal calprotectin level.

Invasive bacteria and parasites also produce inflammation. Infections caused by Clostridium difficile (C. diff.) subsequent to antibiotic use have become increasingly common and virulent.

Image source: Escherichia coli, Wikipedia, public domain.

Not all chronic diarrhea is strictly watery, malabsorptive, or inflammatory, because some categories overlap.

References:

Evaluation of Chronic Diarrhea. Juckett G, Trivedi R. Am Fam Physician. 2011 Nov 15;84(10):1119-1126.
Skin patch vaccine to prevent travelers' diarrhea
Image source: Colon (anatomy), Wikipedia, public domain.

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