When I see reference to diarrhea or constipation, it's almost always in reference to the under- or over-reabsorption of water, respectively, in the large intestine. However, if the large intestine plays a much smaller role in water reabsorption than the small intestine (80% vs 20%), why is the small intestine rarely implicated in these disorders? Is it because the small intestine usually doesn't malfunction in this way and malfunctions are much more common in the large intestine?

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    $\begingroup$ Suppose, small intestinal pathology occur, fat absorption won't take place, osmotic load will be delivered to colon and then osmotic diarrhoea occurs. Eg of such small intestinal pathology: celiac, Whipple, tropical sprue, chron, TB etc $\endgroup$ – Sikander Aug 10 '20 at 5:44
  • $\begingroup$ Well, a lot of diarrhoeal diseases are because of small intestinal disorders too. (See ncbi.nlm.nih.gov/pmc/articles/PMC3621726 for a detailed discussion.) Something similar holds for constipation: msdmanuals.com/home/digestive-disorders/…. $\endgroup$ – Adhish Aug 15 '20 at 10:34

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