Overview of the mechanisms causing diarrhea

(A) Normal — During normal function, Na⁺ and nutrient absorption drives fluid absorption, with a small basal amount of electrolyte (Cl^–)-driven fluid secretion, allowing efficient reabsorption of fluid, leading to minimal fluid loss via feces.
(B) Loss of nutrient absorption, or the presence of nonabsorbable solutes (diet-induced or "osmotic" diarrhea) — Loss of nutrient absorption because of damage or loss of the required transporter, or the presence of nonabsorbable osmoles in the lumen (eg, PEG 3350), prevents fluid absorption and promotes fluid secretion into the intestinal lumen, leading to diarrhea.
(C) Increased secretion or reduced absorption of electrolytes (electrolyte transport-related or "secretory" diarrhea) — Excessive anion-driven fluid secretion and reduced electrolyte-driven fluid absorption, as occurs in certain infections such as cholera, leads to accumulation of fluid in the intestinal lumen and diarrhea.
(D) Rapid intestinal transit (motility-related diarrhea) — Increased intestinal motility results in reduced time to absorb electrolytes and nutrients, leading to excessive unabsorbed substrates in the intestine and reduced fluid absorption, leading to diarrhea.