What is the pathophysiology of osmotic diarrhea?

Updated: Oct 31, 2018
  • Author: Stefano Guandalini, MD; Chief Editor: Carmen Cuffari, MD  more...
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Answer

In osmotic diarrhea, stool output is proportional to the intake of the unabsorbable substrate and is usually not massive; diarrheal stools promptly regress with discontinuation of the offending nutrient, and the stool ion gap is high, exceeding 100 mOsm/kg. In fact, the fecal osmolality in this circumstance is accounted for not only by the electrolytes but also by the unabsorbed nutrient(s) and their degradation products. The ion gap is obtained by subtracting the concentration of the elec­trolytes from total osmolality (assumed to be 290 mOsm/kg), according to the formula: ion gap = 290 – [(Na + K) × 2].


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