A runny bowel movement may not be entirely fluid like is seen with acute conditions such as gastroenteritis or food poisoning. While water accounts for some 75% of fecal weight, stool should still have a firm but not hard, solid to semi-solid consistency. Passing stool should not be associated with passing copious quantities of water/fluid through the anus except in diarrhea.

It is often difficult to distinguish a normal bowel movement from slight changes that may not fit into the category of diarrhea. One of the ways to assess different forms of stool is by using the Bristol Stool Chart where 7 types of stool/bowel movements are described. Read more on types of feces. A runny bowel movement is generally accepted as being diarrhea. The definition of diarrhea stands as passing more than 200ml/200g of stool in 3 or more movements within a 24 hour period. While frequent bowel movements may not fall within the clinical definition of diarrhea especially if it is of a normal consistency or slightly loose, watery stool is usually considered as diarrhea.

Excessive Water in the Bowels

Water in the bowels comes from the foods and fluids that are consumed. Moderate amounts of water also constitute the mucus that lubricates the gastrointestinal lining, digestive juices secreted for breaking down foods and in the bile passed out of the gallbladder. While water is passed from the tissue spaces of the gut wall and enter into the lumen, most of this water is reabsorbed particularly in the colon.

Excessive quantities of water that is retained or passed into the bowels than the water that is absorbed lead to diarrhea. This can be explained by one or more mechanisms of diarrhea.

Osmotic Diarrhea

In this type of diarrhea, the concentration of solutes in the lumen of the bowel is very high thereby disturbing the osmotic gradient that is responsible for water absorption from the bowels. Water remains in the bowel and affects the normal stool consistency. Osmotic diarrhea is seen with :

  • Malabsorption syndromes
  • Food intolerance
  • Antacids with magnesium
  • Laxatives
  • Increased gastrointestinal motility reducing contact time for digestion and absorption

Secretory Diarrhea

With secretory diarrhea, the amount of water and electrolytes that is passed out into the gut lumen is greater than the water that is absorbed. This is seen with :

  • Food poisoning
  • Heavy metal toxins and other forms of chemical and plant poisons
  • Some types of laxatives
  • Various types of drugs used for a number of conditions (not only gastrointestinal)
  • Hormone-secreting (endocrine) tumors

Exudative Diarrhea

This is also known as inflammatory or infectious diarrhea. Most cases are due to an infection and the stool may contain blood or pus. Various types of microorganisms, particularly bacteria, viruses and protozoa, damages the absorptive functions on the bowel lining. It also destroys the epithelial lining so that blood and serum (exudate) can leak out into the lumen of the gut. This type of diarrhea is also compounded by the action of white blood cells against the invading pathogen. Chemical mediators of inflammation secreted by these immune cells causes secretory diarrhea. Exudative diarrhea is seen with primarily with gastroenteritis.


Article reviewed by Dr. Greg. Last updated on March 25, 2011