Diarrhea Due To Unknown Causes (Idiopathic Diarrhea)

Normal Gastrointestinal Motility

Food that is consumed undergoes both mechanical and chemical digestion in the alimentary tract. Nutrients are absorbed in the process and water and mucus are mixed with the ingested food at almost every part of the alimentary tract. Large amounts of water are then absorbed in the large intestine thereby transforming the liquid residual matter that enters the colon into a firm yet soft material by the time it reaches the descending colon. This residual and waste material is known as feces and is subsequently evacuated from the body during defecation. In order to facilitate this process, food, chyme (digested food) and residual nutrients with waste has to pass from the mouth to the anus. This movement is achieved by the action of the muscles throughout the alimentary tract in a coordinated process known as peristalsis.

Gastrointestinal motility is largely the same in every part of the alimentary tract as described under esophageal motility, intestinal motility and colonic motility. It is also similar in the stomach although the action of the pyloric pump in gastric emptying regulates the passage of chyme into the duodenum of the small intestine. The time that it takes for food to move from the mouth to the anus is known as the bowel transit time. It should not exceed 72 hours although the first remnants of any food or drink should appear in the stool as early as 12 to 14 hours after ingestion. This is dependent on gastrointestinal motility and further compounded by water intake, quantity of ingested food in a day and fiber in the diet.

Diarrhea or Frequent Stools

Diarrhea is defined as the passing of more than 200 milliliters (ml) or 200 grams (g) of stool in a 24 hour period. This is usually within three or more bowel movements and the stool may be loose, watery and poorly formed. Sometimes very frequent bowel movements with well-formed stool may occur and does not typically appear as what one would consider to be diarrhea. This is in contrast to normal bowel movements and bowel habit, which can be as frequent as once or twice a day or less frequent but up to three bowel movements or more within a one week period. Any disturbance in bowel habit that leads to more frequent movements than normal is generally considered to be diarrhea by most people, irrespective of the weight or volume. It is also not uncommon or watery or poorly formed stool to also be referred to as diarrhea or more correctly diarrhea stool, although it does not fit the “strict” definition of diarrhea. Read more on the different types of stool as illustrated on the Bristol stool chart.

Types of Diarrhea

There are four types of diarrhea which vary based on the underlying mechanism (pathophysiology) that contributes to loose or watery stool, which is more frequently passed than normal and of a higher volume/weight than normal.

  • Osmotic diarrhea which arises with excessive solutes in the lumen of the gut that hampers water absorption.
  • Secretory diarrhea which is due to a significantly larger amount of water being secreted into the lumen of the gut than the quantity of water being absorbed in the large intestine.
  • Infectious / inflammatory diarrhea is disruption, damage or destruction of the absorptive epithelium lining the gut as a result of an injury, inflammation without injury or infection.
  • Rapid gastrointestinal motility accounts for diarrhea associated with a faster than normal bowel transit time without any of the mechanisms discussed above.

Idiopathic Diarrhea

Any condition that occurs for unknown reasons is termed idiopathic. Diarrhea is no different. However, in the large majority of cases where a person may feel that the diarrhea is occurring for no known reason, there is an undiagnosed cause of the diarrhea. Diarrhea is the most common gastrointestinal symptom. It is not a disease. Diarrhea may occur with any disruption in lifestyle, diet or even temporary psychological stress but is almost always acute. It lasts for than 5 days and often resolves spontaneously within 2 to 3 days. While this usually does not warrant further investigation. A differential diagnosis can be reached on the medical history and clinical examination without further specialized investigation.

However, diarrhea that is persisting or recurrent is termed chronic diarrhea and should be investigated thoroughly. Apart from the medical history and clinical examination, stool tests, radiological imaging and laboratory studies may be necessary. It is highly unlikely that chronic diarrhea will occur for no known reason. As stated above, it usually persists because it has not been diagnosed as yet. In the event that all available diagnostic techniques reveal no abnormality, there is a possibility that diarrhea is due to a functional bowel disorder. The two most likely such disorders that would account for diarrhea is irritable bowel syndrome (IBS) and functional diarrhea.

Prior to diagnosing a functional bowel disorder as the cause of diarrhea, infectious, inflammatory and food intolerance/malabsorption  and cancer needs to be excluded. The various other causes of chronic diarrhea, even if rare, also need to be investigated and excluded.


This may be associated with actual pathogens causing irritation of the bowel lining or the toxins of the microorganisms. Viral pathogens account for the majority of acute infections like gastroenteritis which presents with acute nausea, vomiting and diarrhea. Viral gastroenteritis is often termed the ‘stomach flu‘ and resolves within 48 to 72 hours with no need for any additional measures beyond supportive treatment like appropriate rehydration. Bacterial gastroenteritis is the second most common cause of acute infectious diarrhea. It is also the main cause of food poisoning, where the enterotoxin of the bacteria contaminates food and leads to vomiting and diarrhea. Read more on food infection vs food intoxication.

Fungal infections of the gastrointestinal tract that may cause diarrhea are not common. It is more frequently seen in immunocompromised patients as it arises as an opportunistic infection. This is more likely with conditions like HIV/AIDS and poorly controlled diabetes mellitus. Human intestinal worms or parasitic worms known as helminths can infest the gut. It is typically a long term infestation that may be asymptomatic initially but eventually lead to weight loss, diarrhea and liver complications.

Fungi and parasitic worms (helminths) are more likely to present as chronic diarrhea whereas viruses and bacteria tend to cause a more acute infection.


Non-infectious inflammatory disorders that cause diarrhea are common and may vary in pathophysiology. Not all conditions are isolated to the bowels and in chronic settings, it is more commonly associated with autoimmune or immune-mediated reactions. Inflammatory bowel disease is a typical example of a chronic inflammatory disease of the bowels that presents with diarrhea. In the acute setting, toxicity associated with alcohol, prescription medication, illicit substances, poisons and other ingested irritants may be the cause of inflammation and diarrhea.

Food Intolerance and Malabsorption

Food intolerance and malabsorption of certain nutrients may arise for a number of reasons. Most cases are inherited as is seen with lactose intolerance although underlying immune mechanisms may account for other instances like with gluten intolerance in celiac disease. Intolerance, maldigestion and malabsorption may also occur secondary to other disorders particularly of the organs of the digestive tract like with pancreatitis. This typically presents as diarrhea that is triggered by consuming specific foods which most patients are able to identify.


Diarrhea is one of the symptoms of colorectal cancer although it may also present with constipation. With the incidence of colorectal cancer rising globally, it should be considered as a possible cause of diarrhea when other pathologies have been excluded. Sometimes diarrhea may occur with carcinoid syndrome even if the malignancy is not within the gastrointestinal tract. Although the clinical presentation varies depending on the site of the malignancy, cancer patients may report changes in bowel habit despite the primary tumor not involving the gastrointestinal tract nor is it associated with carcinoid syndrome. However, this is highly variable and should not be considered as a conclusive indication of cancer. Diarrhea may also be seen a side effect of chemotherapy used in the treatment of cancer.

Functional Bowel Disorders

Functional bowel disorders are diagnosed when there are no other underlying abnormalities that can account for the changes in bowel habit, yet there are gastrointestinal signs and symptoms. While the cause is largely unknown, it is believed to be associated with deranged bowel motility, more specifically rapid gastrointestinal motility. Certain foods, stress and other diseases may trigger or exacerbate the condition but are not the underlying cause. Psychological factors are important components of these conditions.

The most prevalent functional bowel disorder is irritable bowel syndrome (IBS). It is a condition that is characterized by abdominal discomfort or pain with a change in bowel habit. IBS may be termed diarrhea-predominant IBS or constipation-predominant IBS. In some instances diarrhea and constipation may alternate although there are intervals of normal bowel habit without any discomfort or pain.

Functional diarrhea is a term for diarrhea that occurs without abdominal pain or discomfort (excludes IBS) which arises for unknown causes (no abnormality detectable). It is applied to the passage of loose, watery or mushy stools and in this scenario, it is the stool form that is of greater importance rather than frequency of bowel movement. There is a single criteria for diagnosing functional diarrhea and that is the loose, watery or mushy stools without pain in at least 75% of the stools. This should be present for 3 months or more with onset at least 6 months prior to diagnosis.

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