Diarrhea is a very common problem that has been experienced by any of us many times. It is usually a short-term infection-related issue that spontaneously withdraws but chronic diarrhea should not be underestimated as it may be related to some serious diseases (not only) of gastrointestinal tract.


There are many causative factors of diarrhea, only the most common and most clinically important shall be mentioned in this article.

Bowel infections

Infectious inflammation of the digestive tract is probably the most common cause of acute diarrhea. Quite regular cause is poisoning by a toxin produced by staphylococcus that often infects the food products. The point is that cooking kills the bacteria but the toxin stays intact as it is resistant to temperature. Few hours after food ingestion symptoms such as nausea, vomiting and diarrhea occur. Especially in children collectives there are typical epidemics of diarrhea caused by viral particles (rotavirus, norovirus). Severe and less frequent causative agents of infectious diarrhea are bacteria such as salmonella (salmonellosis), Campylobacter (campylobacteriosis), Shigella (dysentery) and vibrio cholerae (cholera). Bacterial diarrhea is usually longer-lasting and there are often present blood and mucus in stool.


Note: Infections of the digestive tract also include presence of parasites such as tapeworms. These parasites may cause chronic diarrhea, loss of appetite and weight loss.

Inflammatory bowel diseases (IBD)

This group of autoimmune diseases includes Crohn's disease and ulcerative colitis. Although both diseases are very different, they have one thing in common - the bowel wall has been for some reason attacked by our own immune system. The diseases have a chronic course; periods of relative calm are alternated with periods of bouts with abdominal pain, diarrhea and loss of appetite. Blood, mucus and sometimes pus are usually present in stool.


There is also disease known as microscopic colitis that causes chronic diarrhea and that is newly counted into IBD group.

Celiac disease

This is also an autoimmune disease of intestines that is mostly focused on duodenum and rest of the small intestine. The cause is an immune reaction to a substance known as gluten, which is contained in cereals and a wide variety of food products. Antibodies against gluten damage the intestinal wall and cause local chronic inflammation. Celiac disease typically manifests by diarrhea and weight loss but it can have a more insidious course leading to chronic complications such as infertility and increased cancer presence. Given that celiac disease is often relatively inconspicuous, there may be many undiagnosed cases and we estimate the disease frequency up to one of hundred people.

Colon cancer

Some tumors of the large intestine, especially those in its left part, manifest by alternating constipation and diarrhea. This sign should be therefore always suspicious of a more serious disease.

Imbalance of serum ion levels

Diarrhea may occur especially in high serum level of potassium ion (hyperkalemia) and low level of calcium ion (hypocalcemia).

Overactive thyroid gland

Diarrhea is one of many symptoms of thyroid hormones overproduction together with excessive sweating, weight loss, anxiety, palpitations, etc.

Malabsorption of nutrients

There are many possible causes of gastrointestinal malabsorption (see related text). Diarrhea is often present in fat malabsorption that is related to bile flow disorders and pancreatic diseases. Fat present in stool causes diarrheal oily yellowish foul-smelling stool. Such situation is typical for chronic pancreatitis and obstruction of biliary tract (see in article dedicated to jaundice).

Antibiotic side-effect

Antibiotics wipe out the intestinal microflora and thus disrupt the bowel equilibrium; which is often followed by diarrhea. In some cases, antibiotic use causes proliferation of more resistant bacteria such as Clostridium that is followed by occurrence of pseudomembranous colitis.

Food allergy

Diarrhea is one of the possible forms of allergic reactions, especially if the allergen (i.e. the substance causing allergy) is ingested.


Post-irradiative inflammation of colon occurs usually after radiotherapy of pelvic tumors. Diarrhea, often with blood and mucus, is an unpleasant sign of intestinal mucosa.

Functional gastrointestinal disorders

Functional causes are mentioned as almost the last but maybe they would deserve to be in the first place. This is a group of illnesses that do not have a clear organic cause, there is usually present a psychological factor. Diarrhea is usually associated with functional disease known as irritable bowel syndrome. There should be no objective warning sings such as unintentional weight loss, blood in stool, troubles waking from sleep, fever, etc. The diagnosis should be made by excluding organic causes by a cautious examination.

Dietary causes

Many food products have a laxative effect, especially food rich on fiber. Fiber is healthy, but its excessive intake sometimes causes diarrhea. Common lactose intolerance is another dietary cause of diarrhea. This condition is related to the fact that many adult people have insufficient intestinal enzyme that digests the milk sugar (lactose). Undigested sugar molecules stay in intestines and cause diarrhea.

Diagnostic approach

We usually try to diagnose chronic diarrhea (lasting more than 3-4 weeks) and severe cases of acute diarrhea. Medical history is of a great importance. It is important to know the length of troubles, number of bowel movements per day, presence of blood or mucus in stool, occurrence of other symptoms and current medication. Physical examination should include cautious abdominal examination and digital rectal exam. Stool should be thoroughly examined including fecal occult blood testing.


Blood tests are important to evaluate blood count (as anemia can accompany colorectal cancer), blood minerals concentration and CRP level (elevated CRP may mean inflammatory processes in the body).


Possible imaging methods usually include abdominal ultrasound that can be followed by computed tomography. Endoscopic examination (colonoscopy) should be reserved for patient with a high suspicion of colon pathology (tumor, inflammatory bowel disease, infectious colitis, etc).


The therapy depends on the causative factor. When treating diarrhea itself, it is important to assure adequate fluid intake to prevent dehydration. There are also some drugs used to slow the bowel peristalsis but their use is limited to only some cases.

Jiri Stefanek, MD  Author of texts: Jiri Stefanek, MD
 Contact: jiri.stefanek@seznam.cz
 Sources: basic text sources