Diarrhea

  • Author:
    Jay W. Marks, MD

    Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

  • Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

woman with abdominal pain

Quick GuideDiarrhea and Digestive Distress: Problem Foods to Avoid

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What is the definition of diarrhea?

Diarrhea can be defined in absolute or relative terms based on either the frequency of bowel movements or the consistency (looseness) of stools.

Frequency of bowel movements: Absolute diarrhea is having more bowel movements than normal. Thus, since among healthy individuals the maximum number of daily bowel movements is approximately three, although some consider five or more bowel movements a day diarrhea can be defined as any number of stools greater than three, although some consider five or more bowel movements to be diarrhea. "Relative diarrhea" is having more bowel movements than usual. Thus, if an individual who usually has one bowel movement each day begins to have two bowel movements each day, then relative diarrhea is present-even though there are not more than three or five bowel movements a day, that is, there is not absolute diarrhea.

Consistency of stools: Absolute diarrhea is more difficult to define on the basis of the consistency of stool because the consistency of stool can vary considerably in healthy individuals depending on their diets. Thus, individuals who eat large amounts of vegetables will have looser stools than individuals who eat few vegetables and/or fruits. Stools that are liquid or watery are always abnormal and considered diarrheal. Relative diarrhea is easier to define based on the consistency of stool. Thus, an individual who develops looser stools than usual has relative diarrhea--even though the stools may be within the range of normal with respect to consistency.

Why does diarrhea develop?

With diarrhea, stools usually are looser whether or not the frequency of bowel movements is increased. This looseness of stool--which can vary all the way from slightly soft to watery--is caused by increased water in the stool. During normal digestion, food is kept liquid by the secretion of large amounts of water by the stomach, upper small intestine, pancreas, and gallbladder. Food that is not digested reaches the lower small intestine and colon in liquid form. The lower small intestine and particularly the colon absorb the water, turning the undigested food into a more-or-less solid stool with form. Increased amounts of water in stool can occur if the stomach and/or small intestine secrete too much fluid, the distal small intestine and colon do not absorb enough water, or the undigested, liquid food passes too quickly through the small intestine and colon for enough water to be removed.

Another way of looking at the reasons for diarrhea is to divide it into five types.

  1. The first is referred to as secretory diarrhea because too much fluid is secreted into the intestine.
  2. The second type is referred to as osmotic diarrhea in which small molecules that pass into the colon without being digested and absorbed draw water and electrolytes into the colon and stool.
  3. The third type is referred to as motility-related diarrhea in which the intestinal muscles are overactive and transport the intestinal contents through the intestine without enough time for water and electrolytes to be absorbed.
  4. The fourth type is unusual. It is best represented by a condition called collagenous colitis. In collagenous colitis, the mechanism of the diarrhea may be the inability of the colon to absorb fluid and electrolytes because of the extensive scarring of the intestinal lining. Inflammation also may play a role.
  5. The fifth type of diarrhea is referred to as inflammatory diarrhea and involves more than one mechanism. For example, some viruses, bacteria, and parasites cause increased secretion of fluid, either by invading and inflaming the lining of the small intestine (inflammation stimulates the lining to secrete fluid) or by producing toxins (chemicals) that also stimulate the lining to secrete fluid but without causing inflammation. Inflammation of the small intestine and/or colon from bacteria or from non-bacterial ileitis/colitis can increase the rapidity with which food passes through the intestines, reducing the time that is available for absorbing water.

Diarrhea generally is divided into two types, acute and chronic.

  1. Acute diarrhea lasts from a few days up to a week.
  2. Chronic diarrhea can be defined in several ways but almost always lasts more than three weeks.

It is important to distinguish between acute and chronic diarrhea because they usually have different causes, require different diagnostic tests, and require different treatment.

Medically Reviewed by a Doctor on 10/29/2015

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