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, 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 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 concerning consistency.
Why does diarrhea develop?
With diarrhea, stools usually are looser whether or not the frequency of bowel movements has increased. This looseness of the stool-which can vary 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.
- The first is referred to as secretory diarrhea because too much fluid is secreted into the intestine.
- 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.
- 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.
- The fourth type is unusual. It is best represented by a condition called collagenous colitis. In collagenous colitis, the mechanism of 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.
- 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 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.
- Acute diarrhea lasts from a few days up to a week.
- Chronic diarrhea can be defined in several ways, but usually 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.