We have already learned that there are two most common types of irritable bowel syndrome: constipation predominant and diarrhea predominant. Let’s speak about those two in more detail.
Constipation is the condition in which defecation is delayed for a variety of reasons. It may be due to consciously ignoring or preventing defecation or to decreased colonic motility, which most commonly is secondary to aging, emotion, or a low-bulk diet. Bulk refers to the content of cellulose or other undigested materials in the diet, the volume of which is not decreased by absorption. The longer fecal material remains in the large intestine, the more water is reabsorbed, and the harder and drier the feces become, making defecation more difficult and sometimes painful. During this period additional material from the small intestine continues to enter the colon, progressively increasing the volume of its contents.
Many people have a mistaken belief that unless there is a bowel movement every day retention of fecal material and bacteria in the large intestine will somehow poison the body because of toxic products produced by the bacteria. Attempts to isolate such toxic agents from intestinal bacteria have been totally unsuccessful. In unusual cases where defecation has been prevented for a year or more by blockage of the rectum no ill effects from accumulated feces were noted except for the discomfort of carrying around the extra weight of 50 to 100 lb of feces retained in the large intestine. The symptoms of nausea, headache, loss of appetite, and general feeling of discomfort sometimes accompanying constipation appear to come from the distension of the rectum and large intestine. Experimentally inflating a balloon in the rectum of a normal individual produces similar sensations. Thus, there is no physiological necessity for having bowel movements regulated by a clock; whatever maintains a person in a comfortable state is physiologically adequate, whether this means a bowel movement after every meal, or once a day, or once a week.
Cathartics, or laxatives, are sometimes necessary to relieve constipation. Several types are in common use. Cellulose in vegetable matter is a natural cathartic because of its ability to increase intestinal motility by providing bulk which stretches the smooth muscle of the intestinal wall, increasing its sensitivity to the basic electrical rhythm, and thus increasing its contractile activity. Castor oil acts by irritating the smooth muscle of the intestinal tract, increasing its motility. Some cathartics, such as mineral oil, act by lubricating hard, dry fecal material, thus easing defecation. Such agents as milk of magnesia are not absorbed or absorbed only slowly by the intestinal wall; the presence of nonabsorbable solute causes water to be retained in the intestinal tract and along with the increased motility resulting from the in-creased volume helps to flush out the large intestine.
Diarrhea, the opposite of constipation, is characterized by frequent defecation, usually of highly fluid fecal matter. A primary cause is greater intestinal motility with less time for absorption and thus the delivery of a large volume of fluid to the large intestine overloading its capacity to absorb salt and water. Certain foods, such as prunes, stimulate intestinal motility and tend to produce diarrhea. Disease-producing bacteria often irritate the intestinal wall, increase motility of the intestinal tract, and lead to diarrhea. Prolonged diarrhea can result in a serious loss of fluid and salt, especially potassium, from the body as well as upsetting the acid-base balance of the body due to loss of bicarbonate.