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Diarrhea-Predominant Irritable Bowel Syndrome

By Stephen Christensen

Irritable bowel syndrome, an intestinal condition that causes abdominal discomfort and abnormal bowel movements, affects 10 to 15 percent of Americans. Women are more likely to have IBS than men, and 80 percent of people with IBS are between the ages of 18 and 55. Some people with IBS suffer primarily from diarrhea, others are troubled by constipation, and still others alternate between the two. Because treatment for these three patterns may differ, physicians have categorized IBS into diarrhea-predominant, constipation-predominant, and alternating patterns.

Altered Intestinal Activity

Although the exact cause of IBS has not been identified, old notions that it is a psychiatric disorder have been discarded. Genetics appear to play a role, as one-third of patients with IBS have a family history of the disorder. According to a 2012 review in “Clinical and Experimental Gastroenterology,” IBS is believed to stem from an interaction among altered intestinal activity, a heightened sensitivity to intestinal pain and certain environmental stressors. In people with diarrhea-predominant IBS, or IBS-D, intestinal activity appears to be accelerated.

Serotonin Blockade

Serotonin is an important chemical messenger in your nervous system and gastrointestinal tract. When serotonin stimulates receptors in your intestinal wall, intestinal motion increases, and more fluid is secreted into the interior of your bowel. These actions lead to cramping and more frequent bowel movements or even diarrhea. IBS-D is believed to result, in part, from heightened intestinal sensitivity to serotonin, a concept that has led to the development of medications that block serotonin’s actions. Currently, alosetron (Lotronex) is the only serotonin blocker licensed for treating IBS-D in the U.S. Due to potentially serious side effects, such as decreased intestinal blood flow, alosetron is available only through special prescribing programs.


Due to their tendency to slow bowel activity, tricyclic antidepressants have found a place in the treatment of diarrhea-predominant IBS. Among these agents, amitriptyline (Elavil), imipramine (Tofranil) and desipramine (Norpramin) have been tested in clinical trials and have been shown to improve symptoms in patients with IBS-D. The doses required to relieve IBS symptoms are often below those needed to control depression, which may help to limit the dry mouth, dizziness and fatigue that are sometimes associated with tricyclic antidepressants.

Intestinal Bacteria

A number of studies have suggested a link between IBS symptoms and alterations in intestinal bacteria. Treatments designed to normalize the bacterial populations in your gastrointestinal tract may relieve some of the bloating, gas and cramping associated with IBS-D. Probiotics, which are “friendly” microbes that can be administered orally, have been associated with reduced symptoms of IBS -- bloating, cramping and gas -- in some trials. According to the American College of Gastroenterology, probiotic preparations containing Bifidobacteria or several different strains of organisms seem to provide relief in some patients, while those containing only Lactobacillus are ineffective. If your IBS-D is resistant to other treatments, your doctor may prescribe a short course of antibiotics to normalize your intestinal bacteria.

Other Therapies

Dietary modifications are routinely recommended for people with IBS, but it is not clear if such changes are universally helpful. Avoidance of caffeine, wheat, corn and dairy products may provide some relief for people with IBS-D, but it is more important to identify specific dietary items that trigger your symptoms and eliminate them from your menu. Over-the-counter remedies, such as peppermint oil, fiber supplements and loperamide (Imodium), have not produced convincing results in clinical studies, although they may help some people with IBS-D.

IBS is a complex disorder, and its triggers, symptoms and effective remedies vary from one person to another. Even among patients within one IBS category, such as IBS-D, no specific approach is consistently effective. Your physician will help you devise a treatment plan to control your symptoms.

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