How do fiber supplements help with irritable bowel syndrome (IBS)?

Irritable bowel syndrome (IBS) has symptoms that include irregularity in timing and consistency of bowel movements. There can be diarrhea, constipation or both associated with IBS. Fiber can help with these symptoms in several ways. Fiber helps to move the material through the GI tract, which helps with symptoms of constipation. Also, fiber is considered a bulking agent, which helps absorb water and decrease diarrhea. This combination of effects makes fiber an ideal component in the treatment of IBS for some patients. Although a large study did not show a benefit, individual results vary, and some patients find good relief with fiber supplementation.
IBS patients with constipation predominant symptom are recommended to increase dietary fiber as an initial intervention and may be prescribed bulking agents to supplement their fiber intake.

This strategy may increase frequency of bowel movements but may not improve abdominal pain severity.

The amount of fiber should be increased slowly since increased fiber intake may lead to increased gas/bloat and diarrhea symptoms. 
Patients with increased sensitivity to bowel distention have a component of irritable bowel syndrome (IBS). This is generally helped over time by taking a daily dose of a fiber supplement such as Konsyl, Metamucil, Citrucel, or a similar product. These products are available in a powder form (flavored or unflavored) that is mixed with water or juice and taken once daily; they are also available in pill form (calcium polycarbophil), which can be taken two tablets twice a day, with a glass of water each time.

Bloating generally gets somewhat worse for the first month after starting fiber supplements but then begins to improve. It may take three to six months to appreciate the full beneficial effect of these supplements.

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