What is the role of fiber in IBS?
Fiber improves bowel function by limiting stool dehydration and normalizing stool consistency and stool volume. For every gram of fiber (e.g., wheat) ingested, approximately 2.7 g of stool is expelled. In general, fiber can improve symptoms associated with mild constipation. However, fiber is frequently associated with increased gas production, abdominal cramps, and bloating, which are frequently present in individuals with IBS.
A trial comparing 10 g of the soluble fiber psyllium (also known as ispaghula ), 10 g of the insoluble fiber bran, and placebo found patients receiving psyllium husk had a mean improvement in IBS symptom severity, whereas bran was not significantly better than placebo. The improvement seen with psyllium was most notable during the first month of therapy. Approximately 40% of patients in this study stopped participation before the final visit because of worsening in their IBS symptoms. Not surprisingly, the highest dropout rate was among those patients receiving bran during the first month of treatment.
A prudent approach in IBS patients with mild to moderate IBS is to initially instruct them to gradually increase dietary fiber intake to approximately 20 to 25 g per day over several weeks. If adding fiber in the diet fails to relieve symptoms, psyllium should be tried next because of its ability to absorb water. If psyllium is not tolerated, then a trial with the semisynthetic fiber methylcellulose or the synthetic fiber polycarbophil should be considered. Patients who develop gas and distention with fiber should be instructed to reduce the dose of fiber and reduce their consumption of gas-producing foods, such as beans, cabbage, legumes, apples, grapes, and raisins.