Patients suffering from chronic illnesses are wise to consult a physician and work with her or him to find the best treatment approaches. The Nutrition MD Irritable Bowel Syndrome: Nutritional Considerations webpage says that patients suffering from IBS will benefit most from a combination of medical, nutritional, and behavioral approaches such as "Increased insoluble fiber (e.g., bran cereal, whole grain breads): Several studies have revealed that increasing bran fiber intake decreases bloating, constipation, and diarrhea in patients with IBS. . ." Other suggestions are elimination diets, probiotic treatment with Lactobacillus plantarum, Bifidobacterium breve, Streptococcus faecium, & combinations of these with other organisms, and peppermint oil capsules. It might be necessary to try one or more of these nutritional measures under the supervision of your physician to see which treatment is most beneficial to you.
P.S. Nutrition MD is operated by PCRM; information at Nutrition MD is written by Dr. Neal Barnard and members of his team. http://www.nutritionmd.org/consumers/gastrointestinal/ibs_nutrition.html