The investigators will conduct an 18-week (2-week initial run-in phase, 12-week treatment phase, and 4-week follow up phase), 2-arm, randomized, double-blind, controlled Phase 2 trial in 120 subjects with IBS-C. A 2-week run in will be completed in order to ensure the stability of baseline symptom scores and to ensure that all study subjects have the required symptom severity to fulfill the inclusion criteria before randomization and beginning the treatment. The investigators selected a 12-week treatment phase because of the adequacy of this length of time to clinically see the impact of fiber supplementation (as seen in the investigators Phase I trial), and because of the 12-week duration requirement of the FDA for all therapeutic intervention/agents for treatment of IBS. The 4-week follow up phase is to determine the sustainability of the effects of therapy and to obtain further safety data.Potential study subjects will be identified though the Rush University Medical Center (RUMC) clinical databases; advertisements on the RUMC clinical trials and GI websites, RUMC Facebook and other social media sites; and IRB approved flyers placed in the GI clinic and throughout the medical center. All potential candidates will be interviewed on the phone by a clinical coordinator using a structured checklist, at which time the study requirements will also be explained. Subjects will be asked for food allergies, including artificial sweeteners such as sucralose and ascelfame K. Subjects will also be informed that the fiber is made out of corn or psyllium and not other grains. However, it can't be guaranteed that it is free of other food allergy-related contaminants. Also, subjects will be asked to limit intake of foods containing more than 3 grams of prebiotics. A list of these foods will be provided.