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.

Those who are eligible and are willing to participate will be invited for the first study visit. Five visits will be required from each subject.

Visit 1: After the subject has reviewed and signed the informed consent form, the investigators will check to see if they are eligible for this study by a physical exam, questionnaires and a blood test. A physician or their assistant will do a physical exam and take the subjects vitals (such as height, weight, waist circumference and blood pressure).

The Subject Information Form will be an administered questionnaire related to their health, disease, and their family medical history. They will then have 2½ tablespoons of blood drawn by a certified nurse or medical assistant. They will be reminded in advance to fast 12 hours prior to their research visit. The blood tests include: CBC, CMP, CRP, TSH, INR, fasting lipid profile and HbA1c. The subject will be given a 3-day food diary and a 14-day IBS symptom diary to complete before their next visit. They will also be provided with stool collection supplies and instructions to take home. When blood test results are available, those who do not fit the criteria will be called within 7 days after Visit 1 stating that they are ineligible. Among these blood tests, the fasting lipid profile results will be given to the subject after their participation in the study and after completion of the study. The rest of the results will be given to the subject at their next visit. If they continue to be a candidate in this study based on blood results, the potential subjects will fill out the 3-day food diary, continue to fill out the 14-day IBS symptom diary, and complete the stool collection the day before or the morning of Visit 2. The subject will be advised to not keep the stool at room temperature longer than 5 hours. If they anticipate it to be longer than 5 hours before their visit time, then the stool will be kept in the freezer. Both of the diaries and the stool sample are to be brought to Visit 2, which is 2 weeks after their Visit 1. Visit 1 will be about 3 hours in length.

Visit 2: The subject will give the study coordinator their 3-day food diary, 14-day IBS symptom diary, and stool sample. They will provide the date of stool collection, as well as the method of storing the sample. The physician will review their 14-day IBS symptom diary and confirm their eligibility. If they continue to be eligible, the physician will do a physical exam and take their vitals (such as weight, waist circumference and blood pressure). The subject will then be asked to fill out a series of questionnaires relating to their diet (VioScreen Food Frequency Questionnaire), IBS symptoms (IBS-Quality of Life questionnaire \[IBS-QoL\] and IBS Questionnaire by Whorwell and Francis), sleeping/circadian patterns (2 NIH sleep questionnaires, Munich Chronotype Questionnaire, Lark/Hummingbird/Owl questionnaire, food timing questionnaire), the Expectancy and Credibility Questionnaire, and the 34-item tool asking about their GI symptoms and severity. The subject will then be randomized into one of the two groups. Depending on which group they are randomized to, they will be provided with a 6-week supply of NTX-1 OR psyllium fiber products in an identical appearing sachet along with instructions for taking the fiber. They will be provided with a 3-day food diary and a daily and weekly IBS symptom diary to fill out prior to Visit 3. This visit will be about 3 hours in length. The next visit will be 6 weeks after Visit 2.

Visit 3: The subject will give the study coordinator their 3-day food diary, the daily and weekly IBS symptom diary, and any excess fiber product. The physician will review their IBS symptom diary and ensure there are no significant adverse events. The physician will do a physical exam and take their vitals (such as weight, waist circumference and blood pressure). The subject will then be asked to fill out a series of questionnaires relating to their IBS symptoms (IBS-Quality of Life questionnaire (IBS-QoL) and IBS Questionnaire by Whorwell and Francis), sleeping/circadian patterns (2 NIH sleep questionnaires, Munich Chronotype Questionnaire, Lark/Hummingbird/Owl questionnaire, food timing questionnaire), the Expectancy and Credibility Questionnaire, and the 34-item tool asking about their GI symptoms and severity. Next, they will have 2 tablespoons of blood drawn by a certified nurse or medical assistant. The following blood tests will be done: CBC, CMP and CRP. The subject will be reminded in advance to fast 12 hours prior to their research visit. A copy of their blood test results will be given to them at their next visit. Next, they will be given another 6-week supply of the fiber product, the 3-day food diary, a daily and weekly IBS symptom diary to fill out prior to Visit 4 and stool collection supplies. They are asked to collect stool as previously indicated and bring the stool to Visit 4; handling will be identical to previous visit. This visit will be about 2 hours in length. The next visit will be 6 weeks after their Visit 3.

Visit 4: The subject will give the study coordinator their 3-day food diary, the daily and weekly IBS symptom diary, and any excess fiber product. The physician will review their IBS symptom diary and ensure there are no significant adverse events. The physician will complete a physical exam and take their vitals (such as weight, waist circumference and blood pressure). The subject will then be asked to fill out a series of questionnaires relating to their diet (VioScreen Food Frequency Questionnaire), IBS symptoms (IBS-Quality of Life questionnaire (IBS-QoL) and IBS Questionnaire by Whorwell and Francis), sleeping/circadian patterns (2 NIH sleep questionnaires, Munich Chronotype Questionnaire, Lark/Hummingbird/Owl questionnaire, food timing questionnaire), the Expectancy and Credibility Questionnaire, and the 34-item tool asking about their GI symptoms and severity. They will then have 2½ tablespoons of blood drawn by a certified nurse or medical assistant. Subjects will be reminded in advance to fast 12 hours prior to their research visit. The blood tests include: CBC, CMP, CRP, TSH, INR, fasting lipid profile and HbA1c. Among these blood tests, the fasting lipid profile results will be given to the subject after the completion of the study. The rest of the results will be given to the subject at their next visit. They will be provided with a 3-day food diary and a daily and weekly IBS symptom diary to fill out prior to Visit 5. Visit 4 will be about 3 hours in length. The next visit will be 4 weeks after their fourth visit, which is the subjects' last visit.

Visit 5: The subject will give the study coordinator their 3-day food diary and the daily and weekly IBS symptom diary. The physician will review their IBS symptom diary and ensure there are no significant adverse events. The physician will do a physical exam and take their vitals (such as weight, waist circumference and blood pressure). The subject will then be asked to fill out questionnaires relating to their IBS symptoms (IBS-Quality of Life questionnaire (IBS-QoL) and IBS Questionnaire by Whorwell and Francis) and the 34-item tool asking about their GI symptoms and severity. Next, they will have 2 tablespoons of blood drawn by a certified nurse or medical assistant. Subjects will have been reminded in advance to fast 12 hours prior to their research visit. The following blood tests will be done: CBC and CMP. A copy of their blood test results, including the two fasting lipid profile tests, will be given to the subject via e-mail or mail. This visit will be about 2 hours in length.