The objective of this Phase II trial is to test the NTX-1 Fiber in patients with constipation-predominant IBS (IBS-C) and determine if it can reduce symptoms, beneficially modify gut microbiota composition, and improve gut milieu in these patients. The investigators hypothesis for the Phase II study is that NTX-1 Fiber is better tolerated (i.e has less side effects) and is more effective than psyllium (the most commonly used fiber in the US) for symptom improvement and is effective in correcting the abnormal colonic milieu in those with dysbiosis and low SCFA levels.