Trial to Understand Efficacy of Colesevelam in Diarrhea Predominant IBS Patients With Bile Acid Malabsorption

Inclusion Criteria: * Females and males age 18 -75. * An IBS diagnosis based on the Rome III criteria for at least 3 months, with onset at least 6 months previously, of recurrent abdominal pain or discomfort. * Biomarkers serum alpha C4 ≥ 40 ng/mL or FGF19 ≤ 80 pg/mL or fecal bile acid \>2000 micromoles/48h Exclusion Criteria: * IBS patients with known clinically-relevant inflammation. * IBS patient with known bleeding diathesis * History of abdominal surgery Patients participating will not take any of the following disallowed medications for at least 7 days prior to and during the remainder of the study: * Any treatment specifically taken for IBS-D, including loperamide, cholestyramine, alosetron * Drugs with a known pharmacological activity at 5-HT4, 5-HT2b or 5-HT3 receptors * All narcotics * Anti-cholinergic agents * Tramadol * Oral anticoagulants * Antimuscarinics * Peppermint oil * Systemic antibiotics, as well as antibiotics directed at colonic flora such as rifaximin and metronidazole Gastrointestinal preparations: * Anti-nausea agents * Osmotic laxative agents * Prokinetic agents * 5-HT3 antagonists