Inclusion Criteria:  Subjects must meet all of the following inclusion criteria to be eligible for enrolment into the study at baseline visit:  Evidence of a personally signed and dated informed consent document indicating that the subject (or a legal representative) has been informed of all pertinent aspects of the study. Subjects are willing and able to participate in the study, complete subject assessments, attend scheduled clinic visits, and comply with all protocol requirements as evidenced by written informed consent. Male and/or female subjects between the ages of ≥18 and ≤75 years at the time of informed consent.  Diagnosis of IBS following Rome IV criteria (Mearin et al., Gastroenterology 2016)*: Recurrent abdominal pain, on average, at least 1 day per week in the last 3 months, associated with 2 or more of the following criteria:  related with defecation associated with a change in frequency of stool onset associated with a change in form (appearance) of stool * Criteria fulfilled for the last 3 months with symptom onset at least 6 months before diagnosis. Diagnosis of IBS with predominant diarrhea (IBS-D), following Rome IV criteria (Mearin et al., Gastroenterology 2016): more than one fourth (25%) of bowel movements with Bristol stool form types 6 or 7 and less than one-fourth (25%) of bowel movements with Bristol stool form types 1 or 2 or Patient reports that abnormal bowel movements are usually diarrhea (like type 6 or 7 in the picture of Bristol Stool Form Scale) Evidence of a colonoscopy performed 5 years within the enrolment showing no evidence of organic disease Evidence of increased intestinal permeability at 51Cr-EDTA or 99mTc-DTPA assessment performed during screening phase.  Exclusion Criteria:  Subjects presenting with any of the following will not be included in the study:  Subjects with suspect or evidence of organic disease, including but not limited to coeliac disease, inflammatory bowel disease, gastrointestinal neoplasia, unexplained anemia Subjects with a history of colonic or small bowel resection. Subjects with lactose intolerance Use of probiotics or antibiotics or investigational agents within 30 days before baseline Repeated use of anti-inflammatory drugs, including the non-steroidal anti-inflammatory drugs such as aspirin or ibuprofen within 7 days from the enrolment (except for prophylactic use of a stable dose of aspirin up to 325 mg / day for cardiac disease) and use of any product or ingredient that can have an effect on the intestinal permeability. Any major illness/condition or evidence of an unstable clinical condition (eg, renal, hepatic, hematologic, gastrointestinal, endocrine, pulmonary, immunologic, or local active infection/infectious illness) that, in the investigator's judgement will substantially increase the risk to the subject if he or she participates in the study. Normal intestinal permeability at 51Cr-EDTA or 99mTc-DTPA assessment performed during screening Women during pregnancy or lactation.