Inclusion Criteria:  - Patients aged 18 - 75 years; Fulfilling the ROME IV criteria for IBS; Moderate symptom severity as defined by a IBS-SSS > 175; Consumption of lactose containing products.  Exclusion Criteria:  - Clinical suspicion of an organic disorder different from LI or IBS (patients can be included when this disorder had been excluded); Known lactose intolerance; Known inflammatory bowel disorder; Known major intestinal motility disorder; Alcohol (defined as more than 14 U per week) or other substance abuse; Active psychiatric disorder; Known systemic or auto-immune disorder with implication for the GI system; Prior abdominal surgery (with the exception of appendectomy); Any prior diagnosis of cancer other than basocellular carcinoma; Current chemotherapy; History of gastro-enteritis in the past 8 weeks; Intake of antibiotics, pre- or probiotics during the past 8 weeks; Dietary supplements unless taken at a stable dose for more than 8 weeks; Treatment with neuromodulators (one neuromodulator taken at a stable dose for more than 12 weeks is allowed); Treatment with spasmolytic agents, opioids, loperamide, gelatin tannate or mucoprotectants during the past 8 weeks; LFD or low FODMAP diet in the past.