Inclusion Criteria:  age 18-80 years patients diagnosed with irritable bowel syndrome based on the ROME IV. criteria positive LHBT (lactose H2 breath test) and LTT (lactose tolerance test) results negative abdominal ultrasound/CAT scan/MRI results within one year signed the informed consent  Exclusion Criteria:  organic gastroenterological disorders which can explain symptoms (e.g. positive serological screening: anti-gliadin IgG / IgA, anti-tissue transglutaminase IgG / IgA, high level of fecal calprotectin tested by endoscopy or positive colonoscopy for IBD, currently active diverticulitis Alarm symptoms: fever (> 38 Co), anaemia (Hgb < 120 g/l), unintended weight loss (> 4.5 kg / 3 months), gastrointestinal bleeding (hematemesis, hematochesia, melena) cardiac failure (NYHA III-IV) liver cirrhosis (Child-Pugh C) active malignancy major abdominal surgery in the history pregnant or breastfeeding women any circumstances which can lead to false results of LHBT and LTT: cigarette smoking or physical exercise within 2 hours before the test, ingestion of dietary fibers on the evening before the test, recent use of antibiotics, lung disease, baseline H2 concentration in the exhaled air is higher than 20 ppm, not properly treated diabetes mellitus, following lactose restricted or another special diet within 1 week prior to study enrolment (ingestion of less than 12 g lactose - less than 250 ml milk - per day) small intestinal bacterial overgrowth (SIBO): if there is a rapid increase of H2 level in the exhaled air (≥20 ppm above baseline within 90 minutes), SIBO is suspected and antibiotic therapy will be started (peroral rifaximin for 5 days) after negative Helicobacter pylori serology. After this procedure, lactulose H2 breath test will be performed to exclude SIBO (≥20 ppm H2 rise only after 90 min). In case of negative lactulose H2 breath test, another LHBT will be carried out slow oro-cecal transit: clinical signs and typical findings on the tests (the LHBT is normal, but the increase of blood glucose level is less than 1.1 mmol/l and/or no H2 rise during lactulose H2 breath test) milk allergy (positive IgE test)