Inclusion Criteria:  willingness to participate (based on signed informed consent form) irritable bowel syndrome with predominant constipation, per Rome IV criteria (Recurrent abdominal pain on average at least 1 day/week in the last 3 months, associated with two or more of the following criteria: related to defecation; associated with a change in frequency of stool; associated with a change in form (appearance) of stool - Bristol stool scale types 1 or 2. Criteria fulfilled for the last 3 months with symptom onset at least 6 months prior to diagnosis),  Exclusion Criteria:  pregnant or breast-feeding women; intolerance of kombucha or any component of the product; history of abdominal surgery; the use of concomitant medications able to affect bowel motility; general condition of a patient making the appropriateness of his participation in the study questionable; chronic decompensated disorders of any organs and systems; mean daily caloric intake less than 800 kcal and more than 6000 kcal inability to follow standard iso-caloric diet and standard volume of water consumption (1.5-2 litres a day)