This project is a single center, randomized, controlled clinical trial evaluating the effects of DAO enzyme dietary supplement on symptoms of irritable bowel syndrome.

These participants are randomized to either a 4-week group with DAO enzyme dietary supplement or placebo. The participants are required to fill out IBS-SSS and IBS-QOL questionnaires and record their IBS symptom improvement, IBS treatment drug use, compliance and adverse reactions every evening. At the same time, the patients' diet is continuously recorded for 3 days by 24-hour diet review method in Visit 0, Visit 1 and Visit 2, respectively, to inform the patients to avoid large fluctuations in diet structure. On days 0, 14 and 28, the subjects are asked to visit the hospital offline. The staff check with the subjects in detail according to the scale and questionnaire contents, and review the general situation and questionnaire in the previous 2 weeks. On the 7th and 21st day, the staff contact the subjects online to provide guidance and remind the subjects to fill in the scale and questionnaire. In addition, oral mucosal samples, urine and feces will be collected for identifying mutations in the genetic DAO enzyme coding gene, histamine detection and 16sRNA sequencing, respectively.

Inclusion Criteria:

1. Age 18-65 years old;
2. Diagnosed with IBS according to Rome IV diagnostic criteria; Recurrent abdominal pain occurred on an average of at least 1 day per week in the past 3 months and was associated with two or more of the following criteria: i. Related to defecation; ii. Associated with changes in bowel frequency; iii. Correlated with fecal morphological changes. The symptoms appeared at least 6 months before diagnosis, and the above diagnostic criteria were met in the last 3 months.
3. Voluntarily participate in the experiment and sign the informed consent.

Exclusion Criteria:

1. Patients with endoscopy examinations which suggest chronic intestinal diseases other than IBS, such as inflammatory bowel disease, celiac disease, gastric and duodenal ulcers, parasitic or bacterial intestinal infections;
2. Pregnant and lactating women;
3. Patients suffering from serious concomitant diseases, such as liver disease, cardiovascular disease, lung disease, kidney disease, tumor;
4. Alcoholism.