1. To analyze the prevalence of overlap syndromes among NERD or RH, FD, and IBS patients, and to identify and to compare the differences in their characteristics and symptoms, and to determine their risk factors.
2. To identify the differences in clinical features according to sex and gender.
3. To analyze the effect of genetic polymorphisms on overlap FGIDs over long-term follow-up period.

Inclusion Criteria:

FGID group

• No organic diseases in routine laboratory tests, abdominal imaging, esophagogastroscopy, and colonoscopy
• NERD or reflux hypersensitivity (RH): typical heartburn occurring at least once a week without visible esophageal mucosal break detected by endoscopy
• FD or IBS: based on the Rome III criteria

Control group

- Subjects who underwent GI evaluations as part of health check-up or for other problems, such as mild abdominal discomfort or pain for a relatively short period of time, but had no organic problems during GI endoscopy

Exclusion Criteria:

• history of GI surgery except appendectomy
• inflammatory bowel disease
• any malignancy, or systemic diseases requiring chronic medication except for hypertension and diabetes mellitus
• pregnant or lactating women
• hepatic, biliary, or psychiatric disorders requiring medication.

Patient who were diagnosed with NERD/RH, FD, IBS and their overlaps (NERD/RH-FD, NERD/RH-IBS, FD-IBS, NERD/RH-FD-IBS) and healthy controls.