Background: Aggregation of symptoms of abdominal pain or bowel disturbance has been described in the families of patients with irritable bowel syndrome (IBS). This may be due to environmental factors, including learned responses to abdominal symptoms or a genetic contribution to the etiology of IBS.
Objectives: To determine the relative contribution of genetic factors to IBS by evaluating IBS symptoms in monozygotic (MZ) and dizygotic (DZ) twins.
Methods: A total of 4,480 unselected twin pairs identified from a national volunteer twin register were asked to complete a validated questionnaire. IBS was defined by the Rome II criteria.
Results: A total of 5,032 subjects replied (56% response rate). One thousand eight hundred seventy complete twin pairs were evaluable; 888 MZ pairs (82 male pairs, mean age 51, SD 13 (range 19-81) yr) and 982 DZ pairs (69 male pairs, age 52, SD 13 (20-82) yr). The prevalence of IBS was 17% in MZ and 16% in DZ twins. There was no significant difference in casewise concordance rates between the MZ and DZ twins (28%vs 27%, p=NS). Logistic regression analysis revealed that decreasing age and increasing psychosomatic score were independently associated with IBS. Multifactorial liability threshold modeling suggested that a combination of unique and shared environmental factors provided the best model for IBS. In contrast, somatization was shown to be moderately heritable.
Conclusion: Genetic factors are of little or no influence on IBS where the predominant influences appear to be environmental.