Irritable bowel syndrome in twins: genes and environment

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Abstract
Both environmental and genetic factors may contribute to irritable bowel syndrome (IBS). Nutrition in fetal life, an early environmental factor, seems to influence the development of chronic diseases later in life, such as coronary heart disease, hypertension, and non-insulin diabetes. This population based twin study evaluated the association between intrauterine growth, measured by weight and gestational age, and IBS. Structural equation analyses were conducted to analyse genetic and environmental sources of variation in liability to IBS. A postal questionnaire was sent to 12 700 Norwegian twins born between 1967 and 1979. The questionnaire included a checklist of 31 illnesses and symptoms, including IBS. The influence of birth weight on developing IBS was tested in four weight groups. Disease discordant monozygotic (MZ) pairs were analysed to test the association between intrauterine growth and IBS. Concordance for IBS was significantly greater (p = 0.011) in monozygotic (22.4%) than in dizygotic (9.1%) twins. The heritability of IBS was estimated to be 48.4% among females. Birth weight below 1500 g (adjusted odds ratio 2.4 (95% confidence interval 1.1, 5.3)) contributed significantly to the development of IBS, which appeared 7.7 years earlier than in higher weight groups. In the MZ group with birth weights lower than 2500 g, twins with IBS were significantly lighter than twins without disease (190.6 g; p = 0.02). The present study demonstrates that restricted fetal growth has a significant influence on the development of IBS later in life. Weight below 1500 g influences age at onset. Genetic contribution appears to be important for IBS among females.