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Gut. 2006 Sep 28; [Epub ahead of print] Links IBS in twins: genes and environment.Bengtson MB, Ronning T, Vatn M, Harris J. University of Oslo, Norway.BACKGROUND AND AIMS: 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, like 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. METHODS: A postal questionnaire was sent to 12700 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. RESULTS: 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 1500g (adjusted OR=2.4, 95% CI:1.1, 5.3)contributed significantly to development of IBS, which appeared 7.7 years earlier than in higher weight groups. In the MZ group with birth weights lower than 2500g, twins with IBS were significantly lighter than the twins without disease (190,6g, p=0,02). CONCLUSION: 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.PMID: 17008364
 

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I pulled up the full article, it looks like it was a getting enough people in all the categories problem.For the males they didn't have any that fit in one of the categories you need to do the analysis. Looks like the monozygotic (identical) male twins where both of the twins have IBS was what they were missing to be able to do a full analysis on that data set. You also saw gender differences when they did the dizygotic (fraternal) twins as the ones where you had a boy and a girl where both were the same was lower than if you had two girls or two boys.K.
 

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Thanks, Kathleen. I have another question that I could probably figure-out given enough time, but it's probably good for others to see the answer here as well. What exactly does 48% heritability mean in real terms? Does that mean there is only a 48% chance that female twins will both get IBS? Is that high for a heritability score?
 

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Without double checking how that number is calculated, I think it is what portion of variation can be estimated to be the contribution of genetics to determining who has it and who doesn't. They mentioned that there seemed to be a shared environment effect based on how the fraternal twin data came out.I'm not sure how that compares to things like mental illnesses or diabetes or other things where you have both genetics and environment playing a role in the who gets it and who doesn't equation.If something were to twig at 100% that would mean the only thing that deterimines it is genetics. So in that case all monozygotic twins would be concordant (the same) and depending on how the genetics of the illness you'd get some percentage of the fraternal twins getting it.Something that was totally environmental the concordance data would be the same in monozygotic or dizygotic twins because gentics would play no role at all.Let me google some other diseses real quick and see if something pops up about how this compares to other thingsSchizophrenia 82-85%http://archpsyc.ama-assn.org/cgi/content/a...2?view=abstractHeart disease risk factors: cholesterol, HDL, and bad cholesterol, LDL, as well as triglycerides, are 60 percent to 80 percent determined by genetics. http://158.93.191.59/news/2005NewsRel/Snie...avis101405.htmlAlheimers between 58 and 79 percent.http://www.medicalnewstoday.com/medicalnews.php?newsid=37261Celiac is in this range 87% and 57% depending on the prevalence in the population as a whole data.http://gut.bmjjournals.com/cgi/content/full/55/6/803So overall the genetics is less important it seems than for a lot of other conditions, but it does play a role.I think it is interesting that they found low birth weight to be a factor. My guess would be that means something that happens during development plays a big role. Maybe one parameter could how many times your GI tract can withstand a GI infection. So could be low birth weight people who get it earlier than usual can only get for example 3 infections before they start having problems where the same genetics and environment who fully developed before birth could get 6 infections before they have problems.
 
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