IBS clusters in families, and that can have two explanations: shared genes, or a shared environment that runs from diet and gut infections to how a family handles stress and illness. Studies of twins, of whole national populations and of hundreds of thousands of genomes point the same way. Genes play a real but modest part, and much of what makes one person develop IBS lies elsewhere.[1]
Does IBS run in families?
Yes. A nationwide Swedish study compared the relatives of people diagnosed with IBS with the relatives of people without it:[2]
| Relative of someone with IBS | Odds of IBS compared with usual |
|---|---|
| Parent | 1.90 times |
| Child | 1.82 times |
| Sibling | 1.75 times |
| Niece or nephew | 1.27 times |
| Cousin | 1.11 times |
| Spouse | 1.51 times |
The authors read this pattern two ways. A raised risk that extends to second- and third-degree relatives points to a genetic component, while the raised risk in spouses, who share a home but not genes, points to a non-genetic contribution as well.[2] A US study that surveyed relatives directly, rather than relying on patients' reports, found that 50% of the families of people with IBS had at least one other affected relative, compared with 27% of control families.[3]
What do twin studies show?
Identical twins share all their genes and non-identical twins share about half, so a strongly genetic condition should be shared more often by identical twins. That is what researchers found, but only up to a point:
- In a US study of 6,060 twin pairs, concordance (how often both twins had IBS) was 17.2% in identical twins and 8.4% in non-identical twins, which supports a genetic contribution.[4]
- In the same study, non-identical twins with IBS were more likely to have a mother with IBS (15.2%) than a twin with IBS (6.7%), and having a parent with IBS predicted IBS more strongly than having a twin with it. The authors concluded that social learning, meaning what people learn from those around them, has an equal or greater influence than heredity.[4]
- A Norwegian twin study found concordance of 22.4% in identical and 9.1% in non-identical twins, and estimated heritability at 48% in women. Being born very small, under 1,500 g, more than doubled the odds of IBS later in life.[5]
Across twin studies, estimates of how much of IBS is heritable vary widely, from 0 to 57%.[1]
Have scientists found genes for IBS?
A few, each with a tiny effect. The largest genome-wide study, published in 2021, compared 53,400 people with IBS with 433,201 people without it, then checked its results in a separate group of more than 205,000 people with IBS.[1] It found:
- Six genetic regions linked to IBS, near genes including NCAM1, CADM2, PHF2/FAM120A, DOCK9, CKAP2/TPTE2P3 and BAG6. The first four are associated with mood and anxiety disorders, are active in the nervous system, or both.[1]
- Small effects. Each variant raised the odds of IBS by less than 5%, and together the common variants explained only about 6% of the variation in IBS risk. The equivalent figure is 41% for Crohn's disease and 26% for anxiety.[1]
- Shared roots with mood. The genetic risk of IBS overlapped strongly with that of anxiety, neuroticism and depression. Further analyses suggested shared biological pathways, rather than anxiety simply causing gut symptoms.[1]
The authors concluded that the genetic contribution to IBS is modest, and that environmental factors, including gut bacteria, diet, stress and learned behaviors, all of which families can share, probably play a bigger role.[1]
If genes play a small part, what causes IBS?
There is no single cause. UK guidelines list genetics, epigenetic changes (chemical marks that turn genes up or down), gut infections and adverse events in early life as things that may predispose someone to IBS. Chronic stress, psychological symptoms, negative beliefs about symptoms and unhelpful ways of coping can then make symptoms more frequent and severe.[6]
The clearest trigger is a gut infection: in a meta-analysis of 45 studies, 10.1% of people had IBS a year after infectious enteritis.[7] In the genome study, people whose IBS began after an infection had a family history and psychological profile much like other people with IBS, which the authors took to mean that the infection "falls on fertile ground" in people who are already predisposed.[1] For the bigger picture, see What is IBS? and Is IBS an autoimmune disease?
What does a family history mean for you?
- It raises your odds; it doesn't decide your fate. Genes explain only part of IBS risk, and much of what families share is environment.[1][2]
- Treatment works the same way. Diet changes, psychological therapies and medicines are chosen by your symptoms, not your genes. See How is IBS treated?
- Some family histories call for a doctor's visit. If a close relative has had bowel cancer, inflammatory bowel disease or celiac disease, get bowel symptoms checked rather than assuming they're IBS. The same goes for blood in your stool, unintentional weight loss, anemia, fever, symptoms that wake you at night, or symptoms that start after about age 50.[8][9] See IBS red flags.