In the absence of detectable somatic (i.e., physical) abnormalities in irritable bowel syndrome (IBS), it has been considered for a long time to be a psychological disorder caused by anxiety, depression, somatization (physical symptoms caused by psychological stress), or hypochondria. Although recent research has shown that there is no convincing evidence to support an effect of psychological factors on the onset and/or progression of IBS, stress can worsen the symptoms. Although the cause of IBS is not completely understood, several factors appear to be involved in its development, including heritability, environment and social learning, diet, composition of large-bowel bacteria, low-grade inflammation, and abnormal gastrointestinal endocrine cells.

About one-third of IBS patients have a family history of the disorder. Several studies have shown that IBS is common among the relatives of IBS patients. Studies on twins have shown that the chances of one twin having IBS if the other twin has IBS are greater if they are identical twins (i.e., formed from a single egg) than if they are fraternal twins (i.e., formed from two eggs). Moreover, genetic studies have provided evidence for a hereditary component in IBS.

There is some evidence that the illness behavior (i.e., the manner in which an individual monitor the structure and functions of his body, interpret symptoms, take an action, and make use of healthcare facilities) in a family contributes to the causes of IBS. Having a mother with IBS accounts for as much as genetic factors. Of course this raises the question as to whether IBS is caused (1) by the possession of half a set of genes from a mother with IBS or (2) by social learning.

Most patients with IBS find that certain foodstuffs trigger their symptoms, and often make a conscious decision to avoid these foodstuffs, which include milk and dairy products, wheat products, onion, garlic, peas, beans, hot spices, cabbage, smoked products, fried food, and caffeine. The effect of these foods on IBS symptoms is not due to an allergy or intolerance, but other factors as explained below.

Some foodstuffs are rich in short-chain carbohydrates, the so-called fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs; Table 3.1). These carbohydrates are poorly absorbed, and a significant portion enters the distal part of the small bowel and the large bowel. On entering the large bowel, they increase the osmotic pressure and become a substrate for bacterial fermentation, which results in gas production, distention of the abdomen, and abdominal discomfort or pain.