It has been suggested that the symptoms of irritable bowel syndrome (IBS) may be wrongly attributed to lactose intolerance. We examined the relations among IBS, demographic factors, living habits, and lactose intolerance. On the basis of a lactose tolerance test with ethanol, 101 of the 427 healthy subjects studied were lactose maldigesters and 326 were lactose digesters. IBS was diagnosed by means of the Bowel Disease Questionnaire, according to the Rome criteria. The use of dairy products and symptoms experienced after their consumption were recorded. IBS was found in 15% of both the lactose maldigesters and lactose digesters. One-third of the subjects reported intolerance to dairy products containing < or = 20 g lactose. About half of this third were lactose maldigesters and about half were lactose digesters. As explanations for this subjective lactose intolerance, the logistic regression model estimated lactose maldigestion (odds ratio: 10.3; 95% CI: 5.2, 20.4), IBS (4.6; 2.1, 10.1), experience of symptoms other than gastrointestinal ones (2.3; 1.2, 4.5), and female sex (2.1; 1.1, 4.0). Characteristics common to both subjective lactose intolerance and IBS were female sex and the experience of abdominal pain in childhood (P < 0.01). Age, regularity of meals, and the amount of physical activity were not associated with either subjective lactose intolerance or IBS. Of the subjects with IBS, the percentage of lactose maldigesters was the same as in the whole study group (24%) but the number who reported lactose intolerance was higher (60% compared with 27%, P < 0.001). We showed a strong relation among subjective lactose intolerance, IBS, the experience of abdominal pain in childhood, and female sex.
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