Lactose malabsorption and irritable bowel syndrome. Effect of a long-term lactose-free diet.

Lactose malabsorption may induce abdominal symptoms indistinguishable from those of the irritable bowel syndrome (IBS), however the exact relationship between the two conditions and the optimal differential diagnostic workup are still to be defined. We prospectively studied the prevalence of lactose malabsorption (by means of a hydrogen breath test) and the clinical effect of a long-term lactose-free diet in 230 consecutive patients with a suggested diagnosis of irritable bowel syndrome, no organic disease of the GI tract, and no history of milk intolerance. Lactose malabsorption was diagnosed in 157 patients (68.2%). In 48 (43.6%) of the 110 patients who complied with the diet symptoms subsided, in 43 they were somewhat reduced and in 17 they remained unchanged. Symptoms never fully subsided in lactose malabsorbers non-compliant with the diet or in normal lactose absorbers who adhered to a lactose-free regimen. Partial improvement was observed in 20% of these subjects. No relation was demonstrated between pre-trial symptoms and the outcome of the diet. The occurrence of symptoms during the lactose breath test strongly suggested a favorable response to diet, but did not help in predicting whether symptoms would subside or be reduced. Conversely, their absence during the test was not associated with an acceptable negative predictive value. The high prevalence of lactose malabsorption in the patients under study suggests that in Italy IBS and lactose malabsorption are frequently associated. A test for diagnosing lactose malabsorption should always be included in the diagnostic workup for IBS and a long-term lactose-free regimen recommended if the test is positive.