Cognitive behaviour therapies (CBTs) have through several trials been demonstrated to reduce symptoms and disability in irritable bowel syndrome (IBS) patients, but the mechanisms responsible for the changes are still unknown. The aim of this study was to test a theoretical model of CBT and investigate if cognitions and/or behaviour mediated the changes seen in CBT for IBS.

To assess for possible mediating effects, we applied path analysis to the dataset of 149 diagnosed participants randomized to mebeverine hydrochloride plus CBT or mebeverine hydrochloride alone. Primary outcome was symptom severity, while secondary outcomes were work and social adjustment and anxiety.

The path analyses supported mediational paths for all outcomes. Changes in behaviour and cognitions mediated all three outcomes, with models placing behaviour change ‘upstream’ of cognition change having best fit. The analyses of model fits revealed best fit for the anxiety model and hence provide increased confidence in the causal model of anxiety.

Changes in behaviour and cognitions mediate the change in CBT given to IBS patients. The results strengthen the validity of a theoretical model of CBT by confirming the interaction of cognitive, emotional and behavioural factors in IBS.