Psychological Factors Predict Response to a Low Fermentable Oligo‐, di‐, Monosaccharide and Polyol Dietary Intervention in Irritable Bowel Syndrome: A Prospective Cohort Study

BACKGROUND: The low fermentable oligo-, di-, monosaccharide and polyol (FODMAP) diet (LFD) effectively manages irritable bowel syndrome (IBS), but predictors of treatment response remain unknown. OBJECTIVE: This study investigated whether psychological factors predict symptom improvement and quality of life (QoL) outcomes following a LFD intervention. METHODS: Adults with Rome IV-defined IBS underwent a three-phase LFD over 6 months. Primary outcomes were IBS symptom severity and QoL. Validated questionnaires assessed depressive, gastrointestinal-specific anxiety (GSA), and somatic symptoms, illness perceptions, and treatment expectations. Latent class growth analysis (LCGA) and cross-lagged panel models (CLPM) were used to identify symptom trajectories and examine directional relationships between psychological factors and outcomes, respectively. RESULTS: 112 participants (89% female, median age 30 ± 17 years) completed the study. LCGA identified distinct IBS symptom severity and QoL trajectories during the LFD. Higher baseline treatment credibility and expectancy predicted favourable symptom improvements but were unrelated to membership in the QoL trajectory. Elevated GSA, psychological distress (depression, anxiety, stress), and negative illness perceptions increased the likelihood of poorer outcomes. CLPM revealed that lower GSA and higher personal control preceded subsequent symptom reductions. Higher treatment expectancy predicted improved QoL and symptom outcomes over time, while QoL improvements reduced stress and GSA. CONCLUSION: Lower baseline GSA anxiety and higher treatment expectations consistently predict better response to all phases of the LFD. These findings will help clinicians identify optimal candidates for dietary intervention versus alternative treatments.