Short-Term Effects of a Low-FODMAP Diet, Adapted NICE Dietary Advice, and Adapted NICE Dietary Advice Combined with Intermittent Fasting on Symptoms and Quality of Life in Irritable Bowel Syndrome: A Three-Arm, Non-Randomized Controlled Dietary Inter

Background/Objectives: Dietary interventions are central to irritable bowel syndrome (IBS) management, but evidence directly comparing a low-FODMAP diet (LF), adapted NICE dietary advice (NICE), and adapted NICE dietary advice combined with intermittent fasting (NICE–IF) is limited. This study compared the short-term effects of LF, NICE, and NICE–IF on IBS symptom severity and disease-specific quality of life. Methods: In this prospective, three-arm, non-randomized controlled dietary intervention study, adults with IBS followed LF, NICE, or NICE–IF for two weeks. IBS symptom severity, disease-specific quality of life, and psychological status were assessed using the Irritable Bowel Syndrome Symptom Severity Score (IBS-SSS), the IBS Quality of Life questionnaire (IBS-QOL), and the Hospital Anxiety and Depression Scale (HADS), respectively, at baseline and after the two-week intervention; stool consistency was monitored using the Bristol Stool Form Scale (BSFS). Between-group differences were additionally examined using analysis of covariance (ANCOVA) adjusted for baseline values. Results: Eighty-one participants completed the study. IBS symptom severity decreased significantly in all groups (all p < 0.001), with a significant between-group difference in change (p = 0.018); the reduction was greater with LF than with NICE (p = 0.017). IBS-specific quality of life improved significantly in all groups (all p ≤ 0.002). Although the overall change differed among groups (p = 0.012), pairwise differences were not significant. After adjustment for baseline values, no significant between-group differences remained for symptom severity (p = 0.294) or quality of life (p = 0.333). Post-intervention stool consistency did not differ among groups (p = 0.485). A significant within-group reduction in anxiety scores was observed only in the NICE group (p = 0.025). Conclusions: All three interventions were associated with short-term improvements in IBS symptoms and quality of life. The greater unadjusted reduction in overall symptom severity with LF than with NICE was not robust to adjustment for baseline severity, and no between-group differences in quality-of-life improvement were observed; these findings require confirmation in larger, fully randomized studies with longer follow-up.