Dietary Polyphenols as Adjunctive Therapy for Inflammatory Bowel Disease: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Background: Dietary polyphenols have been studied as adjunctive therapies for inflammatory bowel disease (IBD), but trials vary in formulation, clinical setting, and outcome definitions. We evaluated efficacy and safety separately for ulcerative colitis (UC) and Crohn’s disease (CD). Methods: We searched MEDLINE/PubMed, China National Knowledge Infrastructure (CNKI), Wanfang, Web of Science, Embase, Cochrane Library, and ClinicalTrials.gov through 20 August 2026. Risk of bias was assessed using Cochrane Risk of Bias 2 (RoB 2). Meta-analyses used MDs, SMDs, or ORs with 95% CIs. Quantitative synthesis was undertaken when studies were sufficiently comparable in clinical setting, outcome definition, and data structure; otherwise, results were summarized using study-level estimates. The protocol was registered with PROSPERO (CRD420261451126). Results: Twenty-one reports representing 19 randomized trials and 1048 participants were included. In conventional UC, polyphenols were associated with lower SCCAI scores (MD −1.32, 95% CI −1.81 to −0.83; I2 = 0%), whereas CRP/hs-CRP did not show a clear pooled effect (SMD 0.29, 95% CI −0.32 to 0.90; I2 = 83.7%). IBDQ-9 improved (MD 5.57, 95% CI 2.88 to 8.26). Clinical response favored supplementation (OR 4.90, 95% CI 3.04 to 7.89), as did induction of clinical remission (OR 5.98, 95% CI 3.08 to 11.59; I2 = 22.6%). Supplementation also favored endoscopic remission (OR 10.81, 95% CI 1.67 to 69.93; I2 = 69.4%), but the estimate was imprecise and sensitive to study-level risk of bias. In CD, the two randomized trials were not pooled due to targeting different clinical settings. Safety reporting was heterogeneous and sparse, with no consistent pattern of increased harm across trials. Conclusions: Curcumin has the most extensive adjunctive evidence base in UC but effect estimates for several outcomes were imprecise or sensitive to study design. Evidence in CD is still lacking.