Exploring the potential of apple (poly)phenols: A systematic review of randomized controlled trials on metabolic diseases prevention

Emerging evidence underscores the significance of fruits and vegetables in ameliorating metabolic disorders. However, a comprehensive understanding of the specific role of apple (poly)phenols, a major component of apples, in preventing and treating metabolic diseases remains to be fully elucidated. This study aimed to conduct a systematic review of Randomized Controlled Trials (RCTs) investigating the therapeutic potential of apple (poly)phenols in the context of metabolic diseases among adults. It also focused on the analysis of recent findings suggesting that polyphenol consumption may modulate gut microbiota composition and thereby impart protection against metabolic disorders. Guided by the preferred reporting items for systematic reviews and meta-analyses (PRISMA) 2020 guideline, we carried out an exhaustive search of the literature within PubMed, Medline, Cochrane, and Web of Science databases, encompassing English-language publications spanning 30 years, until December 31, 2022. Eligible studies were RCTs with an intervention period of over one week, utilizing apple or apple (poly)phenols as the intervention and with primary outcome measures encompassing aspects of metabolic diseases, including plasma lipids, plasma glucose, HbA1c, insulin, gastric inhibitory peptide, C-peptide, systolic and diastolic blood pressure, and C-reactive protein levels. The reviewed literature suggests a protective role of apple (poly)phenols, at an intake of approximately 800 mg/day for 4 weeks or more, against chronic metabolic diseases. Nevertheless, limitations such as the small sample sizes in human studies and the limited number of pertinent studies necessitate further research to substantiate these findings conclusively.