Design of polyphenol-rich diets in clinical trials: A systematic review

Most randomized clinical trials of polyphenols focus on individual foods. Nevertheless, due to their presence in many foods and in order to reflect a real situation, clinical trials based on polyphenol-rich diets are particularly important. This systematic review explores the characteristics of the polyphenol-rich diets used in intervention studies. The bibliography search for English-language scientific papers was performed in the Elsevier Scopus Database and PUBMED in March 2020, and focused on intervention studies with whole polyphenol-rich diets, establishing several exclusion criteria. In studies fulfilling the requirements, information on the design of the polyphenol-rich diet and associated polyphenol intake was extracted and compared. A total of 5 studies were selected. Among them, substantial differences were found in the design of the polyphenol-rich diets, regarding specific instructions and concerning the foods provided. Similarly, although a median daily polyphenol intake of 2,564 mg/day (17,945 mg/week) was obtained from the studies, which corresponds to a nutritional dose, intake values varied widely both for total polyphenols (the difference between studies reached threefold), and for individual polyphenol intake (for hydroxycinnamic acids, a tenfold difference was found between percentile 25 and percentile 75 values). These differences made the comparison of results difficult and may affected the observed health effects. Thus, despite the relevance of studying polyphenol-rich diets as a whole, this systematic review found substantial differences between the studies performed, making direct comparisons difficult.