Endogenous and/or exogenous origin of phenolic metabolites in humans (the ENDOPHENOL project): study protocol for a randomized, controlled, crossover trial

Background: (Poly)phenols are molecules with various structural complexities that are mostly present in fruits, vegetables, and some cereals, and are widely consumed daily. Following ingestion, the phenolic compounds that are not absorbed in the upper gastrointestinal tract—the majority—are metabolized in the colon to form low-molecular-weight phenolic catabolites (LMWP), which potentially make a substantial contribution to the biological activity of (poly)phenol-containing foods. However, the production of LMWP can also occur through the metabolism of the aromatic amino acids phenylalanine and tyrosine as well as via catecholamine pathways. Methods: A randomized, controlled, crossover trial in which healthy adults (n = 30) follow a very-low-(poly)phenol diet and restrict physical activity for 5 days is proposed. On the morning of the third day, subjects will consume either coffee (known source of (poly)phenols) or hot water (control). Discussion: The main objective is the characterization of LMWP in biological samples through metabolomic techniques, attempting to distinguish the possible origin of these metabolites based on the different production of LMWP in the context of a controlled diet with or without a known source of (poly)phenols (primary outcome). Factors leading to interindividual variability and playing a role in phenolic metabolism, such as gut microbiota composition and genetic polymorphisms (secondary outcomes), will also be considered. Emerging evidence suggests that endogenous metabolism also contributes to circulating LMWP, which could lead to an overestimation of their attribution to dietary (poly)phenols. By implementing a long washout and using coffee as the test source, this study aims to capture background phenolic production and better disentangle dietary from endogenous contributions. These insights will support a more accurate understanding of the role of LMWP in nutrition and health. Trial registration: This study was approved by the Territorial Ethics Committee (CET) of Area Vasta Emilia Nord (AVEN) (340/2023/SPER/UNIPR). Clinical Trials registration: NCT06028659. Registered on 23 August 2023.