Gut Microbial Metabotypes Shape Polyphenol Bioactivity: Toward Precision Nutrition Strategies for Human Health

The biological effects of dietary polyphenols are increasingly understood to depend not only on their intake, but also on their bioavailability, host metabolism, and gut microbiota-mediated biotransformation. Because many native polyphenols are poorly absorbed in the upper gastrointestinal tract, a substantial fraction reaches the colon, where intestinal microorganisms convert them into lower-molecular-weight metabolites, including urolithins, phenyl-γ-valerolactones, phenolic acids, enterolignans, and equol. These metabolites may display distinct absorption profiles and biological activities compared with their parent compounds, although their physiological relevance differs according to metabolite class, exposure level, and supporting evidence. Interindividual differences in gut microbiota composition and function contribute to substantial variability in metabolite production, leading to the concept of microbial metabotypes, which classify individuals according to their capacity to generate specific microbial-derived metabolites. This narrative review critically examines gut microbiota-mediated polyphenol biotransformation, major microbial metabotypes, determinants of metabotype variability, and their potential relevance for precision nutrition. Particular attention is given to urolithin, equol, and enterolignan metabotypes while distinguishing relatively well-characterized models from emerging or insufficiently standardized metabolic phenotypes. We also discuss the level of evidence supporting biological effects related to mitochondrial function, endothelial homeostasis, metabolic regulation, inflammation, gut barrier integrity, and gut-brain communication, distinguishing human evidence from preclinical and mechanistic findings. Although metabotype-guided approaches may improve responder stratification in future nutritional studies, their clinical translation remains limited by heterogeneous challenge protocols, non-standardized analytical cut-offs, incomplete validation across populations, and insufficient long-term intervention data. Therefore, microbial metabotypes should currently be considered as promising functional biomarkers for research and stratified trial design rather than established tools for routine personalized dietary prescription.