From rumen to milk: Dietary polyphenols in dairy cows—A critical review

Polyphenols represent a large and structurally diverse family of plant secondary metabolites with bioactive properties. In ruminants, these compounds can influence rumen fermentation, microbial ecology, and nutrient metabolism, offering potential benefits for animal health, productivity, and environmental sustainability. This review synthesizes evidence on the fate of dietary polyphenols in dairy cows from ingestion to their possible secretion into milk. It outlines the main dietary sources and classes of polyphenols, their microbial biotransformations in the rumen, and subsequent host metabolism involving absorption, conjugation, and systemic circulation. Particular attention is given to the mechanisms of mammary uptake and secretion, where most compounds appear as conjugated metabolites such as glucuronides, sulfates, and urolithins rather than parent forms. Although the transfer efficiency from feed to milk is typically low, consistent detection of isoflavone derivatives, phenyl-γ-valerolactones, urolithins, and hippuric acid demonstrates the feasibility of diet-to-milk modulation. Factors affecting bioavailability and transfer include polyphenol structure, dietary matrix, dose, rumen microbiota composition, animal physiology, and feed processing. Advances in high-resolution analytical techniques have improved the characterization of these low-abundance metabolites, yet large variability among studies persists. I<i>n vivo</i> studies indicate that polyphenol-derived metabolites in bovine milk occur at low ng/mL to low µg/mL levels, with compounds such as equol, enterolactone, urolithins, phenyl-γ-valerolactones and phenolic acids typically detected in the sub-micromolar range. Overall, dietary polyphenols offer promising opportunities to improve ruminant health and produce milk with enhanced functional quality, but quantitative and mechanistic research is still required to optimize feeding strategies and understand their contribution to milk bioactivity.