Dietary Polyphenols in Type 2 Diabetes: A Metabolite-Centric Review of Human Evidence

Dietary polyphenols are studied as potential adjuncts for type 2 diabetes (T2D), although many reviews still rely on an antioxidant framework that does not adequately reflect human exposure. This review organises the evidence around two metabolite waves, namely early postprandial phase-II conjugates produced by host enzymes within 0 to 4 h of a polyphenol-rich meal, and microbiome-derived catabolites including urolithins and γ-valerolactones that peak at 6 to 24 h. This biphasic interpretation is advanced as a hypothesis-generating framework rather than as an established explanation, and it offers a testable account of why polyphenol effects may be meal-contingent, metabotype-dependent and heterogeneous across clinical trials. Using randomised controlled trials and meta-analyses published between 2015 and 2025, we review the human evidence for anthocyanins, catechins, stilbenes, cocoa flavanols and olive phenolics, with briefer coverage of isoflavones, curcuminoids and ellagitannins. Anthocyanins show the most consistent glycaemic signal, with a best available pooled estimate of about 0.3% for HbA1c (-0.31%, median exposure eight weeks) derived from trials that did not verify metabolite exposure, an effect that is modest and lies at the lower bound of what is generally regarded as clinically meaningful. Green tea catechins and high-phenolic extra-virgin olive oil are associated with comparable postprandial gains, whereas cocoa flavanols and resveratrol more reliably improve vascular and inflammatory intermediates than glycaemia itself. Most interventions were well tolerated at the nutritional doses tested. Polyphenols are better regarded as meal-timed dietary adjuncts to standard T2D care than as primary glycaemic agents.