Bioaccessibility and Bioavailability of Phenolic Compounds Recovered from Spent Coffee Grounds: From Green Extraction to Gastrointestinal Fate and Bioactivity

Spent coffee grounds (SCG), about six million tons per year, retain much of the phenolic fraction of roasted coffee: mono- and dicaffeoylquinic acids, feruloylquinic acids, lactones and phenolics linked to melanoidins and cell-wall polysaccharides. Recovery is usually judged by Folin–Ciocalteu or radical-scavenging assays, which measure reducing capacity, not phenolic content, and ignore how much is released, absorbed, and converted into redox-active metabolites. This review integrates evidence on SCG composition, recovery technologies, digestion, intestinal transport, colonic fermentation, redox mechanisms, and in vivo data, distinguishing direct SCG evidence from coffee-derived evidence and stating what cannot be transferred. Free chlorogenic acids are partly to largely bioaccessible but poorly absorbed intact; in the only study fractionating spent coffee extracts, about half of the caffeic acid-containing phenolics were linked to macromolecules, a fraction expected but not shown to reach the colon. By analogy with coffee, systemic exposure should be dominated by sulfated hydroxycinnamates and microbial catabolites with limited radical-scavenging capacity; luminal antioxidant effects lack direct evidence of local redox protection, and Nrf2 induction has not been shown for SCG-derived material in designs establishing pathway dependence. No human pharmacokinetic or redox-biomarker study with SCG phenolics exists. We propose a tiered workflow, reporting checklist, and research priorities.