Upcycling Agro-Industrial Biomass into Bioactive Oligosaccharides for the Circular Economy: Beyond the Prebiotic Lens with a Focus on Receptor Interactions and Regulatory Perspectives

Oligosaccharides represent an emerging class of bioactive compounds with diverse and structure-dependent functionalities. While traditionally explored for their prebiotic effects on gut microbiota, growing evidence demonstrates that oligosaccharides can directly interact with host receptors in the intestinal and immune systems, modulating inflammation, mucosal immunity, and epithelial barrier function. In the context of a circular bioeconomy, these molecules offer a sustainable and value-added route for the upcycling of agro-industrial and forestry biomass, particularly through the conversion of polysaccharides into structurally tailored oligosaccharides. This review synthesizes current knowledge on direct effects of plant-derived oligosaccharides on intestinal and immune system receptors. We also explore enzymatic and green chemistry-based strategies for oligosaccharide extraction and production, emphasizing their relevance to developing functional ingredients with targeted functionalities. Furthermore, we critically assess challenges and opportunities for translating this knowledge into practical applications, with a focus on regulatory aspects, outlining a roadmap toward the implementation of receptor-active oligosaccharides in precision nutrition and functional food innovation.