A review on insoluble-bound phenolics in plant-based food matrix and their contribution to human health with future perspectives

Diets rich in phenolics are associated with multiple health benefits. The majority of related studies have focused nearly exclusively on soluble free and conjugated phenolics in plant-based foods, whereas a large amount of insoluble phenolics bound to food matrix in the remaining solid residues are neglected. The fate of bound phenolics in human digestive tract and health implications are not well understood. In this review, we provide a thorough literature review regarding the occurrence, chemistry and potential health implications of the insoluble-bound phenolics in food matrix. Research gaps were identified and future perspectives on insoluble-bound phenolics have been proposed. Significant amount of phenolic compounds occurs in insoluble forms through covalently binding to cell wall structural components in plant food matrix. These bound phenolics must be released by various pretreatments such as hydrolysis by enzyme, alkali and acid, or other assisted technologies before they can be identified and characterized, and there is no standard method currently. The health benefits of insoluble-bound phenolics are not only largely governed by their intrinsic content in food matrix, but their bioaccessibility and bioavailability in human body. Insoluble-bound phenolics obtained via hydrolysis possess antioxidant and anti-inflammatory activities in vitro, and have been implicated in different health benefits. Recent studies have suggested their involvement in modulating gut microbiota and intestinal immune response as a dietary component; however, the roles of bound phenolics and phenolic metabolites in intestinal inflammation and gut health and the interplay mechanisms are still poorly understood. This contribution provides an update on the insoluble-bound phenolics in food matrix and new perspectives for future studies.