Dietary fiber: Definitions, physicochemical properties, mechanisms of action, and health effects

Dietary fiber (DF) comprises a heterogeneous group of nondigestible food components with diverse physiological effects throughout the gastrointestinal tract. Its health relevance stems from its physicochemical properties, interactions with the gut microbiota, and effects on digestive and metabolic processes. Yet DF intake remains insufficient in many populations. This review synthesizes evidence on the relationships among DF structure, physicochemical properties, physiological mechanisms, and health effects. Literature on DF definitions, classification, physicochemical properties, physiological mechanisms, microbiota interactions, and health effects was identified through major scientific databases, supplemented by technical and regulatory documents, and synthesized narratively. Recent publications were prioritized, while earlier studies were included when necessary to address fundamental concepts and seminal evidence. The physiological effects of DF depend substantially on fiber type and functional characteristics, particularly viscosity, water-holding capacity, gel-forming ability, and fermentability. These properties influence gastrointestinal function, glycemic and lipid responses, and interactions with the gut microbiota. Fermentation of specific fibers promotes the production of short-chain fatty acids, including butyrate, which may contribute to intestinal and metabolic regulation. Evidence also supports adequate DF intake as part of healthy dietary patterns, although recommendations vary among organizations. DF should be evaluated according to its specific structural and physicochemical characteristics rather than as a uniform dietary component. Integrating fiber properties with gastrointestinal mechanisms, microbiota interactions, and host responses may support more targeted dietary strategies and the development of evidence-based fiber-containing functional foods. Novelty: This review presents an integrated structure–function perspective that extends beyond the traditional soluble/insoluble classification. It emphasizes the influence of fiber structure, processing, dose, food matrix, and individual characteristics on physiological responses. It also highlights current challenges in fiber characterization, microbiota variability, and the development of evidence-based functional foods and personalized nutrition strategies. Keywords: Dietary fiber; physicochemical properties; colonic fermentation; gut microbiota; short-chain fatty acids; gastrointestinal function; metabolic health; functional foods.\