Guidelines for dietary fiber evaluation: A proposed standard integrating glycan structural diversity

Dietary fiber is increasingly regarded as an essential nutrient for metabolic and microbial health.1 However, dietary fiber evaluation remains largely quantity based. Current evaluation systems often overlook the glycan structures that determine how fiber behaves in food matrices, during digestion, and within the gut ecosystem. This limitation is particularly evident in refined carbohydrate-dominant diets, where intensive processing reduces the structural diversity of matrix-embedded and microbiota-accessible glycans reaching the colon. This creates a “glycan gap”: foods with similar fiber content can deliver fundamentally different glycan structures to the gut microbiota, driving distinct microbial utilization patterns and host metabolic responses (Figure 1A).2,3,4 Closing this glycan-structure gap requires fiber quality to be evaluated through the chemical, physicochemical, ecological, and evidence-linked properties of dietary glycans. Here, we propose the dietary glycan index (DGI) as a staged, evidence-updated framework that organizes these properties into comparable dimensions of fiber quality. By complementing existing fiber-intake recommendations, the DGI can help distinguish qualitative differences among fiber-rich foods, ingredients, and dietary patterns.

The physiological effects of dietary fiber depend strongly on glycan structure, which links fiber chemistry to physiological function (Figure 1B).5 Structural variation influences how glycans behave before and after they reach the gut. Within foods, glycan architecture contributes to matrix organization, hydration, viscosity, and interactions with starch, lipids, proteins, minerals, and bile acids. During digestion, these properties affect nutrient diffusion, substrate release, transit behavior, and the fraction of glycans that remains available in the lower gastrointestinal tract. In the colon, linkage configuration, polymer architecture, solubility, and matrix embedding influence enzymatic accessibility, microbial substrate allocation, and the spatial and temporal pattern of glycan utilization.2,3 These structure-dependent effects indicate that dietary fiber evaluation should consider both glycan structural diversity and the matrix conditions that shape glycan availability and activity.

A practical DGI should translate dietary glycan diversity into measurable criteria that connect food chemistry with microbial and host physiology. The DGI can provide a framework for developing shared reference resources for food glycans and for linking glycan structures to biological activity and evidence strength. We propose four core DGI scores: structural diversity, matrix accessibility, functional expression, and evidence confidence (Figure 1C). Together, these scores separate four questions that are often mixed in fiber evaluation: what glycan structures are present, how they are presented in the food matrix, what biological activities they support, and how strong the supporting evidence is.