Abstract: Whole grains retain the intact structures of the endosperm, germ, and bran, and are rich in bioactive substances such as dietary fiber and polyphenols, demonstrating significant effects on improving metabolic health, particularly delaying postprandial glycemic responses. Starch is the major nutrient component of whole grains, and its digestion rate is closely related to its glycemic index. Compared with refined grains, whole grains possess a natural matrix barrier formed by their intact kernel structure and endogenous non-starch components, thereby delaying starch digestion. However, current research has primarily focused on the effect of exogenous additives or individual components on starch digestion, while insufficient attention has been paid to the barrier effects arising from the in-situ matrix structures and components of whole grains. This review summarizes the intrinsic properties of the multi-scale matrix structure of grains, and elucidates the spatial distribution within the hierarchical structures of whole grains (such as starch granule-associated complexes, endosperm cells, and the aleurone layer) and their endogenous components (e.g., proteins, lipids, non-starch polysaccharides, and polyphenols), confirming that they are crucial factors determining the digestive properties of whole grains. Furthermore, it analyzes the changes in hierarchical structures and chemical components under processing conditions. The review further elaborates how endogenous components, including proteins, lipids, non-starch polysaccharides, and polyphenols, modulate starch digestion behavior by regulating gelatinization characteristics, molecular structure, digestive enzyme activity, and substrate accessibility. This review provides a scientific basis for the precise processing and nutritional modulation of whole grain products.

Key words: whole grains; structural barrier; endogenous components; starch; digestion