# Current Research on the Effects of Non-Digestible Carbohydrates on Metabolic Disease
## 6 Conclusions  
A high intake of NDCs, such as dietary fibers and resistant starch, is strongly associated with a reduced risk of MD, including CVD and T2 DM, because of their physical and fermentation properties. In particular, the properties of NDCs, such as water retention, fecal bulking, viscosity, and fermentation in the gut, are important for reducing the risk of MD by decreasing blood glucose and lipid levels, increasing satiety and insulin sensitivity, and modifying the gut microbiome. Moreover, SCFAs produced by certain gut bacteria mainly contribute to reducing the risk of MD by controlling satiety hormones and energy metabolism, decreasing inflammation, and enhancing the immune system. However, these mechanisms are not sufficient to explain the differences in the impact of NDCs on MD according to the type and source of the NDCs and the answers to many questions about how NDCs suppress the development of MD still remain unclear. In particular, the study on the structural property of NDCs, the effects of NDCs on the gut microbial ecosystem, and the biological networking of gut metabolites produced by fermentation of NDCs have been limited. In structural property, the structures of NDCs and their sizes after partial digestion by the GI system are associated with various health benefits [120], but the study on structural property on MD has been rarely conducted except for the degree of esterification in pectin [60]. In the gut microbial ecosystem, although the microbial profiles are significantly different according to individual NDC and the metabolites profiles they produce are also different [121], the factors related to the fermentation property of NDCs except for their physical property mentioned in this review and other metabolites produced by fermentation except for SCFAs have been rarely investigated [122]. In the biological networking of gut metabolites, various metabolites can be produced during gut fermentation, but studies over the past decade have focused only on SCFAs [120]. Gut metabolites can transfer to the whole body, including the brain, liver, kidney, lung, and skin, via blood and the central nervous system, and can affect many physiological functions associated with the risk of MD through biological networking [123]. However, the biological networking of other gut metabolites has been rarely investigated. Although there remain gaps in understanding how NDCs reduce the risk of MD, this review showed how NDCs regulate the incidence of MD by focusing on mechanisms by which the physical and fermentation properties of NDCs in the GI system, and we believe that a better understanding of the relationship between NDC intake and MD is imperative to improve NDC intake guidelines for MD.  
Conceptualization, H.-J.K.; writing--original draft preparation, S.C., Q.-A.N., and H.-J.K.; writing--review and editing, H.-J.K.; visualization, S.C.; supervision, H.-J.K. All authors have read and agreed to the published version of the manuscript. This research was supported by the Research Program (E0210600-01) of the Korea Research Food Institute (KFRI) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2020R11A3072463). Not applicable. Not applicable. Not applicable.  
The data presented in this study are available on request from the corresponding author. The authors declare no conflict of interest.