# Current Research on the Effects of Non-Digestible Carbohydrates on Metabolic Disease
## 5 NDCs and MDs
### Diabetes  
The relationship between NDC intake and type 2 diabetes mellitus (T2 DM) has been clinically investigated for decades. Many recent meta-analyses and clinical studies have shown that a high intake of NDCs, especially dietary fiber, for \(>\)1 month lowered the risk of developing T2 DM and might have therapeutic effects in patients with T2 DM [88, 89], although some studies have shown no significant effects of dietary fiber on T2 DM [90]. Randomized studies of 15 studies from 1980 to 2010 suggested that an increasing dietary fiber diet reduced fasting blood glucose and glycosylated hemoglobin (HbA1C) levels in patients with T2 DM [91, 92]. Similar results were reported in a meta-analysis of 28 randomized controlled trials (\(n=1394\)) on T2 DM patients with a viscous fiber diet at a median dose of approximately 13.1 g/day [93]. However, the effect of NDCs on the risk reduction of T2 DM depends on the type and intake of NDCs. In particular, soluble fibers with viscous and/or gel-forming properties, such as psyllium, \(\beta\)-glucan, and pectin, have been associated with lower postprandial glucose and blood cholesterol levels because the increased viscosity of intestinal contents by soluble fiber can delay gastric emptying, reduce the accessibility of digested enzymes, including amylase and lipase, and slow the intestinal absorption of nutrients, such as monocarbohydrates and cholesterol [94]. Delayed gastric emptying in the stomach can enhance satiety and consequently lower energy intake, resulting in an increase in fat oxidation, eventually leading to a decrease in body weight [95]. In this mechanism, various hormonal responses associated with satiety and insulin sensitivity, which are relevant factors contributing to diabetes, can be affected by viscous soluble fibers. Moreover, soluble fibers can be easily fermented in the gut, resulting in the production of various metabolites, especially SCFAs, and changes in the gut microbiome [39; 96]. SCFAs can be absorbed via GPR41/43 metabolism in the gut and used as an energy source [41]. Absorbed SCFAs can increase satiety, decrease fat accumulation, and increase glucose tolerance via modification of lipid metabolism and insulin sensitivity, and consequently, can decrease the risk of T2 DM [95; 6]. In addition to the high production of SCFAs, the population of the healthy gut microbiome can be increased by the intake of soluble fibers, which can improve inflammation and the immune system associated with many diseases, including T2 DM [39]. Unlike soluble fibers, insoluble fibers with non-viscous properties are mostly poorly fermented in the gut and thus produce fewer SCFAs than soluble fibers [92]. However, accumulated insoluble fibers in the gut decrease gut transit time and increase fecal bulk because of their water-holding and swelling capacities [96]. Decreased gut transit time and increased fecal bulk due to insoluble fibers interfere with the absorption of glucose and cholesterol, resulting in the reduction of blood glucose and cholesterol levels [94; 97]. Moreover, similar to soluble fibers, insoluble fibers can modify the population of the gut microbiome, reduce inflammation, increase insulin sensitivity, and consequently, reduce the risk of T2 DM [94; 97]. However, the difference between the effects of soluble and insoluble fibers on T2 DM is not clear, and the mechanism is currently unclear, although there is an accumulation of scientific evidence on soluble fibers. Many studies have suggested that soluble NDCs are more effective in reducing the risk of T2 DM than insoluble NDCs, but recent studies have shown contrasting results. Prospective cohort studies have shown that a high dietary fiber diet (>25 g/day in women and >38 g/day in men) reduces the risk of developing T2 DM by 20-30%. In particular, a high intake of whole grains and insoluble cereal fibers improved diabetes risk, but soluble fiber did not [98].