Gamma-Polyglutamic Acid-Rich Natto Suppresses Postprandial Blood Glucose Response in the Early Phase after Meals: A Randomized Crossover Study

Untargeted metabolomics revealed the effect of soybean metabolites on poly(γ-glutamic acid) production in fermented natto and its metabolic pathway

BACKGROUND

Natto mucus is mainly composed of poly(γ-glutamic acid) (γ-PGA), which affects the sensory quality of natto and has some effective functional activities. The soybean metabolites that cause different γ-PGA contents in different fermented natto are unclear.

RESULTS

In this study, we use untargeted metabolomics to analyze the metabolites of high-production γ-PGA natto and low-production γ-PGA natto and their fermented substrate soybean. A total of 257 main significantly different metabolites with the same trend among the three comparison groups were screened, of which 114 were downregulated and 143 were upregulated. Through the enrichment of metabolic pathways, the metabolic pathways with significant differences were purine metabolism, nucleotide metabolism, fructose and mannose metabolism, anthocyanin biosynthesis, isoflavonoid biosynthesis and the pentose phosphate pathway.

CONCLUSION

For 114 downregulated main significantly different metabolites with the same trend among the three comparison groups, Bacillus subtilis (natto) may directly decompose them to synthesize γ-PGA. Adding downregulated substances before fermentation or cultivating soybean varieties with the goal of high production of such substances has a great effect on the production of γ-PGA by natto fermentation. The enrichment analysis results showed the main pathways affecting the production of γ-PGA by Bacillus subtilis (natto) using soybean metabolites, which provides a theoretical basis for the production of γ-PGA by soybean and promotes the diversification of natto products. © 2023 Society of Chemical Industry.

High poly-γ-glutamic acid-containing natto improves lipid metabolism and alters intestinal microbiota in mice fed a high-fat diet

Several beneficial effects of poly-γ-glutamic acid (γ-PGA) have been reported. To test whether natto, a fermented soy food rich in γ-PGA, can improve intestinal microbiota content and lipid metabolism in a high-fat diet, we compared the intestinal microbiota content, plasma, liver, and fecal contents, and changes in gene expression in the livers and large intestines of a group of mice fed a high-fat diet supplemented with cooked soybeans (SC group) and a group fed a high-fat diet supplemented with natto (NA group) for 42 days; high-fat diet-fed mice were used as a control (Con group). Hepatic lipid levels were significantly lower, the fecal bile acid and lipid levels were significantly greater, and the Bacteroidetes/Firmicutes ratio was significantly higher in the SC and NA groups as compared to Con group. Additionally, plasma glucose and triglyceride levels, the expression of liver fatty acid synthase, and the relative abundance of Lactobacillaceae was significantly higher in the NA group than in the Con group. Although both natto and cooked soybeans impacted the metabolic response to a high-fat diet, the addition of natto had a greater effect on glucose and lipid metabolism. γ-PGA may play an important role in natto functionality.

Fermented soybean foods and diabetes

The number of patients with type 2 diabetes mellitus is increasing, and its prevention and management are important. One of the factors contributing to the increased incidence of type 2 diabetes mellitus is the change in dietary habits, including a Westernized diet. Fermented foods are foods that are transformed by the action of microorganisms to produce beneficial effects in humans and have been consumed for thousands of years. The production and consumption of fermented soy foods, including natto, miso, douchi, cheonggukjang, doenjang, tempeh, and fermented soy milk, are widespread in Asian countries. This review focuses on fermented soybean foods and summarizes their effects on diabetes. Fermentation increases the content of ingredients originally contained in soybeans and adds new ingredients that are not present in the original soybeans. Recent studies have revealed that fermented soybean food modifies the gut microbiota-related metabolites by modifying dysbiosis. Furthermore, it has been reported that fermented soybean foods have antioxidant, anti-inflammatory, and anti-diabetic effects. In recent years, fermented foods, including fermented soybeans, have shown various beneficial effects. Therefore, it is necessary to continue focusing on the benefits and mechanisms of action of fermented foods.