Effects of a high-γ-polyglutamic acid-containing natto diet on liver lipids and cecal microbiota of adult female mice

Natto is a traditional Japanese fermented soy product high in γ-polyglutamic acid (γ-PGA), whose beneficial effects have been reported. We prepared high-γ-PGA natto and compared the dietary influence on liver lipids and cecal microbiota in mice fed a diet containing it or a standard diet. The mice were served a 30% high-γ-PGA natto diet (PGA group) or standard diet (Con group) for 28 days. Liver lipids, fecal lipids, and fecal bile acids were quantified. Cecal microbiota were analyzed by PCR amplification of the V3 and V4 regions of 16S rRNA genes and sequenced using a MiSeq System. Additionally, the cecal short-chain fatty acid profile was assessed. The results revealed that the liver lipid and triglyceride contents were significantly lower (p<0.01) and amounts of bile acids and lipids in the feces were significantly higher in the PGA group than in the Con group. The cecal butyric acid concentration was observed to be significantly higher in the PGA group than in the Con group. Principal component analysis of the cecal microbiota revealed that the PGA and Con groups were distinct. The ratio of Firmicutes/Bacteroidetes was found to be significantly low in the PGA mice. The results revealed a significantly higher relative abundance of Lachnospiraceae (p<0.05) and significantly lower relative abundance of Coriobacteriaceae (p<0.01) in the PGA group. Analysis of the correlation between bacterial abundance and liver lipids, cecal short-chain fatty acids, fecal lipids, and fecal bile acids suggested that intestinal microbiota can be categorized into different types based on lipid metabolism. Hepatic lipid accumulation typically facilitates the onset of nonalcoholic fatty liver disease (NAFLD). Our findings suggest that high-γ-PGA natto is a beneficial dietary component for the prevention of NAFLD.