The Japanese diet is a healthy dietary pattern, and the oral or gut microbiota have been identified as the main factors underlying the beneficial effects of the Japanese diet. However, epidemiological studies on Japanese dietary patterns calculated from daily eating habits in the general population yielded inconsistent findings. This study aimed to determine the association between the 12-component modified Japanese Diet Index (mJDI12) and the oral and gut microbiota in the general population of a rural area in Japan. After propensity-score matching, 396 participants (198 each in the low and high mJDI12 groups) were picked out. One year after the follow up survey, we reclassified the subjects and compared the low and high mJDI12 groups again. Participants with a high mJDI12 had a higher relative abundance of butyric acid-producing bacteria in their gut microbiota. Moreover, the significantly higher dietary fiber intake in the high mJDI12 group suggested that the high intake of dietary fiber contributed to an increase in butyric acid-producing bacteria in the gut. In contrast, in individuals with a high mJDI12, only Allpprevotella was decreased in the oral microbiota. Thus, the Japanese dietary pattern can have beneficial effects by improving the oral and gut microbiota.

Background/Objective: This study investigated the relationship between the composition of oral microbiota and metabolic dysfunction-associated steatotic liver disease (MASLD) in the general population. Methods: In total, 712 participants in a health check-up project were divided into four oral microbiota patterns by principal component analysis and cluster analysis; they were included in Neisseria, Streptococcus, Fusobacterium, and Veillonella groups. The Neisseria group had the largest number of patients and was used as a reference group to compare the incidence of MASLD and cardiometabolic criteria with the other groups. Results: In a multivariate analysis, the Veillonella group was a risk factor for MASLD independent of cardiometabolic criteria compared with the Neisseria group. The correlation between oral bacterial species and MASLD-related items showed that Neisseria was negatively correlated with controlled attenuation parameters, body mass index, waist circumference, hemoglobin A1c, alanine aminotransferase, and fatty liver index. Veillonella showed a positive correlation with controlled attenuation parameters, waist circumference, body mass index, blood pressure, triglycerides, alanine aminotransferase, aspartate aminotransferase, gamma-glutamyl transpeptidase, and fatty liver index, and a negative correlation with high-density lipoprotein cholesterol. In contrast, the Streptococcus and Fusobacterium groups were not clearly associated with MASLD. Conclusions: Maintaining oral hygiene and preventing periodontitis may contribute to preventing MASLD and extending a healthy lifespan.

Water-soluble dietary fiber is primarily a substrate for degradation of short chain fatty acids (SCFAs), particularly butyric acid, by gut microbiota. SCFAs have beneficial effects on the whole body. However, epidemiological studies on the association between water-soluble dietary fiber from daily food intake and butyric acid-producing bacteria are inconsistent. The purpose of this study was to determine the association between levels of water-soluble dietary fiber from daily food intake and gut microbiota, particularly butyric acid producers, in middle-aged and older adults in a rural area in Japan. We examined the effects of water-soluble dietary fiber intake on gut microbiota after adjusting for confounding factors. After propensity score matching, 520 subjects (260 in the low-intake group and 260 in the high-intake group) were selected. One year later after a follow-up survey, we re-classified the participants and again compared low- and high-intake groups. As a result, people with a high intake had a higher relative abundance of butyric acid-producing bacteria. It was also revealed that butyric acid-producing bacteria remained high in the group that maintained high intake the next year. We concluded that continuous intake of water-soluble dietary fiber from daily food is necessary to maintain sufficient amounts of butyric acid-producing bacteria.

The association between liver fibrosis and oral or gut microbiota has been studied before. However, epidemiological studies in the general population are limited owing to the difficulty of noninvasive liver-fibrosis assessment. FibroScan–asparate aminotransferase (FAST) scores can be used to accurately and non-invasively evaluate liver fibrosis. This study aimed to determine the association between liver fibrosis and oral or gut microbiota using the FAST score in the general population. After propensity score matching of 1059 participants based on sex, age, body mass index, homeostasis model assessment of insulin resistance, and triglyceride levels, 125 (non-liver-fibrosis group, 100; liver fibrosis group, 25) were included. The diversity of gut microbiota differed significantly between the two groups; however, no significant differences were noted in their oral microbiota. The liver fibrosis group showed an increase in the relative abundance of Fusobacteria strains and a decrease in the relative abundance of Faecalibacterium, with the presence of Fusicatenibacter in the gut microbiota. Feacalibacterium was not identified as an independent factor of liver fibrosis in adjusting the fatty liver index. In the general population, gut microbiota may be more involved in liver fibrosis than oral microbiota.

Many factors are associated with the development and progression of metabolic dysfunction-associated steatotic liver disease (MASLD); however, genetics and gut microbiota are representative factors. Recent studies have highlighted the link between host genes and the gut microbiota. Although there have been many studies on the separate effects of single nucleotide polymorphisms (SNPs) and gut bacteria on MASLD, few epidemiological studies have examined how SNPs and gut bacteria interact in the development and progression of MASLD. This study aimed to investigate the association between PNPLA3 rs738409, a representative MASLD-related SNP, and gut bacteria in MASLD using a cross-sectional study of the general population. The 526 participants (318 normal and 208 MASLD groups) were grouped into the PNPLA3 rs738409 SNP, CC, CG, and GG genotypes, and the differences in the gut microbiota were investigated in each group. The PNPLA3 rs738409 CC and CG genotypes were associated with decreased Blautia and Ruminococcaceae in the MASLD group. They were negatively correlated with controlled attenuation parameter levels, body mass index, serum blood glucose, and triglycerides. In contrast, there was no association between the normal and MASLD groups and the gut bacteria in the PNPLA3 rs738409, the GG genotype group. This finding implies that dietary interventions and probiotics may be more effective in preventing and treating MASLD in individuals with the PNPLA3 rs738409 CC and CG genotypes. In contrast, their efficacy may be limited in those with the GG genotype.

The beneficial effects of water-soluble dietary fiber on liver fat and fibrosis involve the gut microbiota; however, few epidemiological studies have investigated this association. This large-scale epidemiological study aimed to determine the effect of water-soluble dietary fiber intake on liver fat and fibrosis via gut microbiota for the general population. We divided low- and high-intake groups by median daily water-soluble dietary fiber intake and matched background factors by propensity score matching for sex and age. The high-intake group had lower controlled attenuation parameters, a lower fatty liver index, and a lower mac-2-binding protein glycosylated isomer level than the low-intake group. Furthermore, in the high-intake group, the prevalences of metabolic dysfunction-associated steatotic liver disease and cardiometabolic criteria were significantly lower than in the low-intake group. In the high-intake group, there were increases and decreases in 16 bacterial species. Of them, those belonging to Faecalibacterium and Gemmiger had higher relative abundances than the other species and had a negative correlation with the fatty liver index and its components triglyceride and glutamyl transpeptidase in a multivariate analysis adjusted for confounding factors. On the other hand, Dorea showed a significant negative correlation with liver stiffness measure, even though Dorea was decreased in the high-intake group. Faecalibacterium and Gemmiger are butyrate-producing bacteria, suggesting that water-soluble dietary fiber may inhibit fatty liver via gut butyric acid production.