Firmicutes/Bacteroidetes ratio of the gut microbiota and its association with abdominal obesity and insulin resistance (METS-IR) in Korean adults

The gut microbiota has been increasingly implicated in the pathogenesis of obesity and insulin resistance. The Firmicutes/Bacteroidetes (F/B) ratio may serve as a marker of metabolic health; nonetheless, evidence for its association with abdominal obesity and insulin resistance in Asian populations remains scarce. This retrospective cross-sectional study included 186 Korean adults undergoing health check-ups. Gut microbiota composition was assessed by 16S rRNA gene sequencing, and the phylum-level F/B ratio was calculated. Abdominal obesity was defined using waist circumference criteria for Asian populations, and insulin resistance was evaluated using the metabolic score for insulin resistance (METS-IR). Participants were categorized into F/B ratio quartiles. Associations between the F/B ratio, abdominal obesity, and insulin resistance were examined using linear and logistic regression analyses with adjustment for potential confounders. Higher F/B ratios were inversely associated with waist circumference and METS-IR. Abdominal obesity prevalence decreased progressively across increasing F/B ratio quartiles. In multivariable linear regression analyses, the F/B ratio remained independently associated with lower METS-IR (β =  − 0.911, p = 0.040). In fully adjusted logistic regression models, participants in the highest F/B ratio quartile had significantly lower odds of abdominal obesity than did those in the lowest quartile (OR = 0.17, 95% CI 0.05–0.59). In Korean adults, the gut microbial F/B ratio was inversely associated with abdominal obesity and insulin resistance, as assessed by METS-IR, a composite index reflecting adiposity-related metabolic dysfunction. These findings suggest that gut microbiota composition may be metabolically relevant to visceral adiposity–related metabolic dysfunction and that the F/B ratio could serve as a complementary indicator for metabolic risk stratification.