Novel Targets for Precision Nutrition: Insulin Resistance and Phenotypic Age Mediate the Protective Effect of Gut Microbiota-Targeted Diet on Metabolic Syndrome

Background: The dietary index for gut microbiota (DI-GM) holds promise for improving metabolic status, yet its mechanistic insight into metabolic syndrome (MetS) is unclear. Methods: In this cross-sectional study, we analyzed data from 20,800 participants in the NHANES (2005–2018). The DI-GM was developed based on dietary patterns, and MetS was defined according to the NCEP-ATP III criteria. Global DI-GM scores came from the Global Dietary Database (164 nations), with MetS burden estimation employing Global Burden of Disease data. We explored mediation by phenotypic age, insulin resistance (HOMA-IR), and inflammation. Secondary analyses involved subgroup stratification, restricted cubic splines (RCS), sensitivity testing, propensity score matching, and multiple imputations. Results: Globally, higher national DI-GM correlated inversely with metabolic risk factor frequency and major MetS sequelae. NHANES analysis found each one-unit DI-GM rise was associated with an 11% lower MetS likelihood (adjusted OR = 0.89, 95%CI: 0.87–0.91). RCS indicated a non-linear exposure–response curve, with prevalence reduction stabilizing above DI-GM = 4.972. Stratification showed significant effect modification by race/ethnicity, education, and income (p-interaction < 0.05). Mediation analysis identified significant roles for phenotypic aging (24.77%) and HOMA-IR (47.98%), but not inflammation (0.54%). Conclusions: A DI-GM-aligned diet correlated with lower MetS prevalence, with insulin sensitivity and phenotypic aging accounting for part of this association. Microbiota-targeted nutrition guidelines for MetS should prioritize metabolic health over anti-inflammation.