Dietary Fiber Intake and Phenotypic Age Acceleration in United States Adults: A Nationally Representative Cross-Sectional Study

Background: Biological age reflects physiological aging beyond chronological age and predicts morbidity and mortality.Higher dietary fiber intake has been linked to slower biological aging, but whether this reflects lower systemic inflammation or improved insulin resistance is unclear.The primary aim was to estimate the association between dietary fiber intake and phenotypic age acceleration among United States adults; a secondary, exploratory aim was to examine whether systemic inflammation or insulin resistance statistically accounts for this association.Methods: This cross-sectional study analyzed 3,896 adults aged 20 years and older from the National Health and Nutrition Examination Survey (NHANES) 2017-2018 cycle.Phenotypic age was computed from nine clinical chemistry biomarkers plus chronological age; phenotypic age acceleration (PhenoAgeAccel) was defined as the residual from regressing phenotypic age on chronological age.Dietary fiber was assessed by a 24-hour recall.Survey-weighted linear regression (Day 1 dietary weights, strata, and primary sampling units) was adjusted for age, sex, race/ethnicity, body mass index, income, energy intake, smoking, and physical activity.As an exploratory secondary analysis, the association was decomposed using the productof-coefficients method with the systemic immune-inflammation index (SII) and, in the fasting subsample, HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) as candidate mediators.Results: Each additional 10 g/day of dietary fiber was associated with a 0.66-year lower PhenoAgeAccel (95% CI -0.92 to -0.40; p<0.001) in the fully adjusted model, with a graded dose-response; the association was unchanged after excluding extreme values and did not differ by sex or race/ethnicity.In the exploratory analysis, systemic inflammation statistically accounted for 17.5% of the association (indirect effect -0.115; 95% CI -0.203 to -0.017); however, because the inflammation index shares leukocyte-derived components with the phenotypic age algorithm, a sensitivity analysis removing these shared components attenuated the indirect effect to -0.045, which accounted for 8.7% of the total effect estimated in that sensitivity model.Insulin resistance showed no statistically detectable indirect association in the fasting subsample.Conclusions: Higher dietary fiber intake was associated with lower phenotypic age acceleration in a nationally representative sample of United States adults.Systemic inflammation statistically accounted for a modest share, but the exploratory estimate is roughly halved once shared measurement is accounted for and cannot be interpreted causally given the cross-sectional design.These findings identify dietary fiber as a low-cost, broadly accessible target for prospective and interventional studies of healthy aging.