This study investigates the independent and combined effects of serum 25-hydroxyvitamin D (25(OH)D) concentrations and physical activity (PA) on phenotypic age acceleration (PhenoAgeAccel) in a nationally representative sample of 18,738 US adults from the National Health and Nutrition Examination Survey (NHANES) 2007-2010 and 2015-2018. The analysis uses weighted multivariable logistic regression models to assess the relationship between 25(OH)D and PA with PhenoAgeAccel, and the population attributable fraction (PAF) to estimate the proportion of PhenoAgeAccel that could be avoided if exposure were eliminated. Key findings include: - Higher 25(OH)D levels and adequate PA were significantly associated with lower risk of PhenoAgeAccel, with a multiplicative interaction observed in participants aged ≤65 years. - The combined effect of high 25(OH)D levels (≥80.4 nmol/L) and PA resulted in a 1.291-year reduction in PhenoAge. - The PAF analysis showed that 30.7% of the reduction in PhenoAgeAccel was attributable to both high 25(OH)D levels and adequate PA. The study concludes that higher 25(OH)D levels and PA are associated with a lower risk of biological aging, with enhanced protective effects when combined, particularly in middle-aged and young adults. These findings highlight the importance of outdoor PA in slowing down the aging process.This study investigates the independent and combined effects of serum 25-hydroxyvitamin D (25(OH)D) concentrations and physical activity (PA) on phenotypic age acceleration (PhenoAgeAccel) in a nationally representative sample of 18,738 US adults from the National Health and Nutrition Examination Survey (NHANES) 2007-2010 and 2015-2018. The analysis uses weighted multivariable logistic regression models to assess the relationship between 25(OH)D and PA with PhenoAgeAccel, and the population attributable fraction (PAF) to estimate the proportion of PhenoAgeAccel that could be avoided if exposure were eliminated. Key findings include: - Higher 25(OH)D levels and adequate PA were significantly associated with lower risk of PhenoAgeAccel, with a multiplicative interaction observed in participants aged ≤65 years. - The combined effect of high 25(OH)D levels (≥80.4 nmol/L) and PA resulted in a 1.291-year reduction in PhenoAge. - The PAF analysis showed that 30.7% of the reduction in PhenoAgeAccel was attributable to both high 25(OH)D levels and adequate PA. The study concludes that higher 25(OH)D levels and PA are associated with a lower risk of biological aging, with enhanced protective effects when combined, particularly in middle-aged and young adults. These findings highlight the importance of outdoor PA in slowing down the aging process.