Independent Associations of Vitamin D Levels with Visceral Fat and Metabolic Age in Adults: A Retrospective Cross-Sectional Study

Background/Objectives: Vitamin D deficiency has been associated with obesity and adverse metabolic outcomes. This study aimed to evaluate the associations between serum vitamin D levels and body composition parameters, including visceral fat level, body fat percentage, body mass index, muscle mass, and metabolic age, and to determine whether these relationships were independent of age and sex. Methods: This retrospective cross-sectional study included 164 individuals aged 18–64 years who had serum vitamin D measurements and concurrent body composition analyses between February 2023 and May 2025. Body composition was assessed using bioelectrical impedance analysis (Tanita BC-418), and serum vitamin D levels were measured by electrochemiluminescence immunoassay. Correlations were assessed using Spearman’s rank correlation analysis, and multivariate linear regression analyses were performed after adjusting for age and sex. Results: Serum vitamin D levels showed significant negative correlations with visceral fat level (r = −0.272; p < 0.001), body fat percentage (r = −0.225; p = 0.004), body mass index (r = −0.282; p < 0.001), and metabolic age (r = −0.254; p = 0.001). No significant association was found between serum vitamin D levels and muscle mass (p = 0.409). In multivariate regression analysis, serum vitamin D levels remained independently associated with visceral fat level (p = 0.017), body fat percentage (p = 0.015), and metabolic age (p = 0.026), whereas the association with body mass index was no longer significant (p = 0.114). Conclusions: Low vitamin D levels were independently associated with increased visceral adiposity, higher body fat percentage, and advanced metabolic age. Across all multivariable regression models, age consistently emerged as the strongest predictor, while vitamin D levels remained independently associated with visceral fat, body fat percentage, and metabolic age.