Sex Disparities in the Processes Underlying Aging: Mitochondrial DNA Copy Number Associations with Dynapenia, 25-Hydroxyvitamin D₃ Levels and Quality of Life in Older Adults

Background/Objectives: Mitochondrial dysfunction, often reflected by a decline in mitochondrial DNA copy number (mtDNA-CN) in peripheral blood cells (PMBCs), is a key hallmark of biological aging and is linked to numerous adverse health outcomes, including frailty and cardiovascular disease. Furthermore, emerging evidence suggests that vitamin D may influence mitochondrial dysfunction. This cross-sectional study aims to investigate the associations of mtDNA-CN with muscular strength, self-rated health, and serum 25-hydroxyvitamin D₃ (25(OH)D₃) levels in a community-dwelling elderly population. Methods: A total of 149 elderly outpatients (≥65 years) from Soria, Spain, were included in this cross-sectional study. Muscular strength was assessed using the hand grip strength (HGS) test, and self-rated health-related quality of life (QoL) was measured using the EuroQoL five-dimension questionnaire (EQ-5D). Genomic DNA was extracted from peripheral blood, and mtDNA-CN was quantified using quantitative real-time PCR (qPCR). Serum 25(OH)D₃, intact parathyroid hormone (iPTH), phosphorus, calcium, albumin and other mineral metabolism markers were measured. Statistical analyses, including Spearman correlations and multivariate logistic regression, were performed to assess associations, with stratification by sex. Results: In the total population, a marginally significant positive correlation was observed between mtDNA copy number (mtDNA-CN) and serum 25(OH)D₃ levels (r = 0.210; p = 0.010), which did not remain significant after Bonferroni correction. Among women, lower mtDNA-CN was significantly linked to muscle weakness (p = 0.005), mobility problems (p = 0.009), and a trend toward self-care difficulties (p = 0.016). Multivariate analysis confirmed an independent association with increased mobility impairment risk (adjusted OR = 0.983; 95% CI: 0.97–1.00; p = 0.009). No significant associations were observed between mtDNA-CN and dynapenia or QoL components in the male group. Conclusions: This study identified a marginally significant positive correlation between serum 25(OH)D₃ levels and mtDNA-CN in the total population (r = 0.210; p = 0.010), which did not persist after Bonferroni correction, suggesting an exploratory link between vitamin D status and mitochondrial homeostasis in older adults. In addition, these results highlight sex-specific differences in mtDNA-CN as a potential biomarker of functional decline, particularly of mobility, in women. These findings support the idea that mtDNA-CN could serve as an integrated biomarker and that sex-specific nutrition could be used to promote healthy aging.