Distribution of free 25OHD in elderly population based on LC-MS/MS

Vitamin D deficiency is a global health problem in all age groups, especially in the elderly population. Serum 25(OH)D is the biomarker to assess vitamin D nutrition status. However, the free hormone hypothesis proposes that free vitamin D might be a more reliable marker of vitamin D nutrition status. Thus, the aims of this study were to (1) evaluate the distribution of free 25OHD in elderly individuals, and (2) to assess the association between free 25OHD and total 25(OH)D, 1,25(OH)₂D, 24,25(OH)₂D, calcium (Ca), alkaline phosphatase (ALP), and phosphorus (P) in elderly population. A total of 312 healthy elderly individuals were enrolled in this study and residual serum samples were collected. Free 25OHD, total 25(OH)D, 24,25(OH)₂D, and 1,25(OH)₂D were measured using LC-MS/MS. Other biochemical analytes were measured using automatic analyzers. Our results showed that with an increase in the levels of total 25(OH)D, the levels of 25(OH)D₃, 1,25(OH)₂D, 24,25(OH)D, and free 25OHD increased, whereas the levels of 25(OH)D/24,25(OH)2D decreased. Further, we observed that the level of free 25OHD was significantly positively correlated with the total 25(OH)D (r = 0.226, P < 0.001), 25(OH)D (r = 0.221, P < 0.001), and 24,25(OH)₂D (r = 0.231, P < 0.001) but was negatively correlated with 25(OH)D/24,25(OH)₂D (r = -0.185, P < 0.01). Moreover, the total 25(OH)D, 25(OH)D₃, 24,25(OH)₂D, and 25(OH)D/24,25(OH)₂D were correlated with 1,25(OH)₂D. Furthermore, free 25OHD was positively correlated with creatinine (Cr) (r = 0.227, P <0.001). Our results showed a narrower distribution for free 25OHD than that reported by direct measurement techniques and confirmed the correlation between free 25OHD and total 25(OH)D.