Biomarkers of Vitamin D Sufficiency: Vitamin D metabolite levels do not depend on 25-Hydroxyvitamin D2 levels in healthy Turkish individuals

Objective: Patient-specific factors may influence the dose of supplemental vitamin D necessary for adequacy, and it is important to verify which metabolite is considered clinically meaningful in order to prescribe vitamin D supplements. We assessed the relationship between 25(OH)D2 and free vitamin D levels in healthy Turkish population and those with vitamin D deficiency. Methods: Blood samples were taken from 92 healthy adults over the age of 18, representing both genders (n = 38 males and n = 54 females). Total 25(OH)D concentrations were determined using an immunoassay method called CMIA. In addition, 25(OH)D3 and vitamin D2 levels in serum were measured using liquid chromatography-mass spectrometry (LC-MS). Free 25(OH)D was measured and calculated according to the Bikle method. Results: Among the participants, 54% exhibited total 25(OH)D3 levels below 20 ng/mL. Those with total 25(OH)D3 concentrations of 20 ng/mL or higher had significantly elevated mean serum levels of 25(OH)D3 and free vitamin D compared to those with levels below 20 ng/mL (P < 0.001). In opposition, the mean concentration of 25(OH)D2 did not show significant variation based on the criteria for vitamin D sufficiency. Additionally, serum levels of 25(OH)D2 were consistent across individuals, regardless of their free vitamin D concentrations. Conclusion: The mean concentrations of serum 25(OH)D3 measured by LC-MS and free vitamin D affects the total 25(OH)D3 concentration as calculated by an automated system and indicates vitamin D deficiency. The mean concentration of 25(OH)D2 in serum, however, didn't differ among individuals according to vitamin D deficiency criteria and concentration of free vitamin D, thus does not directly indicate vitamin D deficiency.