Assessment of Serum 3-Epi-25-Hydroxyvitamin D₃, 25-Hydroxyvitamin D₃ and 25-Hydroxyvitamin D₂ in the Korean Population With UPLC–MS/MS

Background: The liquid chromatography–tandem mass spectrometry method (LC–MS/MS) identified an epimeric form of 25-hydroxyvitamin D₃ (3-epi-25-OH-D₃) concentrations. For the first time, the 3-epi-25-OH-D₃ was included in the Korea National Health and Nutrition Examination Survey (KNHANES) IX (from 2022 to 2024). In this study, we evaluated the results of 3-epi-25-OH-D₃ in KNHANES IX to determine its clinical status in Korea. Methods: We measured 3-epi-OH-D₃, 25-OH-D₃, and 25-OH-D₂ concentrations using an Ultra Performance (UP) LC–MS/MS, and assessed the differences in concentration ranges based on gender and age among participants in this national project. Results: The concentration range of 3-epi-25-OH-D₃ among 11,744 Korean participants (5193 male and 6551 female, aged 10–80 with a median age of 53) was 0.50 (lower limit of quantification) to 15.66 ng/mL, with a mean ± SD of 1.15 ± 0.85 ng/mL. The 3-epi-25-OH-D₃ concentration was not significantly different between genders. The concentrations of 25-OH-D metabolites were different across age groups. There was a highly significant correlation between 3-epi-25-OH-D₃ and 25-OH-D₃ concentrations (r = 0.77, R² = 0.5911, slope = 0.0579, p < 0.0001). The minimum and maximum of % 3-epi-25-OH-D₃ were 1.33% and 23.94%, respectively. The % 3-epi-25-OH-D₃ was highest in the highest quartile (Q4) of the total 25-OH-D₃ concentration (p < 0.0001). Conclusions: This is the first large-scale study of 3-epi-25-OH-D₃ concentrations in the Korean population. Accurate measurement of 3-epi-25-OH-D₃ is necessary for the reliable results of vitamin D metabolites in KNHANES.