College of Public Health and Human Sciences

BACKGROUND & AIMS: Vitamin D is associated with many health outcomes and the blood concentration of 25-hydroxyvitamin D [25(OH)D] is commonly measured in clinical practice. A C-3 epimer of this compound, 3-epi-25(OH)D3, has recently been detected in blood samples. Few clinical assays currently detect this epimer and its physiological function is unknown, as are the demographic, behavioral, and physiologic factors that may be correlated with it. We sought to determine the correlation between these factors and 3-epi-25(OH)D3.

METHODS: We conducted a cross-sectional population-based study of 303 non-Hispanic white participants in the Survey of the Health of Wisconsin. Serum 25(OH)D2, 25(OH)D3 and 3-epi-25(OH)D3 were measured by high-performance liquid chromatography tandem mass spectrometry. We measured vitamin D intake from foods and supplements via a food frequency questionnaire, sun exposure by spectrophotometry, waist circumference during a physical exam, and additional demographic and behavioral factors by questionnaire. We calculated the percent of 3-epi-25(OH)D3 out of the total 25(OH)D3.

RESULTS: Summer season (P = 0.009), higher alcohol intake (P = 0.007), and higher vitamin D intake from supplements (P = 0.0004), but not food (P = 0.20), were significantly associated with a higher percent of 3-epi-25(OH)D3 relative to the total 25(OH)D3, although these associations appear to be partially driven by individuals with low 3-epi-25(OH)D3. Moreover, the percent of 3-epi-25(OH)D3 was significantly correlated with the total 25(OH)D3 (r = 0.37, P < 0.0001).

CONCLUSIONS: We report findings from an epidemiologic study of 3-epi-25(OH)D3 and show that individuals with lower total 25(OH)D3 tend to have a lower percent of 3-epi-25(OH)D3 relative to the total. While this is the largest reported sample of adults with measured 3-epi-25(OH)D3, the sample size of 303 is relatively small and replication of our findings is necessary.