Circulating total serum 25-hydroxyvitamin D (25OHD) is the most appropriate biomarker for the assessment of vitamin D status. Total 25OHD is the sum of 25OHD2 and 25OHD3 in serum. While the C3 epimer of 25OHD3 (3-epi-25OHD3) has been detected in serum from adults and infants( 1 , 2 ), very little is known about the bioactivity of the 3-epi series of vitamin D structural analogs. As a result, 3-epi-25OHD3 is not included in the calculation of total 25OHD in serum. The aim of this study was to quantify 3-epi-25OHD3 concentrations in maternal-cord dyads around the time of delivery. A total of 82 cord blood samples were collected and within a week of delivery, matched maternal samples were drawn. Serum 25OHD2, 25OHD3, and 3-epi-25OHD3 were extracted, separated and quantified using an LC-MS/MS method, which is traceable to the US National Institute for Standards and Technology (NIST) higher order reference measurement procedure( 3 ).
Mean (sd) of maternal and cord serum 25OHD3 concentrations were 38.2 (21) and 25.4 (15) nmol/L, respectively (P<0.001). 3-epi-25OHD3 was detected in all maternal and cord samples and while there was no significant difference between them (P=0.056), cord 3-epi-25OHD3 concentrations were 118% of maternal levels on average. The relative percentage of 3-epi-25OHD3 over 25OHD3 was significantly higher in cord samples (P<0.001), see table.
Levels of 3-epi-25OHD3 in mothers and cords tracked the seasonal variation in 25OHD3, see Figure. There were strong associations between maternal and cord concentrations of 25OHD3 (r=0.868) and the 3-epi-25OHD3 (r=0.672), (both P<0.001). In mothers, the association between serum 25OHD3 and the 3-epi-25OHD3 was r=0.902 and in cord serum was r=0.782 (both P<0.001).
The ubiquitous presence of 3-epi-25OHD3 in human serum, the strong correlation with 25OHD3, and the relatively large proportion of the epimer in cord blood are of interest. Although the concentrations of 3-epi-25OHD3 are not currently included in the estimate of total 25OHD, separate quantification of the epimer might be especially important for infants to provide a more complete estimate of total serum 25OHD concentrations, should the epimer have biological function and clinical significance.
Supported by the Higher Education Authority Program for Research in Third Level Institutions (PRTLI Cycle 4)