Plasma Levels of 25‐Hydroxyvitamin D 3 and In Vivo Markers of Cytochrome P 450 3 A Activity in Swedes and Koreans: Effects of a Genetic Polymorphism and Oral Contraceptives

In vitro studies have shown that vitamin D may induce several cytochrome P 450 ( CYP ) enzymes in general and CYP 3 A 4 in particular. The primary aim of this study was to investigate the relationship between plasma levels of 25‐hydroxyvitamin D 3 and suggested in vivo markers of CYP 3 A activity in healthy volunteers from S weden and K orea. Plasma concentrations of 25‐hydroxyvitamin D 3 were analysed in samples from three previously performed studies, and the correlation between these levels and suggested in vivo markers of CYP 3 A activity was investigated by means of nonparametric correlation. In addition, we studied the modulating effects of three vitamin D receptor promoter polymorphisms on the association between 25‐hydroxyvitamin D 3 and CYP 3 A enzyme activity in Swedish subjects. The plasma levels of 25‐hydroxyvitamin D 3 were not significantly associated with CYP 3 A phenotypes in any of the three studies, but after accounting for the vitamin D receptor polymorphism rs4516035, there was a significant positive association between 25‐hydroxyvitamin D 3 and CYP 3 A activity ( p  = 0.004). Swedes (n = 65) had significantly higher 25‐hydroxyvitamin D 3 levels than K oreans (n = 67), 75 nM compared with 31 nM ( p  < 0.001). Swedish women taking oral contraceptives ( OC ) (n = 19) had somewhat higher plasma levels of 25‐hydroxyvitamin D 3 compared with S wedish women not taking oral contraceptives (n = 21), 89 and 72 nM, respectively ( p  = 0.02). In conclusion, our results suggest that the overall influence on the CYP 3 A activity by 25‐hydroxyvitamin D 3 is of marginal importance.