Serum concentrations of vitamin D metabolites in vitamin D supplemented pregnant women. A longitudinal study.

The serum concentrations of the vitamin D metabolites 25-OHD, 1,25-(OH)2D, 24,25-(OH)2D and 25,26-(OH)2D, and of vitamin D binding protein (DBP), were determined longitudinally in 22 vitamin D supplemented pregnant women, and in 17 age-matched non-pregnant women studied during the summer. The pregnant women had higher 25-OHD and 1,25-(OH)2D, similar 24,25-(OH)2D, and lower 25,26-(OH)2D concentrations than the non-pregnant group. The relative concentrations of 24,25-(OH)2D and 25,26-(OH)2D (expressed as the molar ratio of these metabolites to 25-OHD) were lower during pregnancy. The DBP levels were increased in pregnancy, but the calculated free fraction (i.e. not bound to DBP) of the hormonal form of vitamin D, 1,25-(OH)2D, was still persistently higher in the pregnant than in the non-pregnant women. The study suggests that a daily vitamin D supplement of 400 IU satisfies the vitamin D requirement of pregnant women living in a cool climate with limited sun exposure. The increased absolute and relative concentration of 1,25-(OH)2D and decreased relative levels of 24,25-(OH)2D and 25,26-(OH)2D further suggest that the increased intestinal calcium and phosphate absorption, which is known to occur during pregnancy, is at least partially mediated by the vitamin D endocrine system.