Evidence for Maternal and Fetal Differences in Vitamin D Metabolism

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Abstract
Vitamin D metabolism was studied in pregnant, D-deficient rats and their fetuses. D-depleted, pregnant rats were supplemented with [3H]25OHD3 [25-hydroxy-vitamin-D] on the 19th day of pregnancy. The distribution and metabolism of radiolabeled D metabolites was different in maternal and fetal blood, kidneys and small intestine. 24,25(OH)2D3 [24,25-dihydroxy vitamin D] was the predominant dihydroxylated D meabolite in the fetus, whereas 1,25(OH)2D3 was the predominant dihydroxylated D metabolite in the mother. The ratio of 24,25(OH)2D3:1,25-(OH)2D3 was 12-fold greater in fetal plasma than maternal plasma. Maternal nephrectomy reduced the metabolism of [3H]25OHD3 to 24,25(OH)2D3 (43%) and 1,25(OH)2D3 (75%). Plasma levels of these 2 metabolites were unchanged in the fetuses of these animals when compared with levels observed in fetuses from mothers with intact kidneys. These results suggest the possibility of independent control of 25OHD3 metabolism by the feto-placental unit and raise questions as to the possible role of 24,25(OH)2D3 in fetal development.