Evidence for Maternal and Fetal Differences in Vitamin D Metabolism

Summary

Vitamin D metabolism was studied in pregnant, D-deficient rats and their fetuses. D-depleted, pregnant rats were supplemented with [³H]25OHD₃ 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 was the predominant dihydroxylated D metabolite 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%). However, plasma levels of these two 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.