Influence of the Vitamin D-binding Protein on the Serum Concentration of 1,25-Dihydroxyvitamin D₃: SIGNIFICANCE OF THE FREE 1,25-DIHYDROXYVITAMIN D₃ CONCENTRATION

The influence of the serum binding protein (DBP) for vitamin D and its metabolites on the concentration of its main ligands, 25-hydroxyvitamin D₃ (25-OHD₃) and 1,25-dihydroxyvitamin D₃ (1,25-[OH]₂D₃) was studied. The concentration of both 1,25-(OH)₂D₃ and DBP in normal female subjects (45±14 ng/liter and 333±58 mg/liter, mean±SD, respectively; n = 58) increased during the intake of estro-progestogens (69±27 ng/liter and 488±90 mg/liter, respectively; n = 29), whereas the 25-OHD₃ concentration remained unchanged. A positive correlation was found between the concentrations of 1,25-(OH)₂D₃ and DBP in these women.

At the end of pregnancy, the total concentrations of 1,25-(OH)₂D₃ (97±26 ng/liter, n = 40) and DBP (616±84 mg/liter) are both significantly higher than in nonpregnant females and paired cord serum samples (48±11 ng/liter and 266±41 mg/liter, respectively). A marked seasonal variation of 25-OHD₃ was observed in pregnant females and their infants, whereas in the same samples the concentrations of both DBP and 1,25-(OH)₂D₃ remained constant throughout the year.

The free 1,25-(OH)₂D₃ index, calculated as the molar ratio of this steroid and DBP, remains normal in women taking estro-progestogens, however, and this might explain their normal intestinal calcium absorption despite a high total 1,25-(OH)₂D₃ concentration. In pregnancy the free 1,25-(OH)₂D₃ index remains normal up to 35 wk of gestation, but during the last weeks of gestation, the free 1,25-(OH)₂D₃ index increases in both circulations. A highly significant correlation exists between the (total and free) 25-OHD₃ and 1,25-(OH)₂D₃ concentrations in maternal and cord serum both at 35 and 40 wk of gestation.

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