Published in Volume
67, Issue 3
(March 1, 1981)J Clin Invest.
1981, The American Society for
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
Laboratorium voor Experimentele Geneeskunde and Department of Obstetrics, Rega Instituut, Katholieke Universiteit Leuven, Leuven, Belgium
Published March 1, 1981
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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