Effects of vitamin D and diet magnesium on magnesium metabolism.

Effects of 6-9 days of vitamin D3 (D3), 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], 24,25-dihydroxyvitamin D3 [24,25(OH)2D3], or 1,24,25-trihydroxyvitamin D3 [1,24,25(OH)3D3] on Mg metabolism were studied in vitamin D-deficient (-D) rats. Mg absorption expressed as percent intake increased with 1,25(OH)2D3 and 1,24,25(OH)3D3. Urinary Mg (UMg) increased with no change in serum Mg (SMg) or Mg balance. 1,25(OH)2D3 was threefold more potent than 1,24,25(OH)3D3 in raising serum Ca and Mg absorption. In a separate experiment in pair-fed rats given D3, 1,25-(OH)2D3, or 1,24,25(OH)3D3, the diet contained either 0.03 or 0.2% Mg; 1,25(OH)2D3 and D3 lowered SMg after 3 days; UMg increased after 24 h to remain elevated. D3 and 1,25(OH)2D3 augmented Mg absorption; feeding 0.2% Mg lowered Mg absorption in -D animals. All sterols augmented Mg absorption in -D rats; both the earlier action of 1,25(OH)2D3 and 1,24,25(OH)3D3 suggests that 1-hydroxylation is necessary. Suppressed Mg absorption with 0.2% Mg in -D rats suggests two transport processes, with one vitamin D dependent. Higher UMg with decreased SMg with 1,25(OH)2D3 suggests reduced tubular reabsorption.