Effect of vitamin D sterols and dietary magnesium on calcium and phosphorous homeostasis.

Calcium (Ca) and phosphorus (P) metabolism were studied in vitamin D-deficient rats as vitamin D status and dietary magnesium (Mg) were varied. Rats given normal (0.03%) or high (0.2%) Mg diets received either vehicle, vitamin D3 (1,650 pmol/day), or 1,25-dihydroxyvitamin D3 (60 pmol/day) for 9 days. In vitamin D-deficient rats, high dietary Mg lowered intestinal Ca absorption from 40 +/- 5 to 11 +/- 5%; P absorption decreased 50%. Treatment with 1,25-dihydroxyvitamin D3 prevented the Mg-induced fall in absorption; vitamin D3 did so only for the first 6 days. The total Ca and Mg (Ca + Mg) absorbed (mM/day) decreased from 0.85 +/- 0.050 mM/day to 0.14 +/- 0.10 with the high dietary Mg; 1,25-dihydroxyvitamin D3 treatment raised Ca + Mg absorption regardless of diet Mg; high dietary Mg raised serum Ca despite a decreased intestinal Ca absorption and urinary Ca; treatment with either sterol had no added calcemic effect. These results are consistent with two processes for intestinal Ca and P transport: one vitamin D-dependent and the other non-vitsamin D-dependent and inhibited by high dietary Mg. Also high dietary Mg increases serum Ca, perhaps by affecting bone resorption.