ZINC NUTRITIONAL-STATUS MODULATES THE RESPONSE OF 1,25-DIHYDROXYCHOLECALCIFEROL TO CALCIUM DEPLETION IN RATS

Male Lewis rats (n = 27) were fed a nonpurified diet containing 0.9% calcium, 0.7% phosphorus, and 0.005% zinc until 8 wk of age. At this time rats were assigned randomly to one of two groups. Both groups were fed a low calcium, low zinc, purified diet (0.2% calcium, 0.4% phosphorus, <0.00007% zinc), but one group was fed 1.78 mg Zn/(animal.d). The zinc-replete animals were individually matched by weight to the zinc-depleted animals and pair-fed. Balances and plasma concentrations of zinc, calcium, and phosphorus and parathyroid hormone, 25 hydroxycholecalciferol [25(OH)D] and 1,25-dihydroxycholecalciferol [1,25(OH)2D] were determined at the start of calcium depletion and 2 wk later. Calcium and 25(OH)D levels were lower in both groups after calcium depletion. Dietary zinc had no significant effect on calcium or 25(OH)D levels. Phosphorus concentrations were lower after calcium depletion, but phosphorus concentration was higher in the zinc-depleted compared with the zinc-replete group at the end of the experiment. 1,25(OH)2D increased in both groups, but was higher in the zinc-replete than the zinc-depleted group at the end of the experiment. Calcium and phosphorus balances were greater in the zinc-depleted group at the end of the experiment. We conclude zinc depletion diminishes the response of 1,25(OH)2D to calcium depletion in rats. The mechanism is unknown, but may involve nonhormonally mediated changes in gastrointestinal absorption of calcium and phosphorus or an affect of zinc on extraintestinal processes.