1. Vitamin D seems to play an essential role in the pathogenesis of idiopathic hypercalciuria at least in part via intestinal hyperabsorption of calcium. Hyper-absorption of calcium, in turn, might enhance the intestinal uptake of free oxalate, thus leading to hyperoxaluria. To verify this hypothesis we studied 75 calcium-stone-formers subdivided as follows: group 1 (15 patients) with isolated hyperoxaluria; group 2 (25 patients) with hyperoxaluria and hypercalciuria; group 3 (22 patients) with isolated hypercalciuria; group 4 (12 patients) with no metabolic abnormalities.

2. As expected, urinary calcium excretion differed in the various groups (P < 0.001), being highest in groups 2 and 3; urinary oxalate excretion, by definition highest in groups 1 and 2, was even more pronounced in group 2 than in group 1 (P < 0.05). Although in the normal range, the serum 1,25-dihydroxyvitamin D concentration was higher (P < 0.001) in the two hypercalciuric groups (2 and 3), showing peak levels in group 2.

3. When the data from the 75 stone-formers were pooled, there was a positive correlation between the serum concentration of 1,25-dihydroxyvitamin D and urinary calcium excretion (P < 0.001) and urinary oxalate excretion (P < 0.003), the latter relationship also being present when only the two hypercalciuric groups (groups 2 and 3) were considered together (P < 0.05).

4. Our data seem to confirm a relevant role for the vitamin D system in the pathogenesis of calcium nephrolithiasis due to increased intestinal calcium absorption, but also because this in turn induces a greater intestinal absorption of oxalate, thus leading to the occurrence or exacerbation of hyperoxaluria.

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