INFLUENCE OF DIETARY CALCIUM ON SERUM 1,25‐DIHYDROXYVITAMIN D CONCENTRATIONS IN RENAL STONE FORMERS

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Abstract
The role of 1,25-dihydroxyvitamin D (1,25(OH)2D) in the pathogenesis of idiopathic hypercalciuria was studied in 37 renal stone formers who, during two 10-day periods, followed first a normal and then a low Ca diet. The following samples were taken during each diet: 24 h urine; fasting blood and urine; blood and urine following a 1 g oral Ca load. Patients were divided according to serum Ca level, 24 h urinary Ca excretion on the first diet and fasting Ca excretion on the 2nd diet. Eight patients were normocalciuric (NSF), 16 had absorptive hypercalciuria (AH), 5 renal hypercalciuria (RH) and 8 primary hyperparathyroidism. In NSF and AH, a positive correlation was found between the fasting and the 24 h urinary Ca (r = 0.787, P < 0.001), while negative correlations were found between the fasting urinary Ca and the serum parathyroid hormone (r = -0.703, P < 0.001) or the fasting urinary cAMP (r = -0.434, P < 0.01). Patients with RH had higher serum PTH and urinary cAMP levels for a given degree of fasting calciuria mainly on the low Ca diet. Mean serum 1,25(OH)2D was similar in NSF (43.6 .+-. 4.5 pg/ml), AH (43.6 .+-. 2.3 pg/ml) and RH (40.4 .+-. 4.8 pg/ml) on the 1st diet; increases were similar in all groups after 10 days of Ca restriction. A positive correlation was found between the serum 1,25(OH)2D concentrations and the 24 h urinary Ca excretion on the 1st diet in NSF (r = 0.889, P < 0.001) but not in AH or RH. There was no evidence of such correlation with the low Ca diet. No correlation between the calciuric response to Ca loading and the serum concentrations of 1,25(OH)2D was found. Serum concentrations of 1,25(OH)2D may be related to urinary Ca excretion in NSF more than in AH or RH. The factors responsible for the hyperabsorption of Ca in the latter patients remain to be elucidated.