Serum calcitriol and dietary protein intake in idiopathic calcium stone patients

In kidney stone patients, high protein intake and calcitriol overproduction are factors leading to hypercalciuria, but there are conflicting reports on the effects of dietary protein on calcitriol production. To investigate the relationships between serum calcitriol, dietary protein intake, and urinary calcium excretion, 33 male idiopathic calcium stone formers (aged 20-60 years), with normal renal function and on unrestricted diet, have been studied. Dietary protein intake was estimated by the protein catabolic rate determination. Abnormally elevated calcitriol levels were found in 16 patients (48.5%) who had similar levels of serum intact parathyroid hormone and phosphate, creatinine clearance, and calcium and phosphate urinary excretion, but lower protein catabolic rate (82+/-16 vs. 97+/-20 g/day, P<0.05) than the patients with normal calcitriol levels. The calcitriol to intact parathyroid hormone ratio was higher in hypercalciuric than in normocalciuric patients (2.4+/-1.1 vs. 1.6+/-0.8, P<0.05). Calcitriol was positively correlated with plasma calcium (r=0.41, P<0.01) and inversely with protein catabolic rate (r=-0.42, P<0.01). Protein catabolic rate was positively correlated with creatinine clearance (r=0.69, P<0.001) and urinary phosphate excretion (r=0.72, P<0.001). No relationship was observed between calcitriol and creatinine clearance. These results confirm the calcitriol overproduction in calcium stone disease and that the high calcitriol to intact parathyroid hormone ratio is the main feature associated with hypercalciuria. Calcitriol serum levels appear to be unrelated to creatinine clearance, whereas there is an inverse relationship with protein catabolic rate. This suggests that low rather than high dietary protein intake may favor the increase of calcitriol synthesis in male calcium stone formers with normal renal function.