Osteocalcin response to calcium-restricted diet: a helpful tool for the workup of hypercalciuria.

OBJECTIVES: The treatment of hypercalciuria is controversial. This study investigated whether the osteocalcin response (OCR) to a calcium-restricted diet is a potential tool for the differential therapy of hypercalciuria. 56 patients with calcium urolithiasis were investigated.

METHODS: Osteocalcin (OC) was measured on a free diet (OC 1) and after a calcium-restricted diet (400 mg/24 h for 1 week, OC 2). The OCR was expressed as 100 x (OC 2 - OC 1)/OC 1. A calcium load test and vertebral mineral density measurements (L2-L4) were performed additionally.

RESULTS: 27 patients showed diet-dependent hypercalciuria, 15 of them being classified as absorptive hypercalciuria type II (AH II). 12 were normocalciuric (NC). 29 patients had diet-independent hypercalciuria, 22 being classified as renal hypercalciuria (RH), 7 as absorptive hypercalciuria type I (AH I). 22 patients (42%) showed a positive OCR, indicating an increased bone turnover. Vertebral mineral density was lower in patients with positive than with negative OCR. Generally patients with RH or NC showed a positive OCR, patients with AH a negative or no OCR. 2 patients with AH, however, showed highly positive OCR, indicating negative calcium balance. Long-term follow-up on a low calcium diet showed a steady decrease in bone mineral density.

CONCLUSIONS: This demonstrates that the OCR is more reliable to determine the appropriate management of hypercalciuria than calcium load tests. For practical purposes, we suggest treating hypercalciuric patients with positive OCR with thiazides; diet-dependent hypercalciuria with negative OCR should be treated with a low calcium diet.