Determinants of bone mineral density and calcium excretionin idiopathic hypercalciuria

Background. Alterations of enteral and tubular Ca handling, bone mineral density (BMD) and erythrocyte membrane (Ca-Mg)ATPase have been observed in idiopathic hypercalciuria. In the present study factors affecting BMD were analysed in hypercalciuric patients. Methods. Results in 116 hypercalciuric and 109 normocalciuric idiopathic Ca stone formers were compared. Na, Ca and phosphate excretion were measured. BMD was analysed by DEXA at lumbar-spine and femoral sites. (Ca-Mg)ATPase activity was determined by coupled enzyme assay in erythrocytes. Enteral Ca absorption and tubular reabsorption were assessed using Sr oral load test in 98 hypercalciuric and 62 normocalciuric patients. Results. Lumbar-spine BMD z-score was lower in hypercalciuric than normocalciuric patients (-1.05±0.118 vs -0.65±0.114; p<0.02). Na excretion (182±7.2 vs 156±6.6 mmol/24 hours; p<0.02), phosphate excretion (28.39±0.985 vs 24.16±0.759 mmol/24 hours, p<0.001) and creatinine clearance (110±3.7 vs 98±3.5 ml/min, p<0.02) were higher in hypercalciuric patients, but only in those with BMD z-score ≥-2 these three variables resulted increased. In hypercalciuric patients Sr absorption (1.38±0.060 vs 1.17±0.068 mmol·L-1·min-1; p<0.05) and (Ca-Mg)ATPase activity were elevated independently of BMD, while Na excretion was positively correlated with lumbar-spine BMD (r=0.258, p<0.01). Conclusions. Increase enteral calcium absorption is involved in the mechanisms of idiopathic hypercalciuria. Increased Na intake could favour bone mass preservation in hypercalciuric patients, influencing enteral absorption and urinary excretion of calcium and phosphate.