Case-control study of risk factors for idiopathic calcium nephrolithiasis.

We compared epidemiological risk factors and urine excretion of calcium, phosphate, uric acid, urea nitrogen, sodium, potassium, and fluid volume in recurrent idiopathic calcium stone-formers and in a control group of age- and sex-matched normal volunteers. Stone-formers were less likely than normal subjects to have followed a low-calorie diet, but body weight did not differ between the two groups. Daily urine calcium excretion was a graded risk factor for stone formation throughout its range. Daily urine urea nitrogen and potassium excretion were lower in stone-formers than in controls, but excretion of uric acid, sodium, phosphate, and creatinine did not differ. However, there were positive associations between urine calcium excretion and the urine excretion of sodium, urea nitrogen, uric acid, phosphate, and creatinine in stone-formers; and these associations were significantly stronger than in normal subjects. We conclude that urine calcium excretion is a major risk factor for idiopathic calcium stone formation, but cannot confirm such a role for urine uric acid excretion. Total dietary protein intake may be lower in stone-formers than in controls. However, stone-formers may be more sensitive than normal subjects to the calciuric effects of protein and sodium. A strong association of urine calcium excretion with urine phosphate excretion in stone-formers is probably independent of dietary protein intake.