To examine the relationship between body mass index and 24-hour urine constituents in a population of stone-forming patients.

A total of 880 patients who presented to a metabolic stone clinic for initial evaluation were analyzed. Patients were stratified by gender and divided into quartiles of body mass index. Associations between body mass index (BMI) and urine parameters were explored using bivariate and multivariate linear regression.

On bivariate analysis, increasing body mass index was associated with a significant increase in sodium, calcium, citrate, uric acid, magnesium, calcium oxalate, uric acid, and a decrease in pH in men. In women, it was associated with a significant increase in sodium, uric acid, oxalate, uric acid, and decreasing pH. On multivariate analysis, BMI was associated only with increases in sodium and calcium oxalate and decrease in pH in men. In women, multivariate analysis demonstrated positive association between BMI and urine sodium, creatinine, and phosphate and a negative relationship with urine citrate and sulfate.

Increasing body mass index was related to several risk factors for urinary stone disease in this study, including increasing urine sodium and decreasing pH in men and increasing urine uric acid, sodium, and decreasing urine citrate in women. Just as general recommendations for patients with nephrolithiasis include high voided volumes, low dietary sodium, and low animal protein intake, perhaps weight reduction should be included as part of the counseling of stone-formers to optimize 24-hour urine parameters.