Impact of urinary sodium on the urinary risk factors for calcium oxalate nephrolithaisis
Brian H. Eisner, MD1, Michael L. Eisenberg, MD2, Marshall L. Stoller, MD2.
1Massachusetts General Hospital, Boston, MA, USA, 2University of California, San Francisco, San Francisco, CA, USA.
BACKGROUND: Calcium excretion parallels sodium excretion in the proximal tubule of the kidney. It is believed that increased dietary and urinary sodium promote hypercalciuria and increase the risk of nephrolithiasis. A low salt diet, therefore, is commonly recommended for all calcium stone-formers. We have previously demonstrated that a high sodium diet did not significantly increase urinary calcium or supersaturation of calcium oxalate in hypocitraturic stone-formers. The purpose of this study was to examine the relationship between urinary sodium and urinary risk factors for nephrolithiasis in a population of stone-formers who presented for metabolic evaluation.
METHODS: A retrospective review of 969 patients evaluated in a metabolic stone clinic with 24-hour urinalysis was undertaken. Multivariate linear regression analysis was performed to determine the relationship between urine sodium and urine calcium, urine volume, and
urine supersaturation of calcium oxalate (SSCaOx). Multivariate analysis adjusted for the following covariates: age, sex, BMI, thiazide use, and urinary constituents (calcium, sodium, oxalate, volume, citrate pH, uric acid, phosphate, and magnesium.
RESULTS: Tests of linear trend demonstrated the following: urinary calcium and urinary volume increased with increasing urinary sodium (p<0.001 for both). Urinary supersaturation of calcium oxalate (SSCaOX) decreased with increasing urinary sodium (p = 0.05). Multivariate linear regression adjusted for age, sex, BMI, thiazide use, and urinary constituents. Urinary sodium was positively associated with urinary calcium (ß=0.27, 95% CI 0.15-0.39, p<0.001). Urinary sodium was negatively associated with urinary SSCaOx (ß=-0.013, 95% CI -0.015- -0.01,p<0.001). There was a trend towards a positive association between urinary sodium and urine volume (ß=0.001, 95% CI -0.00006-0.002, p=0.07).
CONCLUSIONS: Increasing urine sodium was associated with increasing urine calcium, increasing urine volume, and decreased urine SSCaOx. We hypothesize that higher salt intake promotes thirst and therefore, increases oral fluid intake and hydration. Sodium restriction may not be an appropriate global recommendation for all calcium stone-formers.