Association of urinary pH with body weight in nephrolithiasis.
BACKGROUND
The prevalence of kidney stone disease in the United States is progressively increasing, paralleling the growing rate of obesity. Uric acid nephrolithiasis, a condition associated with a low urinary pH, has been linked to obesity and insulin resistance. Based on these observations, we hypothesized that urinary pH may be inversely associated to body weight in nephrolithiasis.


METHODS
Data were retrieved from 4883 patients with nephrolithiasis who underwent ambulatory evaluation at two established stone clinics in Dallas and Chicago. The patients collected 24-hour urine samples on an outpatient basis, while avoiding any drug that could alter urinary pH. Patients were divided in increasing sextiles of body weight, and urinary pH was adjusted for urinary creatinine and for age.


RESULTS
Urinary pH had a strong, graded inverse association with body weight. Urinary creatinine and age were both found to be significant covariates of urinary pH, while gender was not a significant independent variable after adjustment for urinary creatinine. Mean 24-hour urinary pH, adjusted for age and urinary creatinine, were 6.09, 6.04, 6.01, 5.99, 5.97, and 5.91 for sextiles of body weight in increasing order from Dallas (P for linear trend <0.0001), and 6.18, 6.10, 6.04, 6.02, 5.97, and 5.88 for the sextiles from Chicago (P for linear trend <0.0001).


CONCLUSIONS
We conclude that urinary pH is inversely related to body weight among patients with stones. The results confirm the previously proposed scheme that obesity may sometimes cause uric acid nephrolithiasis by producing excessively acid urine due to insulin resistance.
Maalouf
Naim M
NM
Center for Mineral Metabolism and Clinical Research, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA.
Sakhaee
Khashayar
K
Parks
Joan H
JH
Coe
Frederic L
FL
Adams-Huet
Beverley
B
Pak
Charles Y C
CY
D000328:Adult; D000368:Aged; D001835:Body Weight; D002940:Circadian Rhythm; D003404:Creatinine / Q000652:urine; D005260:Female; D006801:Humans; D006859:Hydrogen / Q000652:urine*; D006863:Hydrogen-Ion Concentration; D007669:Kidney Calculi / Q000473:pathology* / Q000652:urine*; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies
D006859:Hydrogen; D003404:Creatinine