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Purpose: Bariatric surgery is followed by multiple changes of urinary composition with a
propensity toward a lithogenic profile. We prospectively studied patients who
underwent Roux-en-Y gastric bypass (RYGBP) to assess urinary composition and lithiasis incidence.
Materials and Methods: One hundred and fifty one obese patients underwent RYGBP
and were followed for one year. The analysis comprised two study time points:
preoperative (T0) and one year after surgery (T1). They were analyzed for urinary
stones, blood tests and 24h-urinary evaluation. Nonparametric tests, logistic regression
and multivariate analysis were conducted using SPSS 17.
Results: Median BMI decreased from 44.1kg/m2 to 27.0kg/m2 (p=0.0001) in the
postoperative period. Urinary oxalate (24mg versus 41mg; p=0.000) and urinary uric
acid (545mg versus 645mg; p=0.000) increased significantly postoperatively
(preoperative versus postoperative, respectively). Urinary volume (1310ml versus
930ml; p=0.000), pH (6.3 versus 6.2; p=0.019), citrate (268mg versus 170mg;
p=0.000), calcium (195mg versus 105mg; p=0.000) and magnesium (130mg versus
95mg; p=0.004) decreased significantly postoperatively (preoperative versus
postoperative, respectively). Stone formers increased from 16 (10.6%) to 27 (17.8%)
patients in the postoperative analysis (p=0.001). Predictors for new stone formers after
RYGBP were postoperative urinary oxalate (p=0.015) and uric acid (p=0.044).
Conclusions: RYGBP determined profound changes in urinary composition which
predisposed to a lithogenic profile. The prevalence of urinary lithiasis increased almost
70% in the postoperative period. A metabolic evaluation for nephrolithiasis is
suggested for obese patients following RYGBP.