Gastric Bypass Surgery Increases Risk of Kidney Stones

Patients who undergo gastric bypass surgery experience changes in their urine composition that increase their risk of developing kidney stones, according to a new study.

Researchers at the University of Texas (UT) Southwestern Medical Center (Dallas, TX, USA; www.utsouthwestern.edu) conducted a cross-sectional study of urine and serum biochemistry in 19 non-kidney-stone forming patients who had Roux-en-Y gastric bypass (RYGB) surgery bypass, and compared the results to 19 gender, age, and body mass index (BMI)-matched obese controls without a history of nephrolithiasis.

The researchers found that the excretion of oxalate in urine was significantly greater (hyperoxaluria) in the participants who underwent the surgical procedure (47%), compared to those who did not (10.5%). In addition, citrate in the urine was lower (hypocitraturia) in many gastric bypass patients in comparison to the obese nonsurgical group (32% compared to 5%, respectively). However, the researchers also noted the prevalence of hyperoxaluria and hypocitraturia in the surgical patients was accompanied by significantly lower urine calcium than in obese controls, and that the calcium oxalate urine relative super-saturation ratio was not significantly different between the two groups. The study was published in the March 2010 issue of the Journal of Urology.

"Almost half of the patients who had undergone gastric bypass and did not have a history of kidney stones showed high urine oxalate and low urine citrate - factors that lead to kidney-stone formation,” said lead author Naim Maalouf, M.D., an assistant professor of internal medicine. "These findings illustrate that the majority of patients are at risk for kidney-stone formation after RYGB. This complication may not be well-recognized in part because it tends to occur months to years after the bypass surgery.”

Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate, the primary constituent of the most common kind of kidney stones. Citrate is a weak acid, and as such can be used as a component in buffer solutions; the same feature helps it inhibit kidney stone formation.

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University of Texas (UT) Southwestern Medical Center