Gastric Bypass Patients at Risk of Kidney Stones

From Reuters Health Information

NEW YORK (Reuters Health) Feb 16 - Roux-en-Y gastric bypass surgery might increase patients' risk for kidney stones, researchers report in the March Journal of Urology.

As lead author Dr. Naim M. Maalouf told Reuters Health by email, "We found that the amount of oxalate -- a material found in kidney stones -- in urine was elevated in about half of the patients after Roux-en-Y gastric bypass surgery, even in those without kidney stones."

"The amount of citrate -- a compound that inhibits or slows down stone formation -- in the urine was low in two-thirds of the bypass patients," he added.

Dr. Maalouf and colleagues at the University of Texas Southwestern Medical Center, Dallas, compared 19 bariatric surgery patients with matched obese controls. Neither the patients (at an average of 3.5 years after surgery) nor the controls had any history of nephrolithiasis.

Compared to controls, the patients had significantly higher urinary oxalate than controls (45 versus 30 mg/day) and significantly lower urinary citrate (358 versus 767 mg/day). Patients' urinary calcium was also significantly lower (115 versus 123 mg/day).

Overall 47% of patients and 10.5% of controls had hyperoxaluria (urinary oxalate >45 mg/day), and 63% and 5%, respectively, had hypocitraturia (urinary citrate < 320 mg/day).

The relative hypocalciuria in the studied patients, the researchers point out, "offsets these lithogenic risk factors. The true incidence of nephrolithiasis and optimal treatment for lithogenic risk factors in this population remain to be established."

Dr. Maalouf points out, however: "The majority of patients are at risk for kidney stone formation after Roux-en-Y gastric bypass surgery. This complication is not well recognized because it tends to occur months to years after the bypass surgery."

J Urol 2010;183:1026-1030.