Gastric bypass surgery may increase kidney stone risk

Patients who have undergone gastric bypass surgery for morbid obesity have increased risk factors for the formation of kidney stones, according to a study by Mayo Clinic researchers. The study, presented at the American Society of Nephrology annual meeting in San Diego, builds on somewhat conflicting research on bariatric surgery and stone risk presented by two groups at the 2006 AUA annual meeting.

“Gastric bypass surgery appears to lead to changes in the chemical composition of urine that could favor the formation of kidney stones,” said Rajiv Kimar, MD, of the Mayo Clinic College of Medicine, Rochester, MN. “Based upon this information, we suggest that patients take appropriate measures to reduce the potential for kidney stone formation.”

From 21 patients who had undergone gastric bypass surgery 6 or 12 months previously, the researchers obtained urine samples to measure biochemical risk factors for kidney stones. The same risk factors were measured in a group of 20 obese patients who were being evaluated for gastric bypass surgery.

The group who had bypass surgery 12 months previously showed a significant increase in the level of oxalate and reduced levels of urine citrate. Patients who underwent gastric bypass 6 months previously did not yet have significant changes in oxalate or citrate levels.