Patients who have undergone gastric bypass surgery for morbid obesity have increased risk factors for the formation of kidney stones, according to a paper being presented at the American Society of Nephrology's 39th Annual Meeting and Scientific Exposition in San Diego.

'Gastric bypass surgery appears to lead to changes in the chemical composition of urine that could favor the formation of kidney stones,' comments Dr. Rajiv Kumar of Mayo Clinic College of Medicine in Rochester, Minnesota, one of the study authors. '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. All patients had undergone the 'Roux-en-Y' procedure, which is the most common type of gastric bypass surgery. 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 several chemical changes that have been linked to an increased risk of kidney stones. These included a significant increase in the level of oxalate-a chemical that increases kidney stone risk by binding to calcium.

The patients also had reduced urine levels of citrate, which normally plays an important role in dissolving crystals that can lead to kidney stones. The combination of high oxalate and low citrate levels led to an increase in calcium oxalate supersaturation'-a strong risk factor for stones formed by binding of oxalate to calcium.