Patients who have undergone gastric bypass surgery for morbid obesity are more prone to kidney stones formation. Gastric bypass surgery leads to changes in the chemical composition of urine that could favor the formation of kidney stones. Based upon this information, patients should take appropriate measures to reduce the potential for kidney stone formation.

21 patients who had undergone gastric bypass surgery 6 or 12 months previously, got their urine samples obtained to measure biochemical risk factors for kidney stones. All patients had used 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.

Patients who underwent gastric bypass surgery 6 months previously did not yet have significant changes in oxalate or citrate levels. Levels of other chemicals that can contribute to kidney stones such as uric acid and potassium were unaltered from before to after surgery. There were no changes in standard kidney function indicators after gastric bypass.

As the problem of obesity continues to increase around the world, more patients are undergoing gastric bypass operations. Previously, more extensive gastric bypass operations did lead to increased urinary oxalate levels, which led to increased risk of kidney stones. The new study is the first to specifically examine the risk of similar abnormalities following the “Roux-en-Y" procedure, which has become the standard approach to gastric bypass.

These results suggest that individuals undergoing such procedures are potentially at increased risk for developing kidney stones. Gastric bypass patients should talk to their doctors about dietary changes and other steps they can take to help reduce their risk of getting kidney stones.