People who have had gastric bypass surgery seem to be at increased risk of kidney stones because of changes in their urine composition.

Gastric bypass surgery can improve the health of people with severe obesity through rapid weight loss. However, the operation is not without risk. A team at the Mayo Clinic now reveals that those undergoing gastric bypass may be more likely to develop kidney stones.

They took urine samples from 21 patients who had had gastric bypass surgery six to 12 months previously. The same measurements were made on 20 patients who were awaiting surgery. The first group had several biochemical changes in their urine which rendered them at increased risk of kidney stones, compared with the second group. These changes included increased levels of oxalate and reduced levels of citrate. Those patients who had had their bypass six months previously did not yet have significant changes in urine composition. There were no changes in kidney function after gastric bypass. These findings suggest that people having gastric bypass could be at increased risk of kidney stones and therefore might need to discuss steps, such as dietary changes, that they can take to avoid this complication.

Source
American Society of Nephrology meeting 11th November 2006