Kidney stones a risk after stomach bypass surgery (Reuters)

NEW YORK (Reuters Health) - Morbidly obese adults who undergo a particular type of stomach bypass surgery called Roux-en-Y gastric bypass (RYGB) appear to be at increased risk of developing kidney stones earlier than previously thought. The increase in stone risk was evident just three months after the surgery.

"Although this study demonstrates that there is a higher risk for developing kidney stones (after RYGB), it is important to weigh the risk against the many benefits that RYGB has for the morbidly obese patient, including decreasing cardiac (illness) and improving diabetes," Dr. Manoj Monga, from the University of Minnesota Medical School in Minneapolis, noted in a written statement.

RYGB restricts the amount of food the stomach can hold, and a person can eat, by making it smaller, into a "pouch." It also reduces the amount of nutrients absorbed because the pouch bypasses parts of the small intestine.

Although RYGB has proven to be a safe and effective treatment for morbid obesity, the formation of kidney stones has recently raised concerns among patients undergoing RYGB. Kidney stones are small, pebble-like deposits that can result in severe pain and require an operation to remove them.

Monga and colleagues analyzed urine samples obtained 7 days before and 90 days after RYGB surgery in 24 morbidly obese adults.

At only 3 months after surgery, the researchers observed significant increases in calcium oxalate in urine, a known risk factor for the development of kidney stones. They also observed more calcium oxalate in urine than could normally be dissolved, potentially leading to kidney stone formation.

The reason for increased risk of kidney stones following this type of stomach bypass surgery is not entirely clear, the researchers say. It's possible that the anatomic rearrangement caused by the operation establishes a mildly malabsorptive state, which may be responsible for the increase in calcium oxalate in urine. Kidney stones could also result from an alteration in the normal gut bacteria caused by the procedure.

SOURCE: Journal of the American College of Surgeons, June 2008.