Gastric Bypass Surgery May Increase Risk of Kidney Stones

Roux-en-Y gastric bypass (RYGB), a surgical treatment for morbid obesity, appears to increase the risk of calcium oxalate kidney stones, according to a new study.

Researchers at the University of Minnesota Medical School (Minneapolis, USA) conducted a retrospective and cross-sectional study that involved 24 morbidly obese adults who underwent RYGB surgery between December 2005 and April 2007. Patients provided 24-hour urine collections for analysis 7 days before and 90 days after operation. Primary outcomes were changes in 24-hour urinary oxalate excretion and relative supersaturation of calcium oxalate from baseline to three months post-RYGB.

The researcher found that RYGB was associated with a significant increase in urinary oxalate excretion when compared with baseline. The procedure was also linked to relative supersaturation of calcium oxalate in six patients (25%). De novo hyperoxaluria was noted in several patients; the researchers were unable to identify any preoperative factors that predicted which patients would develop de novo hyperoxaluria or the magnitude of the increase in daily oxalate excretion. The study was published in the June 2008 edition of the Journal of the American College of Surgeons.

"This is the first study to demonstrate how rapid the changes in urinary risk for kidney stones occur,” said lead author Manoj Monga, M.D., of the department of urologic surgery. "This was surprising as previous studies have suggested it takes up to one year before the increase in oxalate is noted. The reason for the difference in our observation is that this is the first study to look at the three-month time point.”

In Roux-en-Y gastric bypass (RYGB), the stomach is reduced by creating a small pouch at the top of the stomach using surgical staples or a lap-band. The smaller stomach is connected directly to the middle portion of the small intestine (jejunum), bypassing the rest of the stomach and the upper portion of the small intestine (duodenum). RYGB was developed when patients treated for obesity with jejunoileal bypass (JIB, now replaced with other options) experienced a marked increased risk of hyperoxaluria, nephrolithiasis, and oxalate nephropathy.


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