Bariatric Surgery: Kidney Stone Risk?

Bariatric surgery is rapidly becoming a favorable gastrointestinal procedure, and surgeons have performed more than 500,000 procedures annually worldwide.

The procedures — Roux-en-Y, sleeve gastrectomy, laparoscopic adjustable gastric banding, and biliopancreatic diversion with duodenal switch – have been associated with improved long-term outcomes.

Some studies have associated these procedures with elevated risk for kidney stones; others have not.

A team of researchers from the Mayo Clinic published a meta-analysis assessing the risk of kidney stones following bariatric surgery in the latest issue of Renal Failure. They looked for studies that followed patients for two to four years after surgery.

The researchers found only four studies that met their strict inclusion criteria. One was a randomized controlled trial and the others were cohort studies. Among the studies, they were able to include 11,348 patients.

They determined that the pooled relative risk for kidney stones in patients undergoing bariatric surgery was 1.22.

Patients who had undergone Roux-en-Y gastric bypass were at higher risk than others, with a pooled relative risk of 1.73 compared to a control group that did not have surgery.

Restrictive procedures (ie. laparoscopic banding or sleeve gastrectomy), which are not as effective as Roux-en-Y procedures but seem to cause no hyperoxaluria, were associated with a lower risk of kidney stones (RR of 0.37).

Obesity itself raises risk of kidney stones by increasing urinary calcium and oxalate excretions, predisposing patients to calcium oxalate crystallization. Obesity-related insulin resistance acidifies urine, also creating an environment ripe for stones. Studies have shown that patients who undergo bariatric surgery often continue to have significant hyperoxaluria, especially in the six months following surgery.

The researchers indicate that bariatric procedures can cause fat malabsorption, which can lead to enteric hyperoxaluria and calcium oxalate stone formation. These findings are limited by the small number of studies and inclusion of only one randomized controlled study.