Gastric bypass patients at risk for kidney stones

January 1, 2009

Laparoscopic Roux-en-Y gastric bypass (RYGB), a popular form of weight reduction surgery for morbidly obese patients, significantly elevates the risk of kidney stone formation.

Winston-Salem, NC-Laparoscopic Roux-en-Y gastric bypass (RYGB), a popular form of weight reduction surgery for morbidly obese patients, significantly elevates the risk of kidney stone formation, according to a pair of studies.

The two studies-one from the University of Minnesota, Minneapolis, and a second led by researchers from Wake Forest University Baptist Medical Center, Winston-Salem, NC-had relatively short follow-up periods of 1 year and 6 months, respectively. Owing to their brevity, neither study found an increased incidence of stones among the patients undergoing the surgery, but both found significant increases in oxalate secretion. Both also found a supersaturation of calcium oxalate in a number of patients relatively soon after the surgical procedure.

"The take-home message from our study is that morbidly obese patients should be counseled regarding the increased risks for kidney stones postoperatively," Bryan Hinck, a former research assistant for senior author Manoj Monga, MD, at the University of Minnesota, told Urology Times. "Not that this should dissuade them from the surgery, but rather inform them of the importance of compliant follow-up."

"Based on our findings, patients who have gastric bypass are at increased risk for forming stones," added Bhavin N. Patel, MD, a Wake Forest urology resident who worked on that group's study with Dean G. Assimos, MD, and colleagues. "Indeed, some are at increased risk of nephropathy and perhaps irreversible kidney damage."

At the time he presented the study, Dr. Patel was University Baptist Medical Center, Winston-Salem, NC. Drs. Monga and Assimos are establishing themselves as leaders in this field of research.

The University of Minnesota study prospectively analyzed stone formation risk factors in 16 patients (10 women, six men) undergoing laparoscopic RYGB. The investigators found that mean urine oxalate levels increased from 33 mg/d to 49 mg/d at 1-year follow-up. The percentage of patients demonstrating hyperoxaluria (>45 mg/d) rose from 19% to 50% during the same period. Urinary volume levels vacillated from 1.68 L/d at baseline to 0.93 L/d at 3 months to 1.8 L/d at 1 year. The percentage of patients with low citrate levels (<320 mg/d) rose from 6.25% at baseline to 16.7% at 3 months to 18.75% at 1 year. Dietary records collected preoperatively and postoperatively (at 1 year) showed no significant changes in the consumption of kilocalories, protein, calcium, or sodium.

"We anticipated an increase in oxalate levels but were surprised by how quickly it developed in such a large proportion of patients," Hinck said.

He noted that the percentage of patients with hyperoxaluria more than doubled during the interval between the 3-month and 1-year follow-ups.

Hyperoxaluria prevalence

The Wake Forest study focused on the postoperative prevalence of hyperoxaluria in 59 morbidly obese patients with no history of nephrolithiasis prior to undergoing either RYGB (53 patients) or duodenal switch bypass (six patients). An age-matched, non-stone-forming group of adults served as controls.

Data were drawn from two 24-hour urine specimens taken at 6 months or more post-op. Patients undergoing the switch procedure presented the highest mean oxalate excretion levels, 91 mg/d, compared to 61 mg/d for the RYGB group and 32.9 mg/d for the controls. (The mean oxalate excretion for the overall bariatric surgery group was 64.3 mg/d). Twenty-nine of the patients (49%) demonstrated hyperoxaluria on both collections and 62% demonstrated hyperoxaluria in at least one 24-hour specimen.

Most notable, according to the investigators, was the finding that 25% of the patients demonstrated profound hyperoxaluria (>100 mg/d) in at least one sample. This appeared in three of the six switch patients and 12 of the 53 RYGB patients.