Gastric bypass surgery may increase kidney stone risk

Morbidly obese patients who undergo a particular type of gastric bypass surgery called Roux-en-Y gastric bypass (RYGB) are at increased risk of developing kidney stones within a few months following the procedure, rather than several months to years afterward, according to research published in the Journal of the American College of Surgeons (2008; 206:1145-53).

Urologists at the University of Minnesota, Minneapolis, conducted a prospective, longitudinal study of 24 morbidly obese adults from a university-based bariatric surgery clinic who were scheduled to undergo RYGB between December 2005 and April 2007. Five patients had a history of nephrolithiasis. Patients provided 24-hour urine collections for analysis 7 days before and 90 days after the operation. Primary endpoints were change in the amount of the compound oxalate in the urine and the relative supersaturation of calcium oxalate from baseline to 3 months post-RYGB.

Significant increases were noted in urinary oxalate excretion (31±10 mg/d vs. 4 ±18 mg/d; p=.026) and relative supersaturation of calcium oxalate (1.73±0.81 vs. 3.47±2.59; p=.030) at only 3 months post-RYGB. Six patients (25%) developed de novo hyperoxaluria, with oxalate excretion increasing from 26±12 mg/d to 63±12 mg/d (p=.0046). There were no preoperative patient characteristics predictive of increased risk of kidney stone formation.