BACKGROUND: Patients treated for obesity with jejunoileal bypass (JIB) 
experienced a marked increased risk of hyperoxaluria, nephrolithiasis, and 
oxalate nephropathy developing. Jejunoileal bypass has been abandoned and 
replaced with other options, including Roux-en-Y gastric bypass (RYGB). Changes 
in urinary lithogenic risk factors after RYGB are currently unknown. Our purpose 
was to determine whether RYGB is associated with elevated risk of developing 
calcium oxalate stone formation through increased urinary oxalate excretion and 
relative supersaturation of calcium oxalate.
STUDY DESIGN: A prospective longitudinal cohort study of 24 morbidly obese 
adults (9 men and 15 women) recruited from a university-based bariatric surgery 
clinic scheduled to undergo RYGB 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 3 months 
post-RYGB.
RESULTS: Compared with their baseline, patients undergoing RYGB had increased 
urinary oxalate excretion (31 +/- 10 mg/d versus 41 +/- 18 mg/d; p = 0.026) and 
relative supersaturation of calcium oxalate (1.73 +/- 0.81 versus 3.47 +/- 2.59; 
p = 0.030) 3 months post-RYGB in six patients (25%). De novo hyperoxaluria 
developed. There were no preoperative patient characteristics predictive of 
development of de novo hyperoxaluria or the magnitude of change of daily oxalate 
excretion.
CONCLUSIONS: This prospective study indicates that RYGB is associated with an 
earlier increase in urinary oxalate excretion and relative supersaturation of 
calcium oxalate than previously reported. Additional studies are needed to 
determine longterm post-RYGB changes in urinary oxalate excretion and identify 
patients that might be at risk for hyperoxaluria developing.