Concurrent with the rising prevalence of kidney stones in the United States, obesity continues to dominate the American health landscape. In 2018, over 40% of U.S. adults were obese (body mass index >30) with approximately 9% having severe obesity (body mass index >40).¹

Despite lifestyle and pharmacological efforts to curtail this challenging condition, many patients ultimately require surgical intervention to significantly improve weight loss. From 2011 to 2019, bariatric surgery rates increased approximately 60% to an estimated 256,000 procedures performed annually. Historically, Roux-en-Y Gastric Bypass (RYGB) was the most performed procedure at almost 37% of all surgeries in 2011. Gastric banding (GB) was the second most common (35%), followed by sleeve gastrectomy (SG, 18%). However, GB rates have since declined significantly, and SG overtook RYGB as the most common in 2013. More contemporary data from 2019 demonstrated that 59% of all bariatric procedures were SG, 18% were RYGB, and only 0.9% were GB.²

Although both SG and RYGB have similar weight loss success, SG only restricts gastric capacity while RYGB also reroutes intestinal contents, which leads to malabsorptive side effects. Nelson and colleagues first reported the resultant enteric hyperoxaluria in 23 patients after RYGB.³ Due to fat malabsorption leading to calcium saponification and increased intestinal oxalate absorption, 21 of the 23 patients developed calcium oxalate stones, with 7 recurrent and 14 de novo stone formers. In total, 19 of these patients (90%) ultimately underwent a surgical procedure for kidney stones.³ But this small series was unable to calculate the actual incidence of kidney stones after RYGB.

Using claims data from 2002–2006, Matlaga and colleagues estimated that the overall incidence of kidney stones was approximately 7.7% in 4,639 patients 4 years after RYGB, compared to 4.6% in obese controls (p <0.0001).⁴ In addition, RYGB patients were more likely to undergo stone surgery compared to controls (3.3 vs 0.9%, p <0.01), with a mean time to a stone event of approximately 1.5 years. Similarly, in a single-institution retrospective series of 762 patients who underwent bariatric surgery matched to obese controls, Lieske and colleagues reported higher rates of nephrolithiasis after malabsorptive surgery (11.1 vs 4.3%, p <0.01) at a mean followup of 6 years.⁵

Without intestinal malabsorption, patients who have restrictive procedures do not appear to have an increased risk of kidney stones. In 2 retrospective reviews after GB, only 1.2% of 332⁶ and 1.5% of 201⁷ patients were found to have stones after followup of 42.5 and 30 months, respectively. Also reported was a low 1.2% (1 of 85) rate of stone formation with SG after 27 months of followup.⁶ Although possible that longer followup might demonstrate higher rates, the variations in 24-hour urine parameters help explain the lower risk.

In a prospective study comparing 24-hour urinalyses in 151 patients before and after RYGB, Valezi and colleagues noted multiple abnormalities.⁸ At 12 months after surgery, urine citrate decreased from 268 to 170 mg/day, urine volume decreased from 1.31 to 0.93 L/day, and urine oxalate increased from 24 to 41 mg/day. Urinary uric acid levels also increased 30% during the study period.⁸ Comparatively, after restrictive procedures urinary oxalate is often normal (35 vs 61 mg/day, p <0.001).⁹ But in the few patients who develop stones, meeting urine output goals can be challenging due to the small gastric pouch and inability to ingest large quantities of fluid.

As rates of obesity and severe obesity continue to climb, we will likely see more patients undergoing bariatric surgery. While malabsorptive procedures have significantly higher kidney stone risk, the reduced rates of these procedures and concurrent increase in restrictive procedures suggest we might see less enteric hyperoxaluria due to bariatric surgery over time. As the effects on kidney stone disease can be challenging, understanding the 24-hour urine changes in both restrictive and malabsorptive bariatric procedures is helpful to make impactful prevention recommendations.