PATIENTS undergoing malabsorptive bariatric surgery may face a substantially higher risk of kidney stone formation than those receiving restrictive procedures, according to new research. The findings highlight important differences in post-operative urine chemistry that could influence long-term renal outcomes after weight-loss surgery.
Comparing Restrictive and Malabsorptive Bariatric Surgery
Bariatric surgery is an effective treatment for obesity and its related comorbidities, but it has long been associated with an increased incidence of nephrolithiasis. Because restrictive and malabsorptive procedures alter gastrointestinal physiology in distinct ways, the authors sought to compare their effects on lithogenic risk profiles using detailed 24-hour urine analyses.
In this prospective cross-sectional study, 90 adults who underwent bariatric surgery at a tertiary centre in Hong Kong between 2017 and 2019 were evaluated within 12 months post-operatively. Of these, 35 patients received malabsorptive procedures, while 55 underwent restrictive surgery. Baseline demographics, comorbidities, and post-operative urine chemistry parameters were assessed. Between-group comparisons were adjusted using stabilised inverse probability of treatment weighting (IPTW) to account for confounding factors.
Restrictive Bariatric Surgery Shows Lower Stone-Forming Risk
At one year after surgery, patients in the malabsorptive group showed a markedly more lithogenic urinary profile. They had significantly higher urinary oxalate levels alongside lower urinary creatinine, potassium, calcium, magnesium, citrate, urate, and pH compared with the restrictive group. The prevalence of hyperoxaluria was more than double in malabsorptive patients (51.4% vs. 25.5%), while hypocitraturia (71.4% vs. 36.4%) and acidic urine pH (54.3% vs. 20.0%) were also significantly more common. In contrast, hyperuricosuria occurred more frequently after restrictive surgery.
IPTW-weighted logistic regression confirmed that malabsorptive procedures were independently associated with higher odds of several key stone-promoting abnormalities. These included hyperoxaluria, hypocitraturia, hypomagnesuria, and acidic urine pH, all of which are well-recognised contributors to kidney stone formation. Notably, urine volume and calcium oxalate activity did not differ significantly between groups.
The authors conclude that malabsorptive bariatric surgery confers a greater risk of developing lithogenic urinary changes than restrictive approaches, underscoring the need for vigilant post-operative surveillance. They recommend close monitoring of urinary parameters and a multidisciplinary strategy involving surgeons, nephrologists, and dietitians to mitigate kidney stone risk in this growing patient population.
Reference
Liu AQ et al. 24-hour urine chemistry shows higher stone formation risk after malabsorptive than restrictive type bariatric surgery. Sci Rep. 2026; doi: 10.1038/s41598-026-37440-y.
