# 24-hour urine chemistry shows higher stone formation risk after malabsorptive than restrictive type bariatric surgery

## TL;DR

Malabsorptive bariatric surgery increases kidney stone risk compared to restrictive surgery due to changes in urine chemistry.

## Contribution

This study compares lithogenic risk profiles between malabsorptive and restrictive bariatric surgeries using 24-hour urine chemistry.

## Key findings

- Malabsorptive surgery leads to higher urinary oxalate and lower citrate, increasing stone risk.
- Hyperoxaluria, hypocitraturia, and acidic urine were more common in malabsorptive patients.
- IPTW analysis confirmed higher odds of lithogenic profiles in the malabsorptive group.

## Abstract

Bariatric surgery is effective for obesity management but associated with kidney stone formation. Give the different post-operative physiology between restrictive type and malabsorptive type bariatric surgery, this study aims to compare difference in post-operative lithogenic risk profiles between these surgical types by assessing the postoperative 24-hour urine chemistry profiles. We conducted a prospective cross-sectional study of consecutive adults undergoing bariatric surgery at a tertiary center in Hong Kong between April 2017 and October 2019. A total number of 35 patients underwent malabsorptive and 55 underwent restrictive procedures. Baseline demographics, comorbidities, and postoperative 24-hour urine chemistry were assessed within 12 months after surgery. Abnormal urinary parameters were identified, with between-group comparisons performed using Mann–Whitney U and Chi-squared tests. Propensity scores were estimated using selected covariates, and stabilized inverse probability of treatment weighting (IPTW) was applied. IPTW-weighted logistic regression was used to compare the odds of abnormal urinary parameters between surgical groups. At 12 months, the malabsorptive group demonstrated significantly higher urinary oxalate and lower urinary creatinine, potassium, calcium, magnesium, citrate, urate, pH, and calcium phosphate activity compared with the restrictive group. The prevalence of hyperoxaluria (51.4% vs. 25.5%, p = 0.012), hypocitraturia (71.4% vs. 36.4%, p = 0.001), and acidic urine (54.3% vs. 20.0%, p = 0.001) was higher in the malabsorptive group. Conversely, hyperuricosuria was more common in restrictive patients (29.1% vs. 11.4%, p = 0.049). No significant differences were observed for urine volume, sodium, phosphate, or calcium oxalate activity. IPTW-weighted logistic regression demonstrated that malabsorptive procedures were associated with significantly higher odds of hyperoxaluria (OR 2.95, 95% CI 1.03–8.44), hypocitraturia (OR 4.13, 95% CI 1.40–12.21), hypomagnesuria (OR 3.26, 95% CI 1.11–9.57), and acidic urine pH (OR 3.76, 95% CI 1.33–10.64). Malabsorptive bariatric surgery is associated with more lithogenic urinary profiles than restrictive surgery, particularly hyperoxaluria, hypocitraturia, hypomagnesuria, and acidic urine, underscoring increased risk of postoperative nephrolithiasis. Close monitoring of urinary parameters and multidisciplinary management are recommended to mitigate stone risk.