Analyzing the Impact of Bariatric Surgery in Kidney Function: a 2-Year Observational Study

Obesity is an independent risk factor for chronic kidney disease (CKD). Our aims were: (1) to evaluate the impact of bariatric surgery (BS) on kidney function, (2) clarify the factors determining postoperative evolution of glomerular filtration rate (ΔGFR) and urinary albumin-to-creatinine ratio (ΔUACR), and (3) access the occurrence of oxalate-mediated renal complications.

We investigated a cohort of 1448 obese patients who underwent BS. Those with baseline-estimated GFR (eGFR0) < 30 mL/min or without information about the 2-year post-surgical eGFR (eGFR2) were excluded.

A total of 725 patients were included. At baseline, 38(5.2%) had hyperfiltration with eGFR0 ≥ 125 mL/min/1.73m² (G0), 492 (67.9%) had eGFR0 90-124 mL/min/1.73m² (G1), 178 (24.6%) had eGFR0 60-89 mL/min/1.73m² (G2), and 17 (2.3%) had eGFR0 < 60 mL/min/1.73m² (G3). ΔGFR significantly increased in 96.6% (ΔGFR = 23.8 (IQR 15.9-29.8)) and 82.4% (ΔGFR = 18.6 (IQR 3.6-44.0)) of the subjects with G2 and G3 CKD, respectively (p < 0.001). The variables independently associated with ΔGFR were baseline body mass index (BMI) (positively), high blood pressure (HBP) (negatively), and fasting plasma glucose (FPG) (negatively), as well as FPG variation (positively). An overall prevalence of high UACR (≥ 30 mg/g^-1) of 17.9% was found, with 81.5% of these subjects presenting A2(30-300 mg/g^-1) and 18.5% A3(> 300 mg/g^-1) UACR. UACR significantly decreased after BS (p < 0.001). Significant predictors of ΔUACR were BMI, systolic blood pressure, and HbA1c. Urinary excretion of calcium oxalate crystals was found in 77(11.1%) patients, with only 1 presenting oxalate-mediated renal complications.

ΔGFR seems to be influenced by the initial kidney function, as it decreases in subjects with hyperfiltration but tends to increase in those with kidney dysfunction. These results suggest that BS is associated with improvement of kidney outcomes, without a significant increase in renal complications.