The relationship between kidney function and body mass index before and after bariatric surgery in patients with chronic kidney disease.

Background: Improvements in kidney function post-bariatric surgery may be related to weight loss-independent effects.

Objectives: To characterize the dynamic relationship between body mass index (BMI) and estimated glomerular filtration rate (eGFR) before and after bariatric surgery in patients with chronic kidney disease (CKD).

Setting: Kaiser Permanente Southern California (KPSC) health system.

Methods: We conducted an observational, retrospective cohort study of patients with CKD stage 3 or higher who received bariatric surgery at the KPSC health system between 2007-2015. Bariatric surgery procedures included primary Roux-en-Y gastric bypass (RYGB) or sleeve gastrectomy (SG) procedures. Outcomes consisted of mean trajectory estimates and correlations of BMI and eGFR taken between 2 years before and 3 years after surgery. Multivariate functional mixed models were used to estimate how BMI and eGFR trajectories evolved jointly.

Results: A total of 619 RYGB and 474 SG patients were included in the final analytic sample. The measurements were available before surgery for a median time of 1.9 years for SG and 1.8 years for RYGB patients. Median follow-up times after surgery were 2.8 years for both SG and RYGB patients. The mean age at the time of surgery was 58 years; 77% of patients were women; 56% of patients were non-Hispanic White; the mean BMI was 44 kg/m; 60% of patients had diabetes mellitus; and 84% of patients had hypertension. Compared to the presurgery eGFR declines, the postsurgery declines in eGFR were 57% slower (95% credible interval [CrI], 33%-81%) for RYGB patients and 55% slower (95% CrI, 25%-75%) for SG patients. The mean correlation between BMI and eGFR was negligible at all time points.

Conclusion: Though bariatric surgery slowed declines in eGFR up to 3 years after surgery, changes in eGFR tracked poorly with changes in BMI. This study provides evidence that the kidney-related benefits of bariatric surgery may be at least partly independent of weight loss. Confirming this hypothesis could lead to mechanistic insights and new treatment options for CKD.