Introduction: Despite advances in immunosuppression, long-term graft survival has barely changed in 20 years. Meanwhile, the obesity epidemic has worsened, and obesity is known to lead to glomerulopathy and albuminuria. We sought to quantify the association between body mass index (BMI) and decline in estimated glomerular filtration rate (eGFR) post-kidney transplant (KT) to inform obesity management in KT recipients (KTRs).
Methods: Our study population consisted of adult KTRs transplanted at our center between 01/02-01/23, followed from 6-months post-KT to graft failure, death or administrative censoring at 10-years post-KT. We obtained serial post-KT BMIs and serum creatinines (SCr). The exposure was defined as duration that KTR BMI was >30 an >35 kg/m2 since KT at each measurement of SCr. The mean rate of eGFR decline (and 95% CI) by post-KT BMI (potentially varying over time) was modeled using multi-level mixed effects linear regression, adjusting for follow-up time, with a person-level random intercept and random slope.
Results: Of 1511 KTRs, 33% were female; median (IQR) age at KT was 57 (47-65) years. KTRs were followed for a median (IQR) of 2.5 (1.3-4.7) years. 
Among all KTRs, eGFR declined at an annual rate of 3.34 (95%CI: 2.82-3.87) mL/min/1.73m2 (units). eGFR declined at an annual rate of 2.92 (95%CI: 2.34-3.50) units when BMI≤30 kg/m2, and at 4.19 (95%CI: 3.48-4.89) units when BMI>30 kg/m2. On average, the decline was 1.26 (95%CI: 0.57-1.96) units faster when BMI>30 kg/m2 vs ≤30 kg/m2 (p<0.001). 
Among the 453 KTRs with BMI>30 kg/m2 at KT, eGFR declined at an annual rate of 3.99 (95%CI: 2.47-5.51) units during the time when BMI≤30 kg/m2, and at 2.87 (95%CI: 1.88-3.86) units when BMI>30 kg/m2 (p=0.10). 
Among the 1058 KT recipients with BMI≤30 kg/m2 at KT, eGFR declined at an annual rate of 3.09 (95%CI: 2.45-3.73) units during the time when BMI≤30 kg/m2, 5.93 (95%CI: 4.87-6.99) units when BMI was >30 and ≤35 kg/m2 (difference:  2.84 [95%CI: 1.89-3.80] units faster decline, p<0.001), and 13.61 (95%CI: 9.73-17.49) units when BMI >35 kg/m2 (difference: 7.68 [95%CI: 3.82-11.53] units faster decline compared to BMI≤35 kg/m2, p<0.001).
Conclusions: The trajectory of eGFR decline post-KT differed by post-KT BMI. KTRs who gained weight after KT to BMI>30 kg/m2 had almost double the eGFR decline per year compared to those who maintained a BMI≤30 kg/m2. Weight gain to BMI>35 kg/m2 was especially harmful to eGFR. Identifying BMI as a potentially modifiable risk factor of faster decline in renal function might inform counselling of KT candidates and post-KT weight management. Indeed, obesity management may prove to be an essential component of post-KT care.