Association between serum bilirubin and diabetic retinopathy: results from the NHANES 1999–2018 and Mendelian randomization study

Although serum bilirubin levels have been linked to diabetic retinopathy (DR), their causal role remains uncertain. We aimed to assess the relationship between bilirubin levels and diabetic retinopathy (DR) using both observational analyses and Mendelian randomization (MR) to infer causality. The relationship between bilirubin levels and the risk of DR was analyzed using a cross-sectional study design and MR analysis, respectively. Data for the observational study were obtained from the 1999–2018 National Health and Nutrition Examination Survey (NHANES). Logistic regression was used to evaluate the association between bilirubin levels and DR. For MR, genetic instrumental variables (single-nucleotide polymorphisms, SNPs) associated with bilirubin levels and DR were obtained from the IEU Open genome-wide association studies (GWAS) project. The results of the inverse variance weighting (IVW) analysis were the primary results of the MR analysis. Odds ratio (OR) and 95% confidence interval (CI) were used to report results. A total of 8,274 diabetic patients from NHANES were included, of whom 1,242 (13.57%) had DR. Diabetic patients with serum total bilirubin > 10.3 µmol/L (vs. ≤ 10.3 µmol/L) had a lower prevalence of DR (OR = 0.77, 95% CI: 0.62–0.96). This association was observed in males (OR = 0.62, 95% CI: 0.47–0.81), individuals with chronic kidney disease (CKD) (OR = 0.72, 95% CI: 0.56–0.94), those with body mass index (BMI) ≥ 25 kg/m² (OR = 0.79, 95% CI: 0.63–1.00, P = 0.049), and those aged ≥ 65 years (OR = 0.76, 95% CI: 0.59–0.99, P = 0.043). MR analysis showed that genetically higher serum total bilirubin levels were associated with reduced risk of proliferative DR (PDR) [IVW OR = 0.910 (95% CI: 0.831–0.997), P = 0.044], whereas genetically predicted bilirubin was not related to non-proliferative DR (NPDR) (P = 0.893) or any DR (P = 0.481). This study support a potential causal relationship between higher serum total bilirubin levels and lower risk of PDR.