Association between bilirubin levels and diabetic retinopathy in type 2 diabetes mellitus

Objective To investigate the correlation between serum bilirubin level and the risk of diabetic retinopathy (DR) in patients with type 2 diabetes mellitus of different sexes. Methods A total of 1 304 patients with type 2 diabetes mellitus were included in this study. The clinical data were collected and fundus examination was performed. According to the results of fundus examination, the patients were divided into DR group and Non-DR (NDR) group. The correlation between the levels of serum total bilirubin, direct bilirubin, indirect bilirubin, and the occurrence of DR was analyzed. Results The levels of total bilirubin, direct bilirubin, and indirect bilirubin in DR group were significantly lower than those in NDR group. Univariate analysis showed that the levels of total bilirubin and indirect bilirubin were negatively correlated with the occurrence of DR (P<0.01). There was no significant correlation between the level of direct bilirubin and the occurrence of DR. Smooth curve fitting showed that there was a U-shaped relationship between the levels of total bilirubin and indirect bilirubin and the risk of DR in women, while a negative correlation between total bilirubin, indirect bilirubin and the risk of DR in men. The results of multiple regression analysis showed that in men total bilirubin increased by 1 μmol/L, the risk of DR decreased by 8% (OR=0.92, 95%CI 0.88-0.98, P<0.01). Indirect bilirubin increased by 1 μmol/L, and the risk of DR decreased by 9% (OR=0.91, 95%CI 0.84-0.96, P<0.01). In women, when total bilirubin<12.8 μmol/L, for every 1 μmol/L increase in total bilirubin, the risk of DR decreased by 17%(OR=0.83, 95%CI 0.72-0.95, P<0.01); When total bilirubin≥12.8 umol/L, for every 1 μmol/L increase in total bilirubin, the risk of DR increased by 10%(OR=1.10, 95%CI 1.01-1.20, P<0.05); When indirect bilirubin<9.8 μmol/L, for every 1 μmol/L increase in indirect bilirubin, the risk of DR decreased by 20%(OR=0.80, 95%CI 0.68-0.94, P<0.01); When indirect bilirubin≥9.8 μmol/L, for every 1 μmol/L increase in indirect bilirubin, the risk of DR increased by 13%(OR=1.13, 95%CI 1.01-1.25, P<0.05). Conclusion This study shows that there is a U-shaped relationship between the levels of total bilirubin and indirect bilirubin and the risk of DR in female patients with type 2 diabetes mellitus, and there is a negative correlation between total bilirubin, indirect bilirubin and the risk of DR in male patients. However, there was no significant correlation between direct bilirubin and DR risk. Key words: Diabetes mellitus, type 2; Diabetic retinopathy; Bilirubin