Non-Linear Relationship Between Fasting C-Peptide and Retinopathy in Patients with Type 2 Diabetes Mellitus - A Retrospective Study.

Background: Previous research has demonstrated that fasting C-peptide (FCP) serves as a protective element against diabetic retinopathy. But the protective effect of elevated FCP levels against diabetic retinopathy (DR) remains uncertain when these levels exceed specific thresholds. This research aimed to investigate the intrinsic link between FCP concentration and DR in individuals with T2DM.

Methods: A total of 1661 individuals diagnosed with type 2 diabetes participated in this observational analysis, where DR was assessed as the primary outcome and categorized according to FCP levels. Curve fitting analysis and two-part linear regression models were applied to assess the relationship between DR and FCP, and exploratory analysis was conducted to identify the threshold.

Results: Our study found a non-linear relationship between the two, as well as a threshold effect at an FCP of 4.11 ng/mL. Below the critical value, each 1 ng/mL increase in FCP levels was associated with a 24% lower risk of DR (OR: 0.74, 95% CI: 0.64-0.86). Above the cutoff, the relationship did not reach statistical significance (OR: 1.52, 95% CI: 0.87-2.66).

Conclusion: There was a nonlinear relationship between FCP level and DR risk, which was negatively correlated at first but stabilized at a lower level when FCP>4.11 ng/mL.