Dietary iron intake is nonlinearly associated with the risk of diabetic retinopathy in adults with type 2 diabetes

Objective: To elucidate the association between dietary iron intake and diabetic retinopathy (DR) in type 2 diabetes (T2D) patients. Methods: Participants from the National Health and Nutrition Examination Survey (NHANES) 2005-2008 aged over 40 years with T2D were included. Dietary iron intake was estimated from standardised questionnaires. The presence of DR and vision-threatening DR (VTDR) was determined through retinal imaging. We used logistic regression to assess the relationship between iron intake and DR, and restricted cubic splines to reveal nonlinear links. Results: The study enrolled 1172 T2D adults. We found significant nonlinear associations between dietary iron intake and DR among females ( P = 0.023), but not in males ( P = 0.490). Compared with the lowest quartile of iron intake, the third quartile (13.2-18.1 mg/d) yielded significantly lower odds of developing DR (odds ratio [OR], 0.59; 95% CI, 0.39-0.90) and VTDR (OR, 0.42; 95% CI, 0.19-0.94). Stratified logistic analyses showed that medium-high iron intake was associated with lower risks of DR in females (OR, 0.44; 95% CI, 0.24-0.81), non-Hispanic Blacks (OR, 0.38; 95% CI, 0.17-0.85), and individuals with obesity (OR, 0.45; 95% CI, 0.25-0.82), high HbA1c (OR, 0.56; 95% CI, 0.34-0.93), long diabetes duration (OR, 0.40; 95% CI, 0.21-0.76) or low blood haemoglobin (OR, 0.17; 95% CI, 0.05-0.60). Conclusion: Dietary iron intake was nonlinearly negatively associated with the prevalence of DR and VTDR, showing protective effect against retinopathy of medium-high iron intake in T2D patients. Such associations significantly vary by multiple factors such as age, ethnicity, obesity and glycaemic control.