Serum vitamin B12, vitamin D, and zinc in type 2 diabetes mellitus and their association with diabetic retinopathy: a case-control study

Within complications of type 2 diabetes mellitus (T2DM), diabetic retinopathy (DR) is a frequent microvascular disorder. Recognizing nutritional and clinical factors modifiable through intervention is crucial for prevention; however, earlier studies reported inconsistent findings on micronutrient levels and their role in DR progression. This case–control study was carried out in ophthalmology clinics of Urmia University of Medical Sciences (Iran) between October 2024 and February 2025, enrolling 140 T2DM patients: 70 with confirmed DR and 70 without. DR severity was graded by a vitreoretinal specialist using the International Diabetic Retinopathy Severity Scale, from mild non-proliferative to proliferative stages. Serum vitamin B12, 25-hydroxyvitamin D, and zinc were classified as deficient, insufficient, or sufficient. Demographic (age, sex), clinical (weight, BMI, blood pressure, diabetes duration, insulin use, physical activity), and biochemical parameters (fasting blood sugar, creatinine) were collected. Statistical analyses were performed using chi-square, linear-by-linear association, and binary logistic regression to account for possible confounders. Patients with DR had reduced serum vitamin B12 levels compared with control subjects ( p < 0.001), and this association persisted after adjustment. Longer diabetes duration, higher creatinine, insulin therapy, and lower physical activity were also significantly associated with DR. Zinc status reached significance only in categorical chi-square analysis, while vitamin D showed no difference. Lower vitamin B12, longer diabetes duration, insulin therapy, and reduced physical activity were independently associated with DR. Routine B12 monitoring and managing these modifiable risks may help prevent DR. Larger prospective studies with additional biomarkers and sex-specific analyses are needed. Clinical trial number: Not applicable. This study was observational in design and did not require clinical trial registration.