Serum Zinc Levels in Type 2 Diabetes Mellitus and Their Association with Diabetic Retinopathy and Glycemic Control

Abstract

Background: Zinc is involved in a variety of enzymatic and antioxidant pathways, and hyperglycemia and oxidative stress are thought to contribute to diabetic microvascular injury in the retina. The biological plausibility of the association between circulating zinc and severity of DR and glycemic status is clinically relevant but not enough to conclude one way or the other. Objectives: To evaluate the serum zinc concentration in adults with type 2 diabetes mellitus, to compare the serum zinc concentration among the different grades of diabetic retinopathy and healthy controls, and to investigate the association between serum zinc and glycated hemoglobin (HbA1c). Methods: This hospital-based case-control study was carried out in the ophthalmic outpatient department of Al-Sader teaching hospital in Najaf, and Ibn-Alhaitham eye teaching hospital in Baghdad from August 2016 to March 2017. Patients with type 2 diabetes mellitus (T2DM) (40 without retinopathy, 25 with non-proliferative diabetic retinopathy (NPDR), and 25 with proliferative diabetic retinopathy (PDR)) and age- and sex-matched healthy controls were recruited as the sample. The level of zinc in serum was determined by the colorimetric 5-Br-PAPS method, while diabetic participants had their level of HbA1c measured. Minimal p values of < 0.05 were considered statistically significant and were reported in the source records, including ANOVA, chi-square testing, and Pearson correlation. Results: Mean serum zinc was 87.1±13.9 µg/dL in controls, 63.8±15.0 µg/dL in diabetic participants without retinopathy, 59.8±13.9 µg/dL in NPDR, and 48.3±12.0 µg/dL in PDR. The group comparisons reported revealed that zinc levels in diabetic groups were lower than in the controls (p<0.01). The direct comparisons to the diabetes without retinopathy group were non-significant for NPDR (p=0.311) and PDR (p=0.101). Of diabetics, 68/90 (75.6%) were considered to have low zinc. We found that 26/40 (65.0%) did not have retinopathy, 18/25 (72.0%) had NPDR, and 24/25 (96.0%) had PDR; the reported odds ratio of PDR compared with no retinopathy is 0.08 (95% CI 0.01–0.63; p=0.004), but the coding/reference direction needs to be verified. Low zinc was recorded on 13/29 (44.8%) with optimal glycemic control, 18/24 (75.0%) with suboptimal control, and 37/37 (100%) with poor control (p<0.01). Conclusion: The result indicates low mean serum Zn in diabetic participants; PDR shows the lowest mean serum Zn, and there is a strong descriptive trend between low Zn and poor glycemic control.