Relationship Between Glycated Hemoglobin (HbA1c) and Vitamin D Levels in Type 2 Diabetes Patients: A Retrospective Cross-Sectional Study

Significant links between low serum levels of vitamin D3 and insufficient glycemic control in patients with type 2 diabetes mellitus (T2DM) have been reported previously in the literature. However, there is no exciting evidence on the association between glycated hemoglobin (HbA1c) and vitamin D levels in T2DM individuals in our nation (Yemen). This study aimed to investigate the relationship between HbA1c and vitamin D levels in T2DM patients in a resource-limited setting.

A retrospective cross-sectional study was conducted at the Al-Raffa Center, Ibb, Yemen between June 2018 and September 2023 including 396 patients diagnosed with T2DM. The patient characteristics, comorbidities, HbA1c, and vitamin D levels were gathered from patients' medical profiles. Linear regression analysis was used to find the factors associated with vitamin D deficiency (serum 25(OH)D levels < 20 ng/mL) among T2DM patients. Subsequently, the correlation between HbA1c and vitamin D levels was examined using receiver operating characteristic (ROC) curve analysis.

The mean age was 44.6 ±14.6 years and most of them (n= 227, 57.3%) were female and from a rural area (n= 229, 57.8%). Comorbidities were hypertension, dyslipidemia disease, and cardiovascular disease in 176 (44.4%), 63 (15.9%), and 88 (22.2%) cases, respectively. The mean HbA1c was 8.1 ±2.5%. The mean vitamin D level was 26.9 ±16.5 ng/mL and low vitamin D was present in 260 (65.7%) (vitamin D deficiency in 160 (40.4%) and vitamin D insufficiency in 100 (25.3%) cases). In regression analysis, obesity (>30 kg/m²) (odds ratio (OR): 299.49; 95% confidence interval (CI): 72.66 - 1234.42, p <0.0001), higher HbA1c levels (OR: 1.61; 95% CI: 1.26 - 2.05, p =0.0001), and urban residence (OR: 23.98; 95% CI: 5.62 - 102.42, p <0.0001) were associated with vitamin D deficiency. There was a negative correlation between the vitamin D level and HbA1c which was statistically significant (correlation coefficient r: -0.5452; 95% CI: -0.6109 to -0.4720, p <0.0001). Using the ROC analysis, the serum vitamin D value of ≤18.42 ng/ml was the best cut-off point to predict hyperglycemia (area under the curve: 0.633, 95% CI: 0.672 to 0.770, sensitivity: 52%, specificity: 84.71 %, Yoden's index: 0.3671, p <0.001). Based on this cut-off, 39.4% of individuals (37.5% in the normoglycemic group and 90.9% in the hyperglycemic group) were vitamin D deficient.

In this study, low vitamin D was common among T2DM patients, especially those with poor glycemic control. We observed a link between HbA1c levels, urban residency, and BMI with vitamin D deficiency in T2DM patients. The association was distinguished by low vitamin D levels and elevated HbA1c. Additionally, we found that the serum vitamin D value of ≤18.42 ng/ml was the best cut-off point to predict hyperglycemia in T2DM patients with moderate agreement. To manage their disease, patients with T2DM should take their medications as prescribed and live a healthy lifestyle. This will increase their overall health, especially their vitamin D levels.