Association Between Serum 25-Hydroxyvitamin D Levels and HbA1c/Fasting Plasma Glucose in Type 2 Diabetes Mellitus: Cross-sectional Study, Dezful Diabetes Clinic, Iran

Abstract

Background: Vitamin D deficiency is common among individuals with type 2 diabetes mellitus (T2DM) and has been suggested to affect glucose metabolism. However, the evidence regarding its association with glycemic control remains inconsistent. Objectives: This study aimed to examine the association between serum 25-hydroxyvitamin D (25(OH)D) status and glycemic indices, including HbA1c and fasting plasma glucose, in patients with T2DM. Methods: This cross-sectional study included 180 adult patients with T2DM who attended the Dezful Diabetes Clinic between December 2023 and December 2024. Fasting plasma glucose (FPG), glycated hemoglobin (HbA1c), and serum 25-hydroxyvitamin D [25(OH)D] concentrations were measured using standardized laboratory methods. Vitamin D status was categorized as deficient (< 20 ng/mL), insufficient (20 - 30 ng/mL), or sufficient (> 30 ng/mL). Demographic characteristics were recorded, and glycemic indices were compared according to sex, age group, and vitamin D status. Pearson correlation and multivariable linear regression analyses adjusted for age, sex, Body Mass index (BMI), and antidiabetic medication use were performed. Results: The mean age of the participants was 55.02 ± 10.95 years, and 72.2% were female. The mean FPG and HbA1c values were 154.1 ± 60.7 mg/dL and 8.14 ± 2.10%, respectively. Overall, 35.0% of participants had vitamin D deficiency and 31.1% had insufficiency (combined prevalence, 66.1%). Serum 25(OH)D was significantly inversely correlated with HbA1c (r = -0.23; 95% CI, -0.36 to -0.09; P = 0.002) and FPG (r = -0.17; 95% CI, -0.31 to -0.02; P = 0.022). Mean HbA1c levels were significantly lower in males than in females (7.6 ± 1.7% vs. 8.3 ± 2.2%; P = 0.01) and differed significantly across age groups (P = 0.007). A higher proportion of patients with vitamin D deficiency were classified as diabetic according to HbA1c than were those with sufficient vitamin D levels (85.7% vs. 72.1%; χ2 = 6.11, df = 2, P = 0.047). Conclusions: Vitamin D deficiency was highly prevalent among patients with T2DM and was significantly inversely correlated with HbA1c and FPG, with a significant categorical association with HbA1c-defined glycemic control. Assessment of vitamin D status may be relevant to the clinical management of T2DM; however, longitudinal studies are needed to establish causality.