Type 2 Diabetes Mellitus (T2DM) is a chronic metabolic disorder characterized by insulin resistance and progressive β-cell dysfunction, resulting in sustained hyperglycemia. It constitutes a major global health burden, with rising prevalence, particularly in low- and middle-income countries. India, often referred to as the "diabetes capital of the world," is witnessing a rapid increase in T2DM cases, fueled by sedentary lifestyles, urbanization, and dietary transitions.

Vitamin D, traditionally known for its role in calcium and bone metabolism, has recently gained attention for its extra-skeletal functions, particularly its role in metabolic regulation. Vitamin D receptors are widely expressed in pancreatic β-cells and various immune cells, suggesting a potential link between vitamin D status and glucose metabolism. Emerging evidence indicates that vitamin D may influence insulin secretion, enhance insulin sensitivity, and modulate inflammatory pathways involved in the pathogenesis of T2DM.

Several epidemiological and interventional studies have reported a high prevalence of vitamin D deficiency among individuals with T2DM, with an inverse relationship between serum 25-hydroxyvitamin D [25(OH)D] levels and glycemic indices such as fasting blood glucose and glycated hemoglobin (HbA1c). However, the strength and consistency of this association vary across populations due to differences in geographic location, sunlight exposure, dietary habits, and genetic predispositions.

This study aims to evaluate the correlation between serum vitamin D levels and glycemic control, measured by HbA1c, among patients with T2DM in a South Indian tertiary care setting. Understanding this relationship could offer insights into novel, adjunctive strategies for improving glycemic outcomes through correction of vitamin D deficiency

Study Design and Setting:

This was a cross-sectional, observational study conducted in the Department of General Medicine at Government Medical College (GMC), Rajanna Sircilla, Telangana. The study was carried out over a period of nine months, from June 2024 to January 2025.

Study Population:

A total of 100 adult patients diagnosed with Type 2 Diabetes Mellitus (T2DM), attending the outpatient and inpatient services of the department during the study period, were enrolled. Inclusion criteria comprised patients aged between 30 and 70 years, with a confirmed diagnosis of T2DM as per American Diabetes Association (ADA) guidelines and willing to provide informed consent. Patients with Type 1 Diabetes Mellitus, chronic kidney disease, liver disease, malabsorption syndromes, or those on vitamin D supplementation within the past six months were excluded.

Data Collection:

After obtaining informed written consent, demographic data (age, gender), clinical history, duration of diabetes, and treatment details were recorded. Venous blood samples were collected from all participants to assess serum 25-hydroxyvitamin D [25(OH)D] and glycated hemoglobin (HbA1c) levels.

Biochemical Analysis:

Serum 25(OH)D levels were measured using chemiluminescent immunoassay (CLIA). Levels were categorized as follows:

Deficient: <20 ng/mL

Insufficient: 20–30 ng/mL

Sufficient: >30 ng/mL
Glycemic control was assessed using HbA1c, measured via high-performance liquid chromatography (HPLC). HbA1c <7% was considered as good glycemic control, and ≥7% as poor control.

Statistical Analysis:

Data were compiled in Microsoft Excel and analyzed using SPSS version 25. Descriptive statistics were used for baseline characteristics. The correlation between serum vitamin D and HbA1c was assessed using Pearson’s correlation coefficient. A p-value of <0.05 was considered statistically significant.

Ethical Consideration:

The study protocol was reviewed and approved by the Institutional Ethics Committee of Government Medical College, Rajanna Sircilla, Telangana. Written informed consent was obtained from all participants prior to enrolment.

A total of 100 patients with Type 2 Diabetes Mellitus (T2DM) were enrolled in the study. The cohort consisted of 54 males (54%) and 46 females (46%), with a mean age of 56.8 ± 10.3 years (Table 1).

Table 1: Demographic and Baseline Characteristics (n = 100)

The assessment of serum 25-hydroxyvitamin D levels revealed that the majority of patients were vitamin D deficient, with 62% falling below 20 ng/mL, 24% showing insufficiency (20–30 ng/mL), and only 14% having sufficient levels (>30 ng/mL) (Table 2).

Table 2: Distribution of Vitamin D Status

With respect to glycemic control, based on HbA1c levels, 72 patients (72%) had poor control (HbA1c ≥ 7%), while only 28 patients (28%) demonstrated good glycemic control (HbA1c < 7%) (Table 3).

Table 3: Glycemic Control Based on HbA1c

When analyzing the relationship between vitamin D status and glycemic control, a significant trend was observed. Among vitamin D-deficient patients, 87.1% (n = 54) had poor glycemic control, while only 12.9% (n = 8) achieved good control. In contrast, among those with sufficient vitamin D levels, 71.4% (n = 10) had good glycemic control, and only 28.6% (n = 4) had poor control (Table 4).

Table 4: Vitamin D Status vs. Glycemic Control

The mean HbA1c was found to be highest among the vitamin D-deficient group (8.3 ± 1.2%), followed by the insufficient group (7.4 ± 0.9%), and lowest in the sufficient group (6.9 ± 0.8%), suggesting an inverse relationship between serum vitamin D levels and glycemic control (Table 5).

Table 5: Mean HbA1c by Vitamin D Status

These findings indicate a statistically significant negative correlation between serum vitamin D levels and HbA1c values, supporting the hypothesis that lower vitamin D levels are associated with poorer glycemic control in patients with T2DM.

This observational study aimed to evaluate the correlation between serum vitamin D levels and glycemic control in patients with Type 2 Diabetes Mellitus. The findings revealed a high prevalence (62%) of vitamin D deficiency among the diabetic population, which is consistent with several previous studies conducted in both Indian and international settings. Notably, patients with vitamin D deficiency demonstrated significantly poorer glycemic control, as indicated by higher HbA1c levels, compared to those with sufficient vitamin D levels.

The inverse relationship observed between serum 25-hydroxyvitamin D and HbA1c levels (r = –0.42, p < 0.001) suggests that vitamin D status may play a role in glycemic regulation. This could be attributed to the presence of vitamin D receptors on pancreatic β-cells and peripheral tissues involved in insulin sensitivity. Vitamin D is believed to influence insulin synthesis and secretion, enhance insulin receptor expression, and modulate inflammatory cytokines, thereby improving glycemic control.

Our findings are in line with studies by Pittas et al. and Chiu et al., which reported that lower vitamin D levels were associated with increased insulin resistance and poorer glucose tolerance. Similarly, a study by Boucher et al. highlighted that vitamin D supplementation might improve glycemic indices in vitamin D-deficient individuals with diabetes. However, some randomized controlled trials have shown mixed results, indicating the need for further longitudinal studies to establish causality.

The strength of our study lies in its focused evaluation of the vitamin D–HbA1c relationship in a rural Indian population, which is often underrepresented in diabetes research. However, the study is not without limitations. Its cross-sectional design precludes causal inference. Additionally, seasonal variation in vitamin D levels, dietary intake, physical activity, and sun exposure were not controlled.

This study demonstrated a significant inverse correlation between serum vitamin D levels and glycemic control in patients with Type 2 Diabetes Mellitus. A majority of participants were found to be vitamin D deficient, and poor glycemic control was notably higher among this group. Patients with sufficient vitamin D levels exhibited better HbA1c profiles, suggesting a potential role of vitamin D in glucose metabolism and insulin sensitivity. While the cross-sectional design limits causal inference, the findings emphasize the importance of routine screening for vitamin D deficiency in diabetic patients. Correcting hypovitaminosis D may serve as a supportive strategy in achieving better glycemic outcomes in the management of T2DM.

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