Vitamin D is a crucial fat-soluble vitamin that plays a significant role in calcium homeostasis, bone metabolism, immune function, and various metabolic processes. Recent research has highlighted its importance beyond skeletal health, particularly in glucose metabolism and insulin regulation. The presence of vitamin D receptors in pancreatic beta cells suggests that vitamin D may influence insulin secretion and sensitivity.1-3 Its deficiency has been associated with impaired insulin function, increased insulin resistance, and a higher risk of developing Type 2 Diabetes Mellitus (T2DM). As T2DM continues to pose a significant global health challenge, understanding the impact of vitamin D deficiency on insulin resistance is essential for improving diabetes management.4-6

India, which is among the countries with the highest diabetes prevalence, is also witnessing an alarming rise in vitamin D deficiency. Despite abundant sunlight, factors such as limited outdoor exposure, poor dietary intake, and lifestyle changes contribute to widespread vitamin D insufficiency.7-9 Studies indicate that inadequate vitamin D levels may play a role in metabolic dysfunction by affecting insulin receptor expression, altering beta-cell function, and promoting inflammation, all of which contribute to insulin resistance and poor glycemic control.10-13

Dhanbad, Jharkhand, is experiencing a growing burden of diabetes, yet limited research has explored the relationship between vitamin D deficiency and insulin resistance in this region. Understanding this association could provide valuable insights into potential early intervention strategies, including vitamin D supplementation and lifestyle modifications.

This study, conducted at Shaheed Nirmal Mahto Medical College, Dhanbad, across the Departments of Physiology and Medicine, aims to assess the prevalence of vitamin D deficiency among T2DM patients and its correlation with insulin resistance. By investigating this link, the findings may help in refining treatment protocols, promoting routine vitamin D screening in diabetes management, and enhancing overall patient care.

Study Design and Setting

This cross-sectional study was conducted at Shaheed Nirmal Mahto Medical College, Dhanbad, in the Department of Physiology and Department of Medicine. The study aimed to assess the prevalence of vitamin D deficiency among patients with Type 2 Diabetes Mellitus (T2DM) and its correlation with insulin resistance. The research was carried out over a period of six months following ethical approval from the Institutional Ethics Committee.

Study Population and Sample Size

The study included a total of 156 participants, all of whom were diagnosed with T2DM as per the American Diabetes Association (ADA) diagnostic criteria. The sample size was determined based on a confidence level of 95% and an expected prevalence of vitamin D deficiency among diabetics. Patients were recruited from the outpatient and inpatient departments of Shaheed Nirmal Mahto Medical College, Dhanbad.

Inclusion and Exclusion Criteria

Inclusion Criteria:

• Individuals aged 35 to 70 years diagnosed with T2DM for at least one year
• Patients willing to provide written informed consent
• Those who had not taken vitamin D supplements in the last three months

Exclusion Criteria:

• Patients with Type 1 Diabetes Mellitus (T1DM) or gestational diabetes
• Individuals with chronic kidney disease (CKD), liver disease, or malabsorption syndromes
• Patients on corticosteroids, antiepileptic drugs, or other medications affecting vitamin D metabolism
• Those with any acute illness or recent hospitalization

Data Collection and Biochemical Analysis

Participants were subjected to a detailed clinical history, physical examination, and laboratory investigations. Data were collected using a structured questionnaire, which included demographic details, lifestyle habits, dietary intake, duration of diabetes, and presence of diabetes-related complications.

The following biochemical parameters were assessed:

• Serum 25-hydroxyvitamin D (25[OH]D) levels – measured using electrochemiluminescence immunoassay (ECLIA)
• Fasting plasma glucose (FPG) and postprandial blood glucose (PPG) – analyzed using glucose oxidase-peroxidase (GOD-POD) method
• Glycated hemoglobin (HbA1c) – measured using high-performance liquid chromatography (HPLC)
• Fasting insulin levels – determined by chemiluminescence assay
• Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) – calculated using the formula:

HOMA-IR = (Fasting Insulin [µU/mL] × Fasting Glucose [mmol/L]) / 22.5

Vitamin D status was categorized as follows:

• Deficient: < 20 ng/mL
• Insufficient: 20–29.9 ng/mL
• Sufficient: ≥ 30 ng/mL

Statistical Analysis

Data were analyzed using SPSS software version 25.0. Descriptive statistics were used to present mean ± standard deviation (SD), frequencies, and percentages. The chi-square test was applied to assess the association between vitamin D levels and insulin resistance. Pearson’s correlation coefficient (r-value) was used to evaluate the relationship between serum vitamin D levels and HOMA-IR values. A p-value < 0.05 was considered statistically significant.

Ethical Considerations

The study was conducted in compliance with the Declaration of Helsinki. Ethical approval was obtained from the Institutional Ethics Committee of Shaheed Nirmal Mahto Medical College, Dhanbad. Participants provided written informed consent, and their data were handled with strict confidentiality.

The socio-demographic characteristics of the study participants, as shown in Table 1, reveal that the majority of individuals fall within the middle-aged category (46-55 years: 37.2%), followed closely by older individuals (56-70 years: 35.9%), indicating that Type 2 Diabetes Mellitus (T2DM) predominantly affects individuals in their later years. The gender distribution was skewed slightly towards males (57.7%), which may reflect higher healthcare-seeking behavior or increased prevalence among men. The duration of diabetes among participants was also a significant finding, with nearly 45% having diabetes for 5-10 years, a crucial timeframe during which complications such as insulin resistance and metabolic dysfunctions typically manifest. These demographic trends emphasize the importance of targeted metabolic health interventions in middle-aged and older adults.

Table 1: Socio-Demographic Characteristics of Study Participants

Table 2 highlights the distribution of vitamin D levels, with an alarming 60.9% of participants classified as vitamin D deficient (<20 ng/mL) and only 10.3% having sufficient vitamin D levels (≥30 ng/mL). This high prevalence of deficiency is particularly concerning, given that vitamin D plays a critical role in insulin sensitivity and glucose metabolism. Despite India’s geographic advantage of year-round sun exposure, lifestyle changes, inadequate dietary intake, and reduced outdoor activity have contributed to widespread vitamin D insufficiency. The findings underscore the need for routine vitamin D screening in diabetic patients, as deficiency may exacerbate insulin resistance and lead to poor glycemic outcomes.

Table 2: Distribution of Vitamin D Levels among Participants

The biochemical profile of participants, summarized in Table 3, further supports the presence of poor glycemic control and insulin resistance among the study group. The mean fasting plasma glucose (145.6 mg/dL) and postprandial glucose (198.7 mg/dL) levels were significantly elevated, indicating suboptimal blood sugar regulation. Similarly, the HbA1c level (7.8%), which reflects long-term glucose control, was above the recommended threshold of 7.0%, suggesting inadequate diabetes management. The fasting insulin level (14.2 µU/mL) and HOMA-IR score (3.6) indicate a state of increased insulin resistance, a hallmark of worsening diabetes. These findings suggest that many participants may benefit from enhanced lifestyle modifications, pharmacological interventions, and adjunct therapies such as vitamin D supplementation to improve metabolic control.

Table 3: Biochemical Profile of Study Participants

Table 4 presents the correlation between vitamin D levels and insulin resistance markers, demonstrating a statistically significant inverse relationship between vitamin D levels and fasting insulin (r = -0.42, p < 0.001) as well as HOMA-IR (r = -0.48, p < 0.001). This suggests that lower vitamin D levels are strongly associated with increased insulin resistance, which is a key contributor to poor glycemic control and diabetes progression. The strength of these correlations reinforces previous findings that vitamin D may enhance insulin receptor sensitivity, improve pancreatic beta-cell function, and reduce systemic inflammation, all of which are crucial for better diabetes management. These results highlight the therapeutic potential of vitamin D supplementation in insulin-resistant individuals.

Table 4: Correlation between Vitamin D Levels and Insulin Resistance

The association between vitamin D deficiency and glycemic control, depicted in Table 5, reveals that patients with low vitamin D levels had significantly worse HbA1c outcomes. Among those classified as vitamin D deficient (<20 ng/mL), 82.1% had HbA1c levels exceeding 7.0%, indicating poor blood sugar regulation. In contrast, only 37.5% of participants with sufficient vitamin D levels (≥30 ng/mL) had HbA1c >7.0%, suggesting that higher vitamin D levels may be protective against poor glycemic outcomes. The highly significant p-value (<0.001) further confirms this relationship. These findings emphasize the need for integrating vitamin D assessment into routine diabetes care, as improving vitamin D status could potentially enhance glycemic control and reduce diabetes-related complications.

Table 5: Association between Vitamin D Deficiency and Glycemic Control

This study provides critical insights into the association between vitamin D deficiency and insulin resistance in individuals with Type 2 Diabetes Mellitus (T2DM). The findings highlight a high prevalence of vitamin D deficiency (60.9%) among diabetic patients and establish a strong negative correlation between vitamin D levels and insulin resistance markers (HOMA-IR and fasting insulin levels). These results are consistent with previous studies that suggest vitamin D plays a pivotal role in glucose homeostasis by enhancing insulin receptor sensitivity, reducing inflammation, and improving pancreatic beta-cell function. The evidence presented here reinforces the need for routine vitamin D screening and potential supplementation as an adjunct in diabetes management.

The socio-demographic distribution of the study participants revealed that middle-aged and elderly individuals comprised the majority of the sample, with most having diabetes for over five years. This is a crucial period during which insulin resistance progresses, leading to worsening glycemic control and increased risk of complications. The slightly higher prevalence of T2DM in males (57.7%) could be attributed to genetic predisposition, lifestyle factors, and differences in healthcare-seeking behavior. Given that diabetes-related metabolic dysregulation intensifies over time, targeted early intervention strategies, including lifestyle modifications and nutritional supplementation, may play a key role in delaying disease progression.14-16

The high prevalence of vitamin D deficiency observed in this study is concerning, particularly because India is a tropical country with abundant sunlight. Despite this, lifestyle changes, decreased outdoor activity, pollution, skin pigmentation, and poor dietary intake of vitamin D-rich foods contribute to widespread deficiency. Research suggests that vitamin D enhances insulin action by upregulating insulin receptor expression and modulating inflammatory pathways that influence glucose metabolism. The significant negative correlation between serum vitamin D levels and insulin resistance (HOMA-IR: r = -0.48, p <0.001) in this study aligns with global findings, suggesting that correcting vitamin D deficiency may improve insulin sensitivity and reduce diabetes-related complications.17-20

One of the most significant findings in this study is the association between vitamin D deficiency and poor glycemic control. Among vitamin D-deficient individuals, 82.1% had HbA1c levels above 7.0%, indicating inadequate glucose regulation. In contrast, only 37.5% of participants with sufficient vitamin D levels had poor glycemic control, suggesting a potential protective role of vitamin D against hyperglycemia. Several mechanisms could explain this relationship, including vitamin D's ability to regulate calcium-dependent insulin secretion, suppress pro-inflammatory cytokines, and enhance glucose uptake in peripheral tissues. These findings highlight the need to integrate vitamin D assessment into routine diabetes care protocols, particularly for individuals with uncontrolled diabetes and high insulin resistance.21-23

The biochemical profile of study participants further supports the presence of metabolic dysfunction in vitamin D-deficient individuals. Elevated fasting plasma glucose (145.6 mg/dL), postprandial glucose (198.7 mg/dL), and HbA1c (7.8%) levels indicate poor glycemic control, which is a major risk factor for diabetes-related complications such as neuropathy, nephropathy, and cardiovascular diseases. The elevated fasting insulin levels (14.2 µU/mL) and increased HOMA-IR score (3.6) reinforce the presence of marked insulin resistance, which is a characteristic feature of poorly controlled diabetes. Given that vitamin D deficiency exacerbates insulin resistance, addressing vitamin D insufficiency through supplementation and lifestyle modifications could serve as a preventive strategy to reduce the burden of diabetes-related morbidity.1,21,24

The findings of this study align with existing literature that underscores the crucial role of vitamin D in metabolic health. Several studies have demonstrated that vitamin D supplementation improves insulin sensitivity, reduces inflammatory markers, and enhances beta-cell function, particularly in patients with prediabetes and early-stage T2DM.25-27 While randomized controlled trials are needed to establish optimal vitamin D dosing guidelines for diabetic patients, the strong correlation observed in this study supports the notion that addressing vitamin D deficiency could significantly improve diabetes outcomes.

Clinical and Public Health Implications

The clinical implications of this study are substantial, particularly in a country like India, where both diabetes and vitamin D deficiency are highly prevalent. Routine screening for vitamin D levels in diabetic patients may help in early detection and correction of deficiencies, potentially reducing insulin resistance and improving overall glycemic control. Additionally, public health strategies promoting increased sun exposure, vitamin D-rich diets, and supplementation in high-risk populations could help mitigate the dual burden of diabetes and metabolic dysfunction. Given the strong association between vitamin D levels and insulin resistance, incorporating vitamin D supplementation as part of a comprehensive diabetes management plan could lead to better metabolic outcomes and reduced risk of complications.

Study Limitations and Future Directions

Despite its strengths, this study has a few limitations. The cross-sectional design does not establish causality, and longitudinal studies are required to determine whether vitamin D supplementation directly improves insulin sensitivity over time. Additionally, dietary intake and sun exposure were not quantitatively assessed, which could have provided further insight into the sources of vitamin D deficiency. Future research should explore randomized controlled trials (RCTs) evaluating the impact of vitamin D supplementation on glycemic control, insulin resistance, and long-term diabetes-related outcomes.

This study provides strong evidence linking vitamin D deficiency to increased insulin resistance and poor glycemic control in T2DM patients. The high prevalence of vitamin D deficiency among diabetics, along with its significant correlation with insulin resistance (HOMA-IR) and HbA1c levels, underscores the need for routine vitamin D screening and potential supplementation as part of comprehensive diabetes care. Addressing vitamin D deficiency could play a critical role in improving insulin sensitivity, enhancing glycemic control, and reducing diabetes-related complications, ultimately improving quality of life and long-term health outcomes for diabetic individuals.

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