European Journal of Cardiovascular Medicine

Vitamin D, a fat-soluble ketosteroid, is essential for maintaining bone health, calcium homeostasis, and immune function. Beyond its traditional roles, growing evidence suggests that vitamin D plays a crucial role in glucose metabolism and insulin sensitivity, influencing pancreatic beta-cell function and peripheral glucose uptake. The presence of vitamin D receptors in insulin-responsive tissues such as muscle, adipose, and pancreatic beta-cells indicates its significant contribution to insulin regulation and metabolic health.^1-4 Deficiency of vitamin D has been linked to impaired insulin action, increased insulin resistance, and a higher risk of metabolic syndrome and Type 2 Diabetes Mellitus. Given the increasing prevalence of insulin resistance and diabetes among postmenopausal women, understanding the relationship between vitamin D deficiency and insulin sensitivity in this population is crucial for formulating effective preventive and therapeutic strategies.^5-7

Menopause marks a critical transition in a woman’s life, characterized by declining estrogen levels, which are associated with an increased risk of osteoporosis, cardiovascular diseases, and metabolic dysfunction. Estrogen is known to enhance insulin sensitivity, and its deficiency during menopause contributes to higher fasting glucose levels, increased visceral fat deposition, and worsening insulin resistance.^8-11 Additionally, vitamin D metabolism is influenced by estrogen, and postmenopausal women are at a greater risk of vitamin D deficiency due to aging-related decline in skin synthesis, dietary inadequacies, and reduced sun exposure. These factors create a complex interplay between menopause, vitamin D status, and insulin resistance, making postmenopausal women particularly vulnerable to metabolic disorders.^12-14

India has witnessed a rising burden of both vitamin D deficiency and Type 2 Diabetes Mellitus, with postmenopausal women being a high-risk demographic for both conditions. Despite ample sunlight, factors such as reduced outdoor activity, increased urbanization, cultural clothing practices, and dietary insufficiencies contribute to widespread vitamin D insufficiency.^13-16 Several studies have indicated that correcting vitamin D deficiency may enhance insulin sensitivity, reduce systemic inflammation, and lower the risk of metabolic syndrome in postmenopausal women.^17,18 However, there is limited research exploring this association in the Indian population, particularly in resource-limited settings such as Dhanbad, Jharkhand.

This study, conducted at Shaheed Nirmal Mahto Medical College, Dhanbad, in the Departments of Physiology and Medicine, aims to assess the prevalence of vitamin D deficiency among postmenopausal women and its correlation with insulin sensitivity. By investigating this relationship, the findings could provide valuable insights into potential early intervention strategies, including vitamin D supplementation, dietary modifications, and lifestyle interventions, to improve metabolic health and overall well-being in postmenopausal women. Understanding this association may also help in shaping clinical guidelines for routine vitamin D screening and targeted metabolic interventions in postmenopausal populations.

Study Design and Setting

This study was designed as a descriptive, cross-sectional study to evaluate the relationship between vitamin D deficiency and insulin sensitivity among postmenopausal women. The research was conducted at Shaheed Nirmal Mahto Medical College, Dhanbad, in the Departments of Physiology and Medicine. The study duration was six months, during which data collection, laboratory investigations, and statistical analyses were performed. Ethical approval for the study was obtained from the Institutional Ethics Committee before participant recruitment.

Study Population and Sample Size

The study included a total of 175 postmenopausal women, selected based on specific inclusion and exclusion criteria. The sample size was determined using statistical estimation with a confidence level of 95 percent and an expected prevalence of vitamin D deficiency among postmenopausal women. The participants were recruited from both outpatient and inpatient services at Shaheed Nirmal Mahto Medical College, ensuring representation from various socioeconomic and demographic backgrounds.

Inclusion and Exclusion Criteria

Inclusion criteria:

• Women aged 45 to 70 years who had attained natural menopause at least one year before the study
• Those willing to provide written informed consent and comply with the study requirements
• Women who had not taken vitamin D or calcium supplements in the past three months

Exclusion criteria:

• Women diagnosed with Type 1 or Type 2 Diabetes Mellitus
• Individuals with chronic medical conditions such as chronic kidney disease, liver disorders, thyroid disorders, or gastrointestinal conditions affecting vitamin D metabolism
• Participants on medications known to interfere with vitamin D metabolism, including corticosteroids, anticonvulsants, bisphosphonates, or hormone replacement therapy
• Women with any acute illness, active infections, or recent hospitalization within the past six months
• Individuals with malignancies or autoimmune diseases that could affect insulin sensitivity or vitamin D metabolism

Data Collection and Clinical Assessment

A structured questionnaire was used to collect demographic details, medical history, menopausal duration, lifestyle habits, dietary intake, physical activity levels, sun exposure, and symptoms related to vitamin D deficiency. Each participant underwent a comprehensive physical examination, including anthropometric measurements such as height, weight, body mass index (BMI), and waist-to-hip ratio. Blood pressure and vital signs were also recorded to assess overall health status.

Biochemical Investigations

Participants underwent venous blood sampling after an overnight fast of at least eight hours. The following laboratory parameters were analyzed:

• Serum 25-hydroxyvitamin D (25[OH]D) levels: Measured using an electrochemiluminescence immunoassay, classified as:
  • Deficient: < 20 ng/mL
  • Insufficient: 20–29.9 ng/mL
  • Sufficient: ≥ 30 ng/mL
• Fasting plasma glucose (FPG) and postprandial blood glucose (PPG): Measured using the glucose oxidase-peroxidase (GOD-POD) method.
• Glycated hemoglobin (HbA1c): Assessed using high-performance liquid chromatography to determine long-term glycemic control.
• Fasting insulin levels: Determined by chemiluminescence immunoassay to assess pancreatic beta-cell function.
• Homeostatic Model Assessment for Insulin Resistance (HOMA-IR): Calculated using the standard formula:
  HOMA-IR = (Fasting Insulin µU/mL × Fasting Glucose mmol/L) / 22.5

Additionally, lipid profile parameters including total cholesterol, triglycerides, low-density lipoprotein (LDL), and high-density lipoprotein (HDL) were measured to assess cardiovascular risk factors in relation to insulin resistance.

Statistical Analysis

All collected data were recorded and analyzed using SPSS software version 25.0. Descriptive statistics such as mean values, standard deviations, frequencies, and percentages were used to summarize demographic and biochemical variables. The chi-square test was applied to determine associations between categorical variables, while Pearson’s correlation coefficient was used to assess the relationship between serum vitamin D levels and insulin resistance markers. Linear regression analysis was performed to evaluate the predictive value of vitamin D status on insulin resistance. A p-value of less than 0.05 was considered statistically significant.

Ethical Considerations

This study adhered to the principles outlined in the Declaration of Helsinki. Ethical clearance was obtained from the Institutional Ethics Committee of Shaheed Nirmal Mahto Medical College, Dhanbad, prior to participant enrollment. All participants were provided with detailed information regarding the study’s objectives, procedures, and potential risks. Written informed consent was obtained from each participant before data collection. Confidentiality was maintained by anonymizing participant records and ensuring that collected data were used strictly for research purposes. Participants were also given appropriate counseling regarding their vitamin D status and lifestyle modifications where necessary

The study investigated the relationship between vitamin D deficiency and insulin resistance in postmenopausal women, revealing significant findings across multiple biochemical and metabolic parameters. The results are summarized in the following tables, illustrating key demographic trends, vitamin D distribution, glycemic control, and their associations with insulin sensitivity markers.

Table 1: Socio-Demographic Characteristics of Study Participants

This table provides an overview of the demographic distribution of participants. The highest proportion (48.6%) belonged to the 51–60 years age group, while 25.7% were in the early postmenopausal stage (45–50 years) and another 25.7% in the older postmenopausal category (61–70 years). The duration of menopause showed significant variation, with 40.0% of women experiencing menopause for 6–10 years, an important period for metabolic changes. Additionally, the body mass index (BMI) data indicated that nearly half (48.6%) of participants were overweight, while 22.9% were obese. A predominantly sedentary lifestyle was observed in 51.4% of participants, emphasizing the need for lifestyle modifications in this population.

Table 2: Distribution of Vitamin D Levels Among Participants

A concerning 56.0% of participants were vitamin D deficient (<20 ng/mL), while only 12.6% had sufficient vitamin D levels (≥30 ng/mL). This widespread deficiency in postmenopausal women is alarming, as it suggests a strong association with metabolic disorders, including insulin resistance and poor glycemic outcomes.

Table 3: Biochemical Profile of Study Participants

The biochemical analysis further confirms the presence of metabolic dysregulation. The fasting and postprandial glucose levels (132.8 mg/dL and 186.4 mg/dL, respectively) were elevated, indicating suboptimal glycemic control. The mean HbA1c (7.4%) was above the recommended threshold, suggesting persistent hyperglycemia. Insulin resistance markers, such as fasting insulin levels (12.5 µU/mL) and HOMA-IR (3.1), were also elevated, reinforcing the presence of insulin resistance.

Table 4: Correlation Between Vitamin D Levels and Insulin Resistance

A strong inverse correlation was observed between vitamin D levels and insulin resistance markers. The correlation coefficient between vitamin D and fasting insulin was -0.39 (p <0.001), while the correlation between vitamin D and HOMA-IR was -0.45 (p <0.001), confirming a statistically significant relationship.

Table 5: Association Between Vitamin D Deficiency and Glycemic Control

Participants with vitamin D deficiency had significantly poorer glycemic control. Among those deficient in vitamin D, 81.6% had an HbA1c level >7%, compared to only 22.7% in the vitamin D-sufficient group. The association was statistically significant (p <0.001), reinforcing the link between vitamin D and diabetes progression.

The study highlights a high prevalence of vitamin D deficiency in postmenopausal women, with significant implications for metabolic health. A strong inverse correlation was found between vitamin D levels and insulin resistance, reinforcing its role in glucose homeostasis. Vitamin D-deficient individuals exhibited significantly poorer glycemic control, emphasizing the need for routine screening and possible supplementation as part of diabetes management. Given these findings, future interventions targeting vitamin D optimization may serve as a promising adjunct in improving insulin sensitivity and overall metabolic health in postmenopausal women

This study provides compelling evidence of the significant association between vitamin D deficiency and insulin resistance in postmenopausal women. The findings reveal a high prevalence of vitamin D deficiency (56.0%) in this population, alongside elevated insulin resistance markers such as fasting insulin and HOMA-IR. This underscores the critical role of vitamin D in metabolic health, particularly in the context of menopause, where hormonal changes contribute to worsening insulin sensitivity. The inverse correlation between vitamin D levels and insulin resistance highlights the potential of vitamin D optimization as a therapeutic intervention to mitigate the risk of metabolic disorders in postmenopausal women.

The demographic data from this study reveal that the majority of participants (48.6%) were in the 51–60 years age group, which is a crucial phase for metabolic and hormonal changes. Postmenopausal women experience a decline in estrogen levels, which has been shown to impair glucose metabolism, increase fat accumulation, and exacerbate insulin resistance. Additionally, a predominantly sedentary lifestyle (51.4%) and a high prevalence of overweight (48.6%) and obesity (22.9%) further compound metabolic dysfunction, increasing the risk of Type 2 Diabetes Mellitus and cardiovascular diseases. These findings suggest that targeted lifestyle interventions, including increased physical activity and dietary modifications, are essential in this demographic.^17-21

The biochemical profile of study participants further supports the presence of metabolic disturbances. Elevated fasting glucose (132.8 mg/dL), postprandial glucose (186.4 mg/dL), and HbA1c (7.4%) levels indicate suboptimal glycemic control, which is a major risk factor for diabetes complications such as nephropathy, neuropathy, and cardiovascular diseases. The increased fasting insulin levels (12.5 µU/mL) and HOMA-IR score (3.1) confirm significant insulin resistance, reinforcing the hypothesis that postmenopausal women are at a higher risk of developing metabolic syndrome. Given the known role of vitamin D in improving insulin receptor function and reducing systemic inflammation, the widespread deficiency observed in this study suggests a potential modifiable factor in the management of insulin resistance.^22-24

A key highlight of this study is the strong inverse correlation between vitamin D levels and insulin resistance markers, with Pearson’s correlation coefficients of -0.39 for fasting insulin and -0.45 for HOMA-IR (p <0.001). These findings align with previous research suggesting that vitamin D plays a crucial role in insulin sensitivity through multiple mechanisms. Vitamin D is known to regulate calcium flux in pancreatic beta cells, thereby enhancing insulin secretion. Additionally, it modulates inflammatory pathways that influence glucose metabolism, reduces oxidative stress, and upregulates insulin receptor expression in peripheral tissues. The significant association observed in this study further strengthens the hypothesis that vitamin D deficiency exacerbates insulin resistance and may contribute to poor glycemic control in postmenopausal women.^25-28

The analysis of glycemic control in relation to vitamin D status further reinforces this association. Among vitamin D-deficient individuals, 81.6% had an HbA1c level greater than 7%, indicating poor long-term glucose regulation. In contrast, only 22.7% of those with sufficient vitamin D levels had HbA1c values above this threshold, suggesting a protective role of vitamin D against hyperglycemia. This highlights the potential benefits of vitamin D supplementation in improving glycemic outcomes, particularly in postmenopausal women at risk of developing Type 2 Diabetes Mellitus. Given that estrogen influences vitamin D metabolism, the decline in estrogen levels post-menopause may further exacerbate vitamin D insufficiency, contributing to metabolic dysfunction.^29-31

The high prevalence of vitamin D deficiency in this study is concerning, especially given India’s geographic advantage of year-round sun exposure. Despite this, factors such as reduced outdoor activity, excessive use of sun-blocking agents, cultural clothing practices, and dietary inadequacies contribute to widespread vitamin D insufficiency. This suggests that increasing awareness regarding sun exposure, vitamin D-rich dietary sources, and the potential benefits of supplementation may play a crucial role in improving metabolic health outcomes. Moreover, vitamin D supplementation may offer an inexpensive and effective strategy to enhance insulin sensitivity and reduce the risk of diabetes-related complications in postmenopausal women.^32-35

From a clinical perspective, these findings emphasize the need for routine vitamin D screening in postmenopausal women, particularly those with insulin resistance or at high risk of developing Type 2 Diabetes Mellitus. Incorporating vitamin D assessment into routine metabolic evaluations could enable early identification of deficiency and allow for timely interventions. Additionally, healthcare providers should consider recommending lifestyle modifications, including weight management, increased physical activity, and vitamin D supplementation, as part of a comprehensive strategy to improve insulin sensitivity and overall metabolic health.

Limitations

Despite the strengths of this study, including a well-defined postmenopausal cohort and robust biochemical assessments, there are some limitations. The cross-sectional design precludes establishing causality between vitamin D deficiency and insulin resistance. Longitudinal studies and interventional trials assessing the impact of vitamin D supplementation on insulin sensitivity are needed to confirm these findings. Additionally, dietary intake and sun exposure were not quantitatively assessed, which could provide further insights into the sources of vitamin D deficiency.

In conclusion, this study highlights a significant association between vitamin D deficiency and insulin resistance in postmenopausal women. The high prevalence of vitamin D deficiency, coupled with its strong inverse correlation with fasting insulin and HOMA-IR, underscores the importance of addressing vitamin D insufficiency as part of metabolic disease prevention strategies. Improving vitamin D status through supplementation, dietary modifications, and increased sun exposure may serve as a valuable intervention to enhance insulin sensitivity, improve glycemic control, and reduce the long-term risk of diabetes-related complications in postmenopausal women. Further research, including randomized controlled trials, is warranted to explore the therapeutic potential of vitamin D in mitigating metabolic disorders in this vulnerable population.

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