European Journal of Cardiovascular Medicine

The global burden of diabetes continues to rise, with over 500 million individuals affected worldwide. Pre-diabetes, characterized by elevated blood glucose levels not yet reaching diabetic thresholds, significantly increases the risk of developing T2DM and cardiovascular complications. Insulin resistance and beta-cell dysfunction are central to the pathophysiology of both conditions (Pittas, 2010; Salarinia et al., 2021).

Vitamin D has been implicated in regulating insulin secretion and sensitivity, notably through the presence of vitamin D receptors (VDR) in pancreatic β-cells and peripheral tissues (Rostampour et al., 2020; (Baziar et al., 2014).  Recent research highlights the role of vitamin D in insulin secretion and sensitivity. Vitamin D receptors (VDR) are expressed in pancreatic beta cells and insulin-responsive tissues, suggesting a mechanistic role in glucose metabolism. However, epidemiological data on vitamin D’s efficacy in altering insulin levels have been inconsistent. Simultaneously, regular moderate exercise has consistently demonstrated benefits in improving insulin sensitivity and glycemic control (Mitri et al., 2011; Talaei et al., 2013).

This study aims to explore the synergistic potential of vitamin D supplementation combined with moderate-intensity exercise on plasma insulin levels in individuals at different stages of glucose dysregulation.

Study Design: Randomized, controlled, 6-month interventional trial.

Participants: A total of 200 subjects (100 pre-diabetic, 100 with T2DM) aged 30–65 years were enrolled from outpatient clinics. Inclusion criteria included HbA1c between 5.7–6.4% (pre-diabetes) and >6.5% (T2DM), sedentary lifestyle, and serum 25(OH)D < 30 ng/mL. Exclusion criteria were chronic kidney disease, hepatic dysfunction, current vitamin D supplementation, or engagement in structured exercise programs.

Randomization and Grouping: Participants were randomly assigned to four groups: - Group A: Control - Group B: Vitamin D supplementation only - Group C: Moderate Exercise only - Group D: Vitamin D + Exercise

Interventions: - Vitamin D Supplementation: Oral vitamin D3, 4000 IU/day for 6 months. - Exercise Protocol: 45 minutes of supervised moderate-intensity aerobic exercise (e.g., brisk walking, cycling) five times per week.

Outcome Measures: - Primary Outcome: Change in fasting plasma insulin levels. - Secondary Outcomes: HbA1c, HOMA-IR index, body weight, serum 25(OH)D levels.

Data Collection: Blood samples were collected at baseline, 3 months, and 6 months for insulin and glucose assays. ELISA was used for insulin quantification.

Statistical Analysis: Data were analyzed using SPSS v26. ANOVA and post-hoc Tukey tests were used for group comparisons. Significance was set at p<0.05.

Baseline Characteristics

All groups were comparable at baseline in terms of age, BMI, FBG, HbA1c, and plasma insulin levels.

At baseline, the demographic and clinical characteristics of participants were comparable across all four groups, indicating effective randomization. The mean age ranged from 51.7 ± 7.9 years in Group B (Vitamin D only) to 53.0 ± 9.1 years in Group C (Exercise only), with no statistically significant differences, ensuring age-related metabolic variability was minimized. Body mass index (BMI) values were also consistent across groups, ranging from 27.8 ± 2.1 kg/m² in Group A (Control) to 28.1 ± 2.2 kg/m² in Group D (Combined), classifying most participants as overweight. Fasting blood glucose (FBG) levels at baseline were similar across all groups, with values between 117.8 ± 11.7 mg/dL (Group C) and 119.0 ± 13.1 mg/dL (Group B), consistent with pre-diabetic status as per American Diabetes Association criteria. Glycated hemoglobin (HbA1c) levels ranged narrowly from 6.1 ± 0.3% to 6.2 ± 0.4%, again confirming uniform glycemic status at the start of the study. Fasting plasma insulin concentrations were nearly identical across groups, ranging from 9.3 ± 2.3 μIU/mL in Group B to 9.7 ± 2.0 μIU/mL in Group C, suggesting a similar degree of baseline insulin resistance. Collectively, these findings confirm that all four groups were well-matched at baseline, thereby ensuring the observed outcomes post-intervention were attributable to the study interventions rather than pre-existing differences (Table 1).

Table 1: Baseline Characteristics of Participants

Changes in Outcome Measures

Following the 6-month intervention period, notable changes in key metabolic outcomes were observed across the study groups, as summarized in Table 2. The combined intervention group (Group D), which received both vitamin D supplementation and engaged in moderate aerobic exercise, demonstrated the most substantial improvements across all parameters. Fasting plasma insulin levels increased significantly in Group D, from a baseline mean of 9.6 ± 2.2 μIU/mL to 13.8 ± 2.9 μIU/mL (p<0.01), suggesting enhanced beta-cell function and insulin secretion. In contrast, Groups B (Vitamin D only) and C (Exercise only) showed moderate but statistically significant increases in insulin levels to 11.2 ± 2.5 μIU/mL and 11.5 ± 2.7 μIU/mL, respectively (*p<0.05), while the control group (Group A) exhibited no appreciable change (9.6 ± 2.0 μIU/mL).

Fasting blood glucose (FBG) levels also declined most markedly in Group D, falling to 98.4 ± 8.6 mg/dL (p<0.01), reflecting improved glycemic control and possibly increased peripheral insulin sensitivity. Groups B and C experienced similar reductions in FBG to 110.3 ± 10.5 mg/dL and 108.5 ± 9.9 mg/dL, respectively (*p<0.05), whereas the control group remained unchanged (118.7 ± 12.1 mg/dL). These patterns were mirrored in glycated hemoglobin (HbA1c) outcomes. Group D participants achieved a mean HbA1c of 5.4 ± 0.2%, representing a significant reduction from baseline and indicating a shift from the pre-diabetic to normoglycemic range. Groups B and C also experienced moderate improvements (5.9 ± 0.3% and 5.8 ± 0.3%, respectively; *p<0.05), while Group A showed no significant change (6.2 ± 0.3%).

Insulin resistance, as measured by the Homeostatic Model Assessment of Insulin Resistance (HOMA-IR), improved most significantly in Group D, decreasing to 1.7 ± 0.3 (p<0.01), a value approaching normal insulin sensitivity. Groups B and C exhibited moderate reductions to 2.3 ± 0.5 and 2.2 ± 0.4, respectively (*p<0.05), while the control group retained a higher HOMA-IR of 2.8 ± 0.6. These results collectively indicate that both vitamin D and exercise independently confer metabolic benefits, but their combination yields the most pronounced and clinically meaningful improvements in insulin dynamics, glucose regulation, and insulin sensitivity(table 2).

Table 2: Post-Intervention Outcomes

Statistical Analysis: Group D showed a statistically significant increase in insulin levels compared to all other groups at 3 and 6 months (p<0.01). Group B and C also showed significant improvements compared to control (p<0.05), but less than the combined group.

Subgroup Analysis: Pre-diabetic individuals in Group D had greater relative increases in insulin levels (mean increase: 5.1 uIU/mL) compared to T2DM participants (mean increase: 3.6 uIU/mL). Collectively, the Combined group showed the most significant advancements in insulin levels, HbA1c, and HOMA-IR, outperforming all other groups at the 3 and 6-month marks.

Post-intervention analysis revealed that the combined intervention group exhibited a statistically significant increase in plasma insulin levels compared to control and individual intervention groups (p<0.01). Notably, improvements were accentuated in the pre-diabetic subgroup, echoing findings by other researchers like Pittas and colleagues who have documented the impact of vitamin D on glucose metabolism and insulin resistance (Pittas, 2010; Kostoglou‐Athanassiou et al., 2013). The multivariate analysis underscored potential synergistic interactions between vitamin D and exercise in enhancing insulin secretion, consistent with literature indicating that both interventions independently bolster insulin sensitivity and secretion (Baziar et al., 2014; Talaei et al., 2013).

This study demonstrated that vitamin D supplementation and moderate exercise independently improve plasma insulin levels, and their combination yields an additive effect. These findings are consistent with previous evidence indicating vitamin D’s role in enhancing insulin sensitivity and secretion.

The findings corroborate earlier research, demonstrating independent benefits of vitamin D supplementation and moderate exercise on insulin levels, with additional additive effects when combined (Dadrass et al., 2019; Rahimi et al., 2017).

Vitamin D is thought to act on pancreatic beta cells via the VDR, influencing insulin gene expression and secretion. Furthermore, it modulates systemic inflammation, which may contribute to improved insulin sensitivity. Exercise enhances glucose uptake in skeletal muscle and upregulates GLUT4 transporters, facilitating better glycemic control. (Morris et al., 2024; Aly et al., 2016).

The greater effect in pre-diabetics suggests that earlier intervention may yield more pronounced metabolic benefits. The study also reinforces the safety and efficacy of the chosen vitamin D dosage and exercise regimen (Salarinia et al., 2023; Jafari et al., 2021).

Limitations include the short duration, lack of dietary control, and reliance on self-reported adherence to the exercise protocol. Future studies should explore long-term effects, include larger cohorts, and investigate molecular biomarkers.

Vitamin D supplementation combined with moderate aerobic exercise significantly enhances plasma insulin levels in individuals with pre-diabetes and T2DM. The synergistic effects observed suggest a beneficial and accessible intervention strategy. Early lifestyle modifications incorporating these elements can delay or prevent diabetes progression.

Acknowledgments:

I thank all the faculty, teaching and non-teaching staff of Department of Biochemistry, Government Medical College, Karur and participants of my study for their valuable contribution The authors would like to thank all of the study participants and the administration Department of Biochemistry, Government Medical College, Karur, Tamilnadu, India for granting permission to carry out the research work.

Conflicts of interest: There are no conflicts of interest.

Ethical statement:

Institutional ethical committee accepted this study. The study was approved by the institutional human ethics committee, Government Medical College, Karur. Informed written consent was obtained from all the study participants and only those participants willing to sign the informed consent were included in the study. The risks and benefits involved in the study and the voluntary nature of participation were explained to the participants before obtaining consent. The confidentiality of the study participants was maintained.

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