Type 2 diabetes mellitus (T2DM) is a global health concern characterized by insulin resistance and relative insulin deficiency, leading to increased risks of cardiovascular disease, nephropathy, neuropathy, and retinopathy. Effective glycemic control is crucial for managing T2DM and preventing complications. Vitamin D plays a vital role in various physiological processes, and emerging research suggests its influence on insulin secretion and sensitivity, potentially impacting glucose metabolism. Inadequate vitamin D levels have been associated with an increased risk of T2DM. C-peptide, a byproduct of insulin production, serves as a marker for β-cell function and endogenous insulin secretion. Changes in C-peptide levels reflect the declining β-cell function in T2DM. This study aims to investigate the correlation between vitamin D deficiency, fasting C-peptide levels, and uncontrolled T2DM (HbA1c > 7%), exploring potential biomarkers and therapeutic avenues for managing uncontrolled T2DM. While research exists on vitamin D deficiency and T2DM, few studies have specifically focused on high-risk populations, making this investigation particularly relevant for developing targeted preventive interventions and optimizing existing management strategies. The prevalence of vitamin D deficiency among individuals with T2DM is high and linked to worsened insulin resistance and glycemic control, highlighting the need to understand its interaction with fasting C-peptide levels in uncontrolled T2DM.

Existing literature demonstrates a link between vitamin D deficiency and increased T2DM risk, although studies focusing on high-risk populations are limited. Several studies show associations between vitamin D deficiency, worsened insulin resistance, impaired glycemic control, and increased risk of T2DM. The role of fasting C-peptide levels in assessing β-cell function and predicting the need for insulin therapy is well-established. However, a knowledge gap exists regarding the combined association of vitamin D deficiency and fasting C-peptide levels with uncontrolled T2DM. This study aims to address this gap, potentially identifying therapeutic targets and informing clinical practice.

This retrospective cohort study included individuals diagnosed with T2DM (aged 18+), with available baseline data on vitamin D levels, fasting C peptide levels, and HbA1c. Participants were recruited from a specialized private diabetic clinic in Abha, Saudi Arabia, from April to June 2023. Data were collected from electronic medical records. A convenience sample of 72 participants was used. Baseline assessments included demographic information, clinical history, and measurements of serum vitamin D levels, fasting C peptide levels, BMI, blood pressure, HbA1c, serum creatinine, total cholesterol, and triglycerides. Follow-up assessments were conducted three months later. The primary outcome was uncontrolled T2DM (HbA1c consistently above 7%). Binary logistic regression analyses were performed using SPSS to investigate the association between vitamin D deficiency, fasting C-peptide levels, and uncontrolled T2DM. The study protocol was approved by the Institutional Review Board of King Khalid University.

The study population consisted mainly of individuals aged 36-75 years, with a slight male predominance. Most participants had a shorter diabetes duration (1-10 years), and the majority (100%) were not on insulin therapy. The mean BMI was 30.478, indicating a high prevalence of obesity. Wilcoxon rank tests showed significant improvements in HbA1c, creatinine, and vitamin D levels between the first and second visits, suggesting a positive response to diabetes management strategies. However, triglycerides increased. Regression analysis revealed significant associations between HbA1c, creatinine, triglycerides, and vitamin D with uncontrolled T2DM. Logistic regression analysis showed a non-significant negative association between both vitamin D levels (coefficient -0.097, p=0.181) and C-peptide levels (coefficient -0.222, p=0.797) and uncontrolled T2DM. While higher levels were associated with a lower likelihood of uncontrolled diabetes, this association did not reach statistical significance.

The study findings align with global T2DM prevalence trends and highlight the importance of early diagnosis and personalized management strategies. The high prevalence of obesity emphasizes the need for lifestyle interventions and weight management. Significant improvements in several physiological parameters between visits underscore the effectiveness of diabetes management strategies. However, the non-significant associations between vitamin D and C-peptide levels and uncontrolled T2DM suggest that while these factors may play a role, further research is needed to confirm their clinical significance and establish causal relationships. The lack of statistical significance might be attributed to the relatively small sample size or the influence of confounding variables.

This study suggests a potential, albeit non-statistically significant, association between higher vitamin D and C-peptide levels and a reduced likelihood of uncontrolled T2DM. Further research with larger sample sizes and longitudinal studies is needed to validate these findings and explore underlying mechanisms. The study highlights the importance of monitoring vitamin D levels and fasting C-peptide levels in T2DM management, emphasizing the need for comprehensive assessments and personalized approaches.

The study's limitations include the small sample size, the use of a convenience sample, a lack of information on potential confounding variables (dietary intake, sun exposure, vitamin supplement use), and the retrospective nature of the study which restricts the ability to establish causality. The absence of data on specific dietary intake, sun exposure, and vitamin D supplementation may have influenced the results.