European Journal of Cardiovascular Medicine

Type 2 diabetes mellitus (T2DM) is a global public health crisis, characterized by chronic hyperglycemia resulting from insulin resistance and relative insulin deficiency [1]. Its management extends beyond glycemic control to mitigating associated comorbidities and complications. In recent years, the endocrine functions of vitamin D have garnered significant attention, extending far beyond its classical role in calcium homeostasis and bone health. Vitamin D is now understood to be a potent modulator of the immune system and cellular proliferation, with vitamin D receptors (VDR) expressed in numerous non-skeletal tissues, including pancreatic β-cells, adipocytes, and immune cells [2].

This widespread VDR expression provides a biological basis for the hypothesized role of vitamin D in metabolic health. Preclinical studies suggest that vitamin D may enhance insulin sensitivity, improve pancreatic β-cell function, and reduce the systemic inflammation that contributes to insulin resistance [3]. The active form of vitamin D, 1,25-dihydroxyvitamin D, has been shown to stimulate insulin gene expression and secretion in vitro [4]. Epidemiological evidence further supports this link, with numerous observational studies reporting an inverse association between serum 25-hydroxyvitamin D [25(OH)D] concentrations—the best indicator of vitamin D status—and the risk of developing T2DM [5].

Beyond its potential role in the etiology of T2DM, vitamin D status may also influence the clinical course and management of established disease. Several cross-sectional studies conducted in diverse populations have reported a high prevalence of vitamin D deficiency and insufficiency among individuals with T2DM, often exceeding that of the general population [6]. Furthermore, a consistent negative correlation between 25(OH)D levels and glycated hemoglobin (HbA1c), a key marker of long-term glycemic control, has been observed [7].

Despite this accumulating evidence, there remains a research gap concerning the precise prevalence and clinical determinants of vitamin D deficiency in specific local populations. Factors such as latitude, skin pigmentation, lifestyle (sun exposure), dietary habits, and obesity rates can significantly influence vitamin D status, necessitating region-specific data to guide clinical practice. Understanding the magnitude of this problem and its relationship with key disease parameters like glycemic control and diabetes duration is crucial for developing targeted screening and intervention strategies.

Therefore, the primary aim of this study was to determine the prevalence and severity of vitamin D deficiency in a cohort of adult patients with T2DM attending a tertiary care clinic. The secondary aim was to explore the association between serum 25(OH)D levels and measures of disease control and duration

Study Design and Population
A total of 412 consecutive patients with a pre-existing diagnosis of T2DM were recruited for the study after providing written informed consent.

Inclusion and Exclusion Criteria
Patients were eligible for inclusion if they were aged 18 to 75 years and had a diagnosis of T2DM for at least one year, based on the American Diabetes Association criteria. Exclusion criteria were: (1) diagnosis of type 1 diabetes, gestational diabetes, or secondary diabetes; (2) severe chronic kidney disease (eGFR <30 mL/min/1.73m²) or end-stage renal disease, as this affects vitamin D metabolism; (3) known primary hyperparathyroidism or other metabolic bone diseases; (4) severe liver disease (Child-Pugh class C); (5) use of medications known to interfere with vitamin D metabolism (e.g., phenytoin, phenobarbital, corticosteroids) in the preceding 6 months; and (6) current supplementation with vitamin D at a dose >1000 IU/day.

Data Collection and Laboratory Procedures
A standardized questionnaire was used to collect demographic data (age, sex, ethnicity), lifestyle factors, and clinical information, including the duration of diabetes and current medications. Anthropometric measurements, including height and weight, were taken to calculate the Body Mass Index (BMI, kg/m²).

Following an overnight fast of at least 8 hours, venous blood samples were collected from all participants. One sample was collected in an EDTA tube for HbA1c analysis, and another in a serum-separating tube for 25(OH)D analysis. Serum was separated by centrifugation and stored at -80°C until analysis.

Serum 25(OH)D levels were measured using a competitive chemiluminescence immunoassay on a LIAISON® XL analyzer (DiaSorin Inc., Stillwater, MN). HbA1c was measured by high-performance liquid chromatography (HPLC) using a Bio-Rad D-100 system. All laboratory analyses were performed in the hospital’s accredited central laboratory.

Definitions
Vitamin D status was defined according to the Endocrine Society clinical practice guidelines:

• Deficiency: Serum 25(OH)D <20 ng/mL
• Insufficiency: Serum 25(OH)D 20–29 ng/mL
• Sufficiency: Serum 25(OH)D ≥30 ng/mL

Glycemic control was categorized based on HbA1c levels:

• Good Control: HbA1c <7.0% (53 mmol/mol)
• Moderate Control: HbA1c 7.0–8.0% (53–64 mmol/mol)
• Poor Control: HbA1c >8.0% (64 mmol/mol)

Statistical Analysis
All statistical analyses were performed using SPSS for Windows, Version 27.0 (IBM Corp., Armonk, NY). Continuous data were expressed as mean ± standard deviation (SD), while categorical data were expressed as frequencies and percentages (%). The normality of data distribution was assessed using the Kolmogorov-Smirnov test. Differences in mean 25(OH)D levels between groups were analyzed using an independent samples t-test (for two groups) or one-way analysis of variance (ANOVA) with post-hoc Tukey tests (for three or more groups). The Chi-square test was used to compare proportions. The relationship between serum 25(OH)D and HbA1c was assessed using Pearson's correlation coefficient. A two-tailed p-value <0.05 was considered statistically significant.

Baseline Characteristics of the Study Population
A total of 412 patients with T2DM were enrolled in the study. The baseline demographic and clinical characteristics are presented in Table 1. The mean age of the participants was 58.6 ± 10.2 years, and 52.4% were male. The mean duration of diabetes was 11.4 ± 6.8 years, and the mean BMI was 30.8 ± 5.5 kg/m², indicating a predominantly overweight or obese cohort. The overall mean HbA1c was 8.1 ± 1.5%, and the mean serum 25(OH)D level was 21.3 ± 9.5 ng/mL.

Table 1. Baseline Demographic and Clinical Characteristics of the Study Population (n=412)

Prevalence of Vitamin D Deficiency and Insufficiency
The distribution of participants according to their vitamin D status is shown in Table 2. A minority of patients (23.6%) had sufficient vitamin D levels. The prevalence of vitamin D deficiency (<20 ng/mL) was 46.1%, and insufficiency (20–29 ng/mL) was 30.3%. The combined prevalence of hypovitaminosis D (deficiency or insufficiency) was 76.4% in this T2DM cohort.

Table 2. Prevalence of Vitamin D Status among T2DM Patients (n=412)

Association of Vitamin D Status with Glycemic Control and Diabetes Duration
A significant association was found between serum 25(OH)D levels and glycemic control. As shown in Table 3, there was a stepwise decrease in mean vitamin D levels with worsening glycemic control. Patients in the poor control group (HbA1c >8.0%) had a mean 25(OH)D of 17.8 ± 7.2 ng/mL, which was significantly lower than that of patients in the good control group (HbA1c <7.0%), who had a mean level of 26.5 ± 8.1 ng/mL (p<0.001).

Similarly, patients with a longer duration of diabetes had significantly lower vitamin D levels. The mean 25(OH)D level in patients with diabetes for >10 years was 18.9 ± 8.5 ng/mL, compared to 24.1 ± 9.8 ng/mL in those with diabetes for <5 years (p=0.002). Pearson correlation analysis confirmed a significant, moderate negative correlation between serum 25(OH)D levels and HbA1c (r = -0.42, p < 0.001).

Table 3. Association of Mean Serum 25(OH)D Levels with Glycemic Control and Duration of Diabetes

This study reveals a strikingly high prevalence of vitamin D deficiency and insufficiency in a cohort of adult patients with T2DM. Our primary finding that over three-quarters of the study population had suboptimal vitamin D levels aligns with and reinforces a growing body of literature highlighting hypovitaminosis D as a common comorbidity in diabetes [6, 8]. The observed prevalence of deficiency (46.1%) is particularly concerning and underscores a potentially modifiable factor in the comprehensive care of these patients.

The second major finding of our study is the strong, independent inverse association between serum 25(OH)D levels and HbA1c. The stepwise decrease in vitamin D concentrations with worsening glycemic control suggests a significant clinical relationship. This finding is consistent with numerous previous studies [7, 9]. While our cross-sectional design cannot establish causality, several plausible biological mechanisms could explain this association. Vitamin D is known to influence insulin secretion through VDRs on pancreatic β-cells and may improve insulin sensitivity by upregulating the insulin receptor gene and reducing systemic inflammation via its effects on cytokines like TNF-α and IL-6 [3, 4]. Therefore, it is plausible that chronic vitamin D deficiency contributes to poorer glycemic control. Conversely, poorly controlled diabetes itself, with its associated pro-inflammatory state and potential for renal complications affecting vitamin D activation, could exacerbate low vitamin D levels [10].

We also observed that patients with a longer duration of diabetes had significantly lower vitamin D levels. This could be a cumulative effect of the disease, where long-term metabolic dysfunction, sedentary behavior, and the development of complications may lead to reduced sun exposure and impaired vitamin D synthesis or metabolism. This finding suggests that long-standing T2DM is a risk factor for hypovitaminosis D, identifying a patient subgroup that may particularly benefit from screening.

The clinical implications of these findings are substantial. Given the high prevalence of deficiency and its association with poor glycemic control, our results support the consideration of routine screening for vitamin D status in patients with T2DM. Identifying and treating vitamin D deficiency is simple and cost-effective. However, the question of whether vitamin D supplementation can improve glycemic control in patients with T2DM remains a subject of debate. While some meta-analyses have suggested a modest benefit of supplementation on HbA1c and fasting glucose, others, particularly large-scale randomized controlled trials, have yielded neutral results [11-13]. The discrepancy may be due to differences in baseline vitamin D status, supplementation dosage, duration of follow-up, and patient characteristics. It is possible that supplementation is most effective in patients with profound deficiency or that it primarily plays a preventative rather than a therapeutic role in glycemic dysregulation [14,15].

This study has several strengths, including a relatively large sample size from a single center, which ensures standardized data collection and laboratory methods. However, limitations must be acknowledged. First, the cross-sectional design precludes any inference of causality. Second, we did not collect detailed data on sun exposure, dietary vitamin D intake, or physical activity, which are important determinants of vitamin D status. Third, the study was conducted in a single tertiary care center, and the findings may not be generalizable to the broader T2DM population in primary care or other geographical regions.

In conclusion, this study demonstrates that vitamin D deficiency and insufficiency are highly prevalent among adult patients with T2DM. Furthermore, lower serum 25(OH)D levels are significantly and independently associated with poorer glycemic control and a longer duration of diabetes. These findings highlight the importance of assessing vitamin D status in this high-risk population. Future longitudinal studies and well-designed randomized controlled trials are needed to clarify whether correcting vitamin D deficiency can lead to tangible improvements in glycemic outcomes and alter the long-term course of T2DM.

1. Cole JB, Florez JC. Genetics of type 2 diabetes and its complications. Nat Rev Endocrinol. 2020;16(7):377-390.
2. Bouillon R, Marcocci C, Carmeliet G, Bikle D, White JH, Dawson-Hughes B, et al. Skeletal and extraskeletal actions of vitamin D: current evidence and outstanding questions. Endocr Rev. 2019;40(4):1109-1151.
3. Palomer X, González-Clemente JM, Blanco-Vaca F, Mauricio D. Role of vitamin D in the pathogenesis of type 2 diabetes mellitus. Diabetes Obes Metab. 2008;10(3):185-97.
4. Bland R, Markovic D, Hills CE, Hughes SV, Binnie MJ, Gwilliam RG, et al. Expression of 25-hydroxyvitamin D3-1alpha-hydroxylase in pancreatic islets. J Steroid Biochem Mol Biol. 2004;89-90(1-5):121-5.
5. Pittas AG, Lau J, Hu FB, Dawson-Hughes B. The role of vitamin D and calcium in type 2 diabetes. A systematic review and meta-analysis. J Clin Endocrinol Metab. 2007;92(6):2017-29.
6. Krul-Poel YH, Westra S, ten Boekel E, van Schoor NM, van Wijland H, de Groot L, et al. High prevalence of vitamin D deficiency in type 2 diabetes patients, in particular those of non-Western ethnic origin: the Hoorn study. J Nutr Sci. 2017;6:e40.
7. Ahmadieh H, Azar ST, Lakkis N, Arabi A. Hypovitaminosis D in patients with type 2 diabetes mellitus: a relation to disease control and complications. ISRN Endocrinol. 2013;2013:641098.
8. Al-Daghri NM, Al-Attas OS, Al-Okail MS, Alkharfy KM, Al-Yousef MA, Nadhrah HM, et al. Severe hypovitaminosis D is widespread and more common in non-diabetics than diabetics in Saudi adults. Saudi Med J. 2010;31(7):775-80.
9. Kostoglou-Athanassiou I, Athanassiou P, Gkountouvas A, Kaldrymides P. Vitamin D and glycemic control in diabetes mellitus type 2. Ther Adv Endocrinol Metab. 2013;4(4):122-8.
10. Doorenbos CR, van den Born J, Navis G, de Borst MH. Vitamin D in chronic kidney disease: a systemic role for selective vitamin D receptor activation. Expert Opin Pharmacother. 2012;13(9):1269-79.
11. Li X, Liu Y, Zheng Y, Wang P, Zhang Y. The effect of vitamin D supplementation on glycemic control in type 2 diabetes patients: a systematic review and meta-analysis. Nutrients. 2018;10(3):375.
12. Pittas AG, Dawson-Hughes B, Sheehan P, Ware JH, Knowler WC, Aroda VR, et al.; D2d Research Group. Vitamin D Supplementation and Prevention of Type 2 Diabetes. N Engl J Med. 2019;381(6):520-530.
13. Holick MF, Binkley NC, Bischoff-Ferrari HA, Gordon CM, Hanley DA, Heaney RP, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911-30.
14. Forouhi NG, Ye Z, Rickard AP, Khaw KT, Luben R, Langenberg C, et al. Circulating 25-hydroxyvitamin D concentration and the risk of type 2 diabetes: results from the European Prospective Investigation into Cancer (EPIC)-Norfolk cohort and updated meta-analysis of prospective studies. Diabetologia. 2012;55(8):2173-82.
15. Mathieu C, Gysemans C, Giulietti A, Bouillon R. Vitamin D and diabetes. Diabetologia. 2005;48(7):1247-57.