Type 2 diabetes mellitus is a chronic metabolic disorder characterized by insulin resistance, progressive beta-cell dysfunction, and sustained hyperglycaemia. It remains one of the most common non-communicable diseases encountered in general medicine practice. The current approach to diabetes care extends beyond glucose lowering and includes systematic identification of modifiable metabolic, nutritional, vascular, and lifestyle-related factors that influence long-term outcomes [1]. In routine outpatient settings, micronutrient deficiencies are often under-recognized because their symptoms overlap with fatigue, myalgia, neuropathic discomfort, and nonspecific ill health commonly reported by patients with diabetes.

Vitamin D is a secosteroid hormone with established effects on calcium homeostasis and skeletal health. Serum 25-hydroxyvitamin D [25(OH)D] is the accepted marker for assessing vitamin D stores because it reflects endogenous synthesis and dietary intake. Deficiency is generally defined as serum 25(OH)D below 20 ng/mL, while values between 20 and 29.9 ng/mL are considered insufficient in many clinical and epidemiological frameworks [2,3]. Apart from its skeletal role, vitamin D receptors are expressed in several tissues relevant to glucose metabolism, including pancreatic beta cells, skeletal muscle, adipose tissue, immune cells, and vascular endothelium. Through these pathways, low vitamin D status has been linked with impaired insulin secretion, insulin resistance, inflammation, and adiposity-related metabolic dysfunction [4,8].

Vitamin D deficiency is highly prevalent in India despite abundant sunlight. Urbanization, indoor work, clothing practices, air pollution, darker skin pigmentation, low dietary vitamin D intake, and limited food fortification contribute to this paradox [4]. South Indian data have shown that vitamin D deficiency is common across the glucose tolerance spectrum and is highest among individuals with type 2 diabetes mellitus [5]. A recent systematic review and meta-analysis also reported a high pooled prevalence of vitamin D deficiency among patients with type 2 diabetes mellitus, supporting its relevance as a frequent comorbidity in diabetic populations [6].

Clinical correlates of vitamin D deficiency in diabetes include obesity, poor glycaemic control, peripheral neuropathy, nephropathy, and low physical activity [6-11]. However, associations vary across regions because of differences in sunlight exposure, dietary habits, body composition, laboratory cut-offs, and diabetes care practices. Local outpatient evidence is therefore useful for identifying high-risk groups and for designing practical screening strategies. This study was conducted to estimate the prevalence of vitamin D deficiency among patients with type 2 diabetes mellitus attending the General Medicine outpatient department and to assess its association with age, sex, diabetes duration, BMI, glycaemic control, sun exposure, physical inactivity, biochemical parameters, and selected clinical correlates.

Study design and setting: This was a cross-sectional observational study conducted in the General Medicine outpatient department of Konaseema Institute of Medical Sciences & Research Foundation, Amalapuram, Andhra Pradesh, India. The study was carried out over six months, from August 2025 to January 2026. The study was done in accordance with the principles of observational study reporting recommended by the STROBE statement [14]. Study population: The study included 100 adult patients with type 2 diabetes mellitus attending the outpatient department during the study period. Patients with established type 2 diabetes mellitus, either previously diagnosed or diagnosed according to accepted diabetes criteria, were considered eligible [1]. Patients with type 1 diabetes mellitus, gestational diabetes mellitus, chronic liver disease, advanced chronic kidney disease, malabsorption disorders, pregnancy, lactation, current corticosteroid or antiepileptic drug use, and those receiving vitamin D supplementation within the preceding three months were excluded to reduce major confounding of vitamin D status. Data collection procedure: After obtaining informed consent, demographic details, duration of diabetes, medication history, symptoms, sun exposure, physical activity, and relevant clinical history were recorded using a structured proforma. Height and weight were measured using standard procedures, and BMI was calculated as weight in kilograms divided by height in metres squared. Patients were grouped as normal BMI, overweight, or obese according to routine adult BMI categories used in clinical practice. Sun exposure of less than 30 minutes per day and self-reported physical inactivity were recorded as lifestyle variables. Laboratory and clinical assessment: Fasting blood glucose, postprandial blood glucose, HbA1c, serum calcium, and serum 25(OH)D levels were measured as part of the study evaluation. Vitamin D status was categorized as severe deficiency when serum 25(OH)D was below 10 ng/mL, deficiency when it was 10-19.9 ng/mL, insufficiency when it was 20-29.9 ng/mL, and sufficiency when it was 30 ng/mL or above [2,3]. For association analysis, vitamin D deficiency was defined as serum 25(OH)D below 20 ng/mL. Peripheral neuropathy, hypertension, dyslipidaemia, microalbuminuria, diabetic retinopathy, and fatigue or myalgia were documented from clinical evaluation, available records, and relevant investigations. Statistical analysis: Data were entered into a spreadsheet and analysed using descriptive and inferential statistics. Continuous variables were expressed as mean ± standard deviation. Categorical variables were summarized as frequencies and percentages. Patients were divided into vitamin D deficient and not deficient groups. Pearson chi-square test was used for categorical comparisons, and independent sample t-test was used for comparison of continuous variables. A p-value less than 0.05 was considered statistically significant. Ethical considerations: The study was conducted after approval from the Institutional Ethics Committee. Written informed consent was obtained from all participants before enrolment. Patient confidentiality was maintained throughout data collection, analysis, and manuscript preparation.

A total of 100 patients with type 2 diabetes mellitus attending the General Medicine outpatient department were included in the study. The mean age of the study population was 53.8 ± 10.9 years. Most patients were aged 50 years and above. Males constituted 56% and females constituted 44% of the study population. The mean duration of diabetes was 6.8 ± 4.1 years, and 62% of patients had diabetes for five years or more. Overweight and obesity were observed in 78% of patients. Poor glycaemic control, defined as HbA1c ≥7%, was present in 76% of patients (Table 1).

Table 1. Baseline demographic and clinical profile of patients with type 2 diabetes mellitus

The mean serum 25(OH)D level was 18.7 ± 8.5 ng/mL. Vitamin D deficiency, defined as serum 25(OH)D level below 20 ng/mL, was observed in 64 patients, giving an overall prevalence of 64%. Severe deficiency was present in 18%, while 46% had vitamin D deficiency in the range of 10-19.9 ng/mL. Vitamin D insufficiency was observed in 24%, and only 12% of patients had sufficient vitamin D levels (Table 2).

Table 2. Distribution of vitamin D status among patients with type 2 diabetes mellitus

Note: Overall vitamin D deficiency represents the combined number of patients with severe deficiency and deficiency; it is not an additional category in the distribution.

Vitamin D deficiency was more frequent among patients aged ≥60 years, females, those with diabetes duration more than 10 years, overweight or obesity, poor glycaemic control, low sun exposure, and physical inactivity. Statistically significant associations were observed for diabetes duration greater than 10 years, overweight or obesity, HbA1c ≥9%, sun exposure less than 30 minutes per day, and physical inactivity. Age ≥60 years and female sex showed higher proportions of deficiency but did not reach statistical significance (Table 3).

Table 3. Association of vitamin D deficiency with demographic and clinical variables

Note: p-values for categorical variables were calculated using Pearson chi-square test.

Patients with vitamin D deficiency had higher mean HbA1c, fasting blood glucose, postprandial blood glucose, and BMI compared with those without deficiency. Mean serum calcium was lower among vitamin D-deficient patients. Among clinical correlates, peripheral neuropathy, microalbuminuria, and fatigue or myalgia were significantly more common in patients with vitamin D deficiency. Hypertension, dyslipidaemia, and diabetic retinopathy were numerically more frequent in the deficient group but did not show statistically significant associations (Table 4).

Table 4. Biochemical parameters and clinical correlates according to vitamin D status

Overall, vitamin D deficiency was common among patients with type 2 diabetes mellitus in the outpatient setting. It was significantly associated with poor glycaemic control, longer duration of diabetes, overweight or obesity, reduced sun exposure, physical inactivity, peripheral neuropathy, microalbuminuria, and fatigue or myalgia.

The present cross-sectional study found that vitamin D deficiency was highly prevalent among patients with type 2 diabetes mellitus attending a General Medicine outpatient department, with 64% of patients having serum 25(OH)D levels below 20 ng/mL. This finding is clinically important because vitamin D deficiency is a modifiable abnormality and shares several risk clusters with type 2 diabetes, including obesity, sedentary lifestyle, poor outdoor exposure, and chronic metabolic inflammation. The observed prevalence is close to the pooled estimate of 64.2% reported in a systematic review and meta-analysis of patients with type 2 diabetes mellitus [6]. It is also comparable to South Indian data in which vitamin D deficiency was more frequent among individuals with diabetes than among those with normal glucose tolerance [5].

The association between vitamin D deficiency and poor glycaemic control was a central finding of this study. Patients with deficiency had higher mean HbA1c, fasting blood glucose, and postprandial blood glucose values than those without deficiency. In addition, HbA1c ≥9% was significantly associated with deficient status. Similar inverse relationships between vitamin D and glycaemic parameters have been reported in previous studies and reviews [7,13]. Biological explanations include altered insulin secretion, impaired insulin sensitivity, low-grade inflammation, and indirect effects mediated through adiposity and reduced physical activity [8]. Since the present study is observational, these findings indicate association rather than causation; nevertheless, they support the value of assessing vitamin D status in patients with persistently poor glycaemic control.

Increased BMI was another significant correlate. Overweight and obesity were common in the study population, and vitamin D-deficient patients had a higher mean BMI. This finding is consistent with systematic reviews demonstrating a relationship between adiposity, lower vitamin D levels, and metabolic risk [8,9]. Vitamin D sequestration in adipose tissue, lower outdoor activity among obese individuals, dietary patterns, and dilutional effects in larger body mass are possible explanations. The significant association with low sun exposure and physical inactivity also reflects the lifestyle component of vitamin D deficiency in routine outpatient diabetes care.

The study also identified significant associations between vitamin D deficiency and peripheral neuropathy, microalbuminuria, and fatigue or myalgia. Earlier studies have reported links between low vitamin D levels and diabetic peripheral neuropathy, diabetic nephropathy, and albuminuria [10-12]. Vitamin D has potential roles in neurotrophic support, inflammation modulation, endothelial function, and renin-angiotensin system regulation, which could explain its relationship with neuropathic and renal markers. Hypertension, dyslipidaemia, and diabetic retinopathy were more frequent among deficient patients, but the differences were not statistically significant, possibly due to the modest sample size. These findings support integrating vitamin D assessment into comprehensive diabetes evaluation, especially among patients with long disease duration, high BMI, poor glycaemic control, low sun exposure, neuropathic symptoms, and early renal involvement.

Limitations

This study has limitations. The cross-sectional design prevents causal interpretation between vitamin D status and diabetic correlates. The sample size was limited to 100 patients from a single outpatient department, reducing wider generalizability. Dietary vitamin D intake, seasonal variation, parathyroid hormone levels, and detailed drug-related confounders were not assessed. Some complications were documented from clinical evaluation and available records only.

Vitamin D deficiency was highly prevalent among patients with type 2 diabetes mellitus attending the General Medicine outpatient department, affecting nearly two-thirds of the study population. Deficiency was associated with longer duration of diabetes, overweight or obesity, poor glycaemic control, low sun exposure, physical inactivity, higher fasting and postprandial glucose levels, peripheral neuropathy, microalbuminuria, and fatigue or myalgia. These findings highlight vitamin D deficiency as an important, measurable, and potentially correctable metabolic abnormality in patients with type 2 diabetes mellitus. Screening high-risk patients and encouraging sunlight exposure, physical activity, nutritional improvement, and appropriate supplementation after biochemical confirmation can support comprehensive diabetes care and improve risk-oriented clinical follow-up in outpatient practice.

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