Diabetes mellitus, predominantly type 2, is a major public health problem, with a prevalence projected to rise from 10.5% in 2021 to 12.2% in 2045 if no preventive measures are taken [1] . Chronic hyperglycemia caused by diabetes mellitus leads, in the long term, to dysfunction of various organs, including the eyes, kidneys, nerves, heart, and blood vessels, resulting in complications that may be macrovascular (myocardial infarction, transient ischemic attack, stroke, and limb ischemia) and/or microvascular (retinopathy, nephropathy, peripheral neuropathy) [2] [3] .

Numerous modifiable risk factors for diabetes mellitus have been identified, most of which are related to lifestyle, such as diet and physical activity. Other risk factors have been identified in recent years. Among these emerging factors is vitamin D status, which appears to be a marker of overall health, since a body of consistent epidemiological evidence indicates that adequate vitamin D intake is associated with better health outcomes. In addition to its role in phosphocalcic homeostasis, several other functions have been attributed to vitamin D. It plays a role in insulin synthesis and may be implicated in the development of diabetes independently of natural predisposing factors [4] [5] .

In diabetic patients, vitamin D plays a crucial role, and its deficiency may be responsible for several disorders [6] . Low plasma vitamin D levels are associated with poor glycemic control because vitamin D plays an important role in insulin secretion through several mechanisms [7] - [10] . Beyond vascular complications, vitamin D deficiency may increase the frequency of hospitalizations, respiratory infections, musculoskeletal disorders, and mortality in diabetic patients, as well as the risk of amputation in cases of diabetic foot lesions [11] [12] .

Because of the metabolic and vascular effects of vitamin D, it has gained considerable attention in diabetes research. In Benin, several epidemiological and clinical studies have been conducted on Type 2 Diabetes (T2D), but none have specifically addressed vitamin D. Many T2D patients followed in care centers are unaware of their vitamin D status, as measurement of serum 25-hydroxyvitamin D is not prescribed due to its high cost. This study was therefore undertaken to fill this gap, with the aim of determining the prevalence and factors associated with vitamin D deficiency among patients with T2D in Parakou.

Study setting: The outpatient clinic of the NGO Diabetes Benin in Parakou served as the study site for participant recruitment.

Study design and period: This was a cross-sectional, descriptive, and analytical study with prospective data collection conducted from January 30 to March 3, 2023.

Study population: The study population consisted of patients with type 2 diabetes (T2D) followed at the outpatient clinic of the NGO Diabetes Benin for at least 6 months, aged 18 years or older, and who provided informed consent. Patients with T2D who were unable to answer questions, those with renal failure, anemia, or abnormalities of phosphocalcic metabolism were excluded from the study.

Variables: The dependent variable was 25(OH)D deficiency, defined as a plasma concentration < 20 ng/mL. Vitamin D status was classified as follows: insufficient if [20 - 30[ ng/mL, sufficient if [30 - 100[ ng/mL, and toxic if ≥100 ng/mL. Independent variables included sociodemographic characteristics, comorbidities, lifestyle factors, chronic complications, clinical parameters, and fasting blood glucose.

Measurement of serum 25(OH)D: Quantification was performed using a competitive enzyme-linked immunoassay (ELFA) with final fluorescence detection on a Minividas analyzer.

Data processing and analysis: Data analysis was performed using Epi Info version 7.2.2.6. Qualitative variables were expressed as percentages with confidence intervals, and quantitative variables as means with standard deviations. Logistic regression analysis was conducted to assess associations between vitamin D deficiency and independent variables among T2D patients. Statistical significance was set at 0.05. Explanatory variables were presented as adjusted Odds Ratios (aOR) with their 95% confidence intervals (95% CI).

Ethical and regulatory considerations: Written informed consent was obtained from all participants. The study protocol was approved by the Local Ethics Committee for Biomedical Research of the University of Parakou under reference number 255/2022/CLERB-UP/P/SP/R/SA, dated May 3, 2022.

General characteristics of the study population: A total of 120 patients with type 2 diabetes were included. Their mean age was 60.35 ± 11.28 years, with a predominance of females (66.67%). The mean Body Mass Index (BMI) was 27.60 ± 5.24 kg/m² (range: 17.18 - 43.05 kg/m²). Patients with a BMI ≥ 25 kg/m² accounted for 65.84%, and waist circumference was elevated in 70% of the participants. The prevalence of hypertension was 63.33%. Lifestyle characteristics included alcohol consumption (13.13%), physical inactivity (17.50%), use of covering clothing (70%), and daily sun exposure < 5 hours (65.83%). Clinically, polyuria and polydipsia were observed in 30.00% and 31.67% of patients, respectively, while abnormal blood pressure was noted in 31.67%. Chronic complications included retinopathy (40.00%) and peripheral neuropathy (63.33%). Biochemically, fasting blood glucose was normal in 45.83% of the participants. ( Table 1 )

Prevalence of 25(OH)D deficiency: Among the 120 patients with type 2 diabetes, 76 had plasma 25(OH)D levels < 20 ng/mL, corresponding to a prevalence of 63.33% (95% CI [54.05 - 71.94]). Only 5% of the participants had normal levels, and none had toxic concentrations. The mean plasma 25(OH)D concentration was 17.35 ± 7.21 ng/mL. ( Table 2 )

Factors associated with 25(OH)D deficiency: In multivariate analysis, normal blood pressure (aOR = 0.30, p = 0.0242) and normal fasting blood glucose (aOR = 0.30, p = 0.0118) were identified as protective factors against 25(OH)D deficiency. ( Table 3 and Table 4 )

OR: Odds Ratio; CI: Confidence Interval; P: uncorrected chi-square test and Fisher’s exact test, as appropriate.

aOR: adjusted Odds Ratio; CI: Confidence Interval; P: Uncorrected chi-square test.

This study is the first to address vitamin D status in northern Benin. The method used for vitamin D measurement is among the most recommended. Statistical analyses, particularly multivariate analysis, allowed for the identification of true factors associated with 25(OH)D deficiency. The precautions taken ensured reliable results.

At the end of the study, the prevalence of 25(OH)D deficiency was found to be 63.33%. Similar prevalences have been reported by Hong et al. [13] in South Korea (71.10%), Taderegew et al. [14] in Ethiopia (64.20%), Siddiqee et al. [15] in Bangladesh (68.00%), Abdo et al. [16] in Yemen (65.70%), and Kumar et al. [17] in China (58.57%). Thus, 25(OH)D deficiency is a worldwide problem that spares no country. Higher prevalences have been reported by other authors, including 88.00% and 93.75% by Li et al. [18] in China and Nasr et al. [19] in Iraq, respectively. Preventive measures are therefore necessary and should include not only screening and vitamin D supplementation but also the adoption of health-promoting behaviors such as adequate sun exposure and reducing the frequent use of covering clothing. In the present cohort, the majority of patients reported less than 5 hours of daily sun exposure and frequently wore covering clothing.

Beyond these measures, it is important to identify associated factors. In this study, normal blood pressure and normal fasting blood glucose were protective factors, consistent with findings from other authors. Normal blood pressure was reported as a protective factor against 25(OH)D deficiency by Lui et al. [20] in China. Observational studies in humans have shown that reduced circulating 25-hydroxyvitamin D is associated with increased activity of the renin–angiotensin–aldosterone system and higher blood pressure [21] . According to Alzahrani et al. [22] in Saudi Arabia and Abdo et al. [16] in Yemen, a normal fasting blood glucose level was a protective factor against 25(OH)D deficiency. The role of vitamin D in insulin synthesis is well established and may explain the association between hyperglycemia and 25(OH)D deficiency [4] [5] .

This study faced certain limitations, including the absence of a control group of non-diabetic individuals and the relatively small sample size, which justifies conducting further studies on a larger cohort. It should also be noted that the exclusion of patients with renal insufficiency, whose vitamin D metabolism is impaired, may have influenced the observed prevalence.

Vitamin D [25(OH)D] deficiency is high among patients with type 2 diabetes followed in Parakou, requiring early screening and management. Good glycemic control and normal blood pressure are goals to be achieved given their protective effect against 25(OH)D deficiency.