Study design and setting

This was a facility-based cross-sectional study designed to determine the prevalence and predictors of vitamin D deficiency among Northern Muslim women presenting with chronic musculoskeletal pain. The study was conducted in Port Harcourt, Rivers State, Nigeria, a metropolitan city located in the Niger Delta region (latitude 4°49′N, longitude 7°2′E), characterized by a tropical monsoon climate with year-round sunlight averaging 12 hours per day.

Data collection occurred from March to August 2024 in five primary health care centres and four outpatient clinics. These facilities were purposefully selected because they serve both local residents and a large population of Northern Nigerian migrants, many of whom are Muslim women adhering to cultural dress codes that reduce sun exposure. This urban setting was chosen to reflect real-world environmental, social, and dietary factors influencing vitamin D status in this population.

Study population and sampling

A total of 220 participants (deficient = 143 and non-deficient = 77) were recruited using a consecutive sampling strategy, in which women presenting to selected outpatient clinics for evaluation of musculoskeletal pain were consecutively approached and screened for eligibility. Participants were included if they were female, aged 18 years or older, self-identified as Muslim women of Northern Nigerian origin, had resided in Port Harcourt for at least one year, and reported chronic musculoskeletal pain lasting a minimum of three months, confirmed by both clinical records and self-report. Exclusion criteria were pregnancy or current breastfeeding, known metabolic bone diseases such as rickets, osteomalacia, or Paget’s disease, use of vitamin D supplements exceeding 1,000 IU/day within the previous three months, chronic kidney or liver disease, malignancy, or any other condition affecting vitamin D metabolism, and the use of medications known to interfere with vitamin D metabolism, including anticonvulsants and glucocorticoids. Women who declined or were unable to provide informed consent were also excluded. Written informed consent was obtained from all participants after the study objectives and procedures were explained in either English or Hausa, depending on participant preference.

Ethical approval

This study was reviewed and approved by the Rivers State University Research Ethics Committee (Protocol No: RSU/REC/2025/112) and conducted in accordance with the Declaration of Helsinki (2024 revision).

Data collection procedures

Data collection was carried out by four trained research assistants who were fluent in both English and Hausa to facilitate effective communication with participants. A structured, interviewer-administered questionnaire, which had been pretested among 20 women to ensure reliability and face validity, was used to gather sociodemographic and lifestyle information. Variables collected included age, marital status, education level, occupation, and household income. Anthropometric measurements were obtained using a digital weighing scale (Seca 803, Germany) and a portable stadiometer (Seca 213, Germany), with participants measured in light clothing and without shoes. Body Mass Index (BMI) was calculated as weight in kilograms divided by height in meters squared (kg/m²). Clothing style was categorized as full-body covering, including face veil or niqab, or partial covering, which left the face and hands exposed. Sun exposure was classified as either less than three hours or three or more hours of direct sunlight per week. Dietary assessment focused on the frequency of consumption of vitamin D-rich foods such as oily fish, eggs, and fortified milk, recorded as daily, weekly, monthly, or rarely.

Pain assessment

Pain severity was assessed using a 10-point Visual Analogue Scale (VAS), where 0 = no pain and 10 = worst possible pain. The VAS was explained to participants, and scores were obtained in a quiet consultation room to minimize bias.

Blood collection and processing

Certified phlebotomists collected 5 mL of venous blood from the antecubital vein using sterile 21-gauge needles and EDTA-containing vacutainer tubes, labeling each sample with a unique participant code for blinding. The samples were allowed to clot at room temperature for 5–10 minutes, then centrifuged at 2,795 × g for 5 minutes using a Hettich EBA 200 centrifuge (Germany). The resulting serum was carefully separated into aliquots and stored in cryovials at −70°C until analysis.

Vitamin D analysis

Vitamin D status was determined by measuring serum 25-hydroxyvitamin D [25(OH)D] concentrations using a solid-phase competitive enzyme-linked immunosorbent assay (ELISA) (DIA Source 25-OH Vitamin D Total ELISA Kit, Bio-Rad Laboratories, Hercules, CA, USA) on a continuous data acquisition (CODA) automated EIA analyzer. Inter-assay coefficients of variation (CV) were 4.900% at 26.3 ng/mL and 4.500% at 42.0 ng/mL. Vitamin D status was categorised based on established guidelines as follows: Deficient: < 20 ng/mL; Insufficient: 20–29 ng/mL, and Sufficient: ≥ 30 ng/mL. All procedures followed the manufacturer’s guidelines. Quality control procedures were followed throughout the analysis to ensure the reliability and reproducibility of results.

Statistical analysis

Data were analyzed using IBM SPSS Statistics version 25 (IBM Corp., Armonk, NY, USA). Descriptive statistics (means, standard deviations, frequencies, and percentages) were used to summarize participant characteristics and vitamin D status categories. The prevalence of vitamin D deficiency was calculated with 95% confidence intervals. Associations between categorical variables (marital status, education level, clothing style, outdoor activity, and dietary intake) and vitamin D status were assessed using chi-square tests. Continuous variables such as age and BMI were compared between groups using independent-samples t-tests. Baseline characteristics (e.g., demographics, pain scores, sun exposure) were compared between groups (deficient vs. non-deficient) using stratified analysis to control for potential confounders, including prior low-dose vitamin D supplementation (< 1,000 IU/day). Multivariate logistic regression was conducted to identify independent predictors of vitamin D deficiency, adjusting for potential confounders. Differences in mean musculoskeletal pain scores across vitamin D status categories (deficient, insufficient, sufficient) were evaluated using one-way analysis of variance (ANOVA) with post hoc Tukey’s tests for pairwise comparisons. Statistical significance was set at p < 0.05.

Sociodemographic and clinical characteristics of participants

Table 1: The sociodemographic and clinical characteristics of participants. A total of 220 women were included. The mean age was 36.8 ± 11.2 years.

Associations of vitamin D deficiency with full-body covering, low outdoor activity, and low intake of vitamin D-rich foods

Univariate analysis (Table 2) revealed significant associations of vitamin D deficiency with full-body covering, low outdoor activity, and low intake of vitamin D-rich foods.

Independent predictors of vitamin D deficiency

Multivariate logistic regression (Table 3) identified independent predictors.

Mean serum 25-hydroxyvitamin D levels

Mean serum 25-hydroxyvitamin D [25(OH)D] levels differed significantly across groups (Table 4).

Self-reported musculoskeletal pain severity by vitamin D status

Participants with a deficiency reported higher musculoskeletal pain scores (Table 5).

Frequency of consumption of vitamin D-rich foods among participants

Dietary sources of vitamin D were generally low (Table 6).

Socio-demographic and clinical characteristics of participants

This study enrolled 220 Northern Muslim women with chronic musculoskeletal pain residing in Port Harcourt, Rivers State. The mean age was approximately 37 years, and a large proportion (80.91%) were married. Over half (58.18%) reported no formal education, and a striking 70.00% practised full-body covering, while 75.00% engaged in outdoor activity for less than three hours per week. In the absence of a direct control group, our findings were compared with existing population-level data on vitamin D deficiency in Nigeria. Previous studies have reported prevalence rates of vitamin D deficiency ([25(OH)D] < 20 ng/mL) ranging from approximately 35% to 65% among urban adults in Nigeria [8]. In comparison, our study population of Northern Muslim women residing in Port Harcourt exhibited a prevalence of 65.0%, suggesting a slightly elevated but broadly similar rate relative to general urban populations. High prevalence of vitamin D deficiency has been reported in women in different regions of Nigeria, which is in line with the findings of our study. In Northern Nigeria, 83.00 to 87.50% of women were reported to be vitamin D deficient [7, 9]. Cultural clothing practices and reduced outdoor exposure may contribute specifically to higher risk in this subgroup, warranting targeted public health interventions [6, 7, 9].

Associations of vitamin D deficiency with full-body covering, low outdoor activity, and low intake of vitamin D-rich foods

Univariate analyses demonstrated significant associations between vitamin D deficiency and full-body covering, reduced outdoor activity, and low dietary intake of vitamin D-rich foods. Participants with full-body covering were significantly more likely to be deficient (79.72% vs. 51.95%, p < 0.001), consistent with prior studies linking concealing clothing to reduced UVB-mediated cutaneous vitamin D synthesis [8, 9]. Similarly, limited outdoor activity correlated with higher deficiency rates (91.60% vs. 55.84%, p < 0.001), suggesting that behavioural factors limit sun exposure even in equatorial regions [10]. Dietary insufficiency also emerged as a significant correlate, reinforcing the importance of food sources in populations with low sun exposure. These findings suggest multiple modifiable factors contributing to the deficiency in this group [11–13].

Independent predictors of vitamin D deficiency

Multivariate logistic regression identified full-body covering, low outdoor activity, and low dietary intake of vitamin D-rich foods as independent predictors of deficiency. Specifically, full-body covering was associated with a threefold increased odds of deficiency (aOR = 3.1), while low outdoor activity (aOR = 2.7) and inadequate dietary intake (aOR = 2.4) also significantly predicted deficiency. These results suggest that cultural clothing practices and behavioural factors independently contribute to risk, even after controlling for age and BMI [12, 14–16].

Mean serum 25-hydroxyvitamin D levels [25(OH)D]

The mean serum [25(OH)D] concentrations varied significantly by clothing style, outdoor activity, and dietary intake. Women practising full-body covering had markedly lower mean levels (15.9 ng/mL) compared to those with partial covering (22.4 ng/mL, p < 0.001). Likewise, participants with < 3 hours of outdoor activity weekly had lower levels (16.5 ng/mL) than those with ≥ 3 hours (22.2 ng/mL). Low intake of vitamin D-rich foods was also associated with lower mean serum levels (16.2 ng/mL vs. 21.5 ng/mL, p < 0.001). These differences underscore how behavioural and cultural practices jointly shape vitamin D status, even in sun-rich regions, by limiting effective UVB exposure and dietary intake [12, 14–16].

Self-reported musculoskeletal pain severity by vitamin D status

Participants with vitamin D deficiency reported significantly higher mean musculoskeletal pain severity scores (VAS 7.3) compared to those with insufficient (5.9) or sufficient (4.8) vitamin D status (p < 0.001). This aligns with evidence suggesting a role for vitamin D in musculoskeletal health and pain modulation [17, 18]. While causality cannot be inferred from this cross-sectional design, the clear gradient in pain scores by vitamin D status suggests that deficiency may contribute to the severity of chronic pain among these women.

Frequency of consumption of vitamin D-rich foods among participants

Dietary assessment revealed generally low consumption of vitamin D-rich foods among participants. For example, only 24.55% consumed fatty fish ≥ 2 times per week, 32.73% regularly ate egg yolks, and fewer than one in five reported regular intake of fortified dairy products or cereals. Vitamin D supplement use was extremely rare (5.45%). These findings highlight a major dietary gap that likely exacerbates deficiency risk, particularly for individuals with limited sun exposure due to clothing or lifestyle [19–22].

Limitations

This study has several important limitations. First, its cross-sectional design prevents establishing causality between vitamin D deficiency and chronic musculoskeletal pain severity. Second, vitamin D intake was assessed through self-reported dietary frequency without precise quantification, which may introduce recall bias. Third, the sample was limited to Northern Muslim women residing in an urban area of Port Harcourt, potentially limiting generalisability to rural settings or other ethnic and religious groups. Additionally, other potential determinants of vitamin D status, such as skin pigmentation, seasonality, and genetic factors, were not assessed. Despite these limitations, the study provides valuable insights into the high burden of vitamin D deficiency and its correlates in a culturally defined, at-risk population.

Conclusions

This study demonstrates a high prevalence of vitamin D deficiency (65.0%) among Northern Muslim women with chronic musculoskeletal pain residing in Port Harcourt, Rivers State, Nigeria. Independent predictors of deficiency included full-body covering, low outdoor activity, and low dietary intake of vitamin D-rich foods. Furthermore, vitamin D deficiency was associated with significantly higher self-reported pain severity. These findings underscore the multifactorial nature of vitamin D deficiency in this population, shaped by cultural clothing practices, behavioral factors, and limited dietary sources. Addressing this public health issue requires culturally sensitive strategies to improve awareness, promote safe sun exposure, and enhance dietary vitamin D intake.

Recommendations

Based on these findings, the following recommendations are proposed:

• 1.

  Healthcare providers should routinely screen at-risk women and offer guidance on sun exposure, diet, and supplements.

• 2.

  Public health authorities should run culturally sensitive awareness campaigns and support food fortification.

• 3.

  Community and religious leaders should promote safe sun exposure within cultural norms.

• 4.

  Nutrition stakeholders should improve access to vitamin D-rich and fortified foods.

• 5.

  Researchers should explore long-term outcomes and test targeted interventions.