Vitamin D deficiency is recognized as one of the most common public health problems worldwide. Low levels of vitamin D have been associated with a wide range of chronic and infectious conditions, including, diabetes (Hyppönen et al., 2001), cardiovascular disease (Lee et al., 2008), cancer (Holick, 2006a), multiple sclerosis (VanAmerongen et al., 2004), osteoporoz (Lips, 2001), autoimmune diseases (Cantorna et al., 2004), and microbial infections (White, 2008).

The role of vitamin D was first noted by Whistler in the 17th century in relation to rickets (O’riordan and Bijvoet, 2014). By the early 1900s, its fat-soluble nature, its production through ultraviolet (UV) sunlight exposure, and its antirachitic properties were described. The discovery of the specific forms vitamin D2 (ergocalciferol) and provitamin D3 (cholecalciferol) followed in the 1930s (DeLuca, 2014).

Vitamin D plays a critical role in bone metabolism as well as in various cellular and immunological processes. Inadequate sun exposure due to cultural clothing styles or excessive use of sunscreens, along with insufficient dietary intake or lack of supplementation, are common reasons for this vitamin D deficiency (Lips, 2007). Tropospheric ozone has also been linked to reduced synthesis of vitamin D. Other factors such as skin pigmentation, age, body weight, malabsorption disorders, and conditions affecting vitamin D metabolism can also influence serum levels (Holick, 2006b).

Currently, plasma (or serum) 25-hydroxyvitamin D [25(OH)D] is considered the best indicator of vitamin D status (Manicourt and Devogelaer, 2008), reflecting total input from cutaneous synthesis, diet, and supplements (Holick, 2007). The circulating concentration of 25(OH)D represents the cumulative effect of dietary intake and sunlight exposure, and it is widely accepted as a reliable marker of vitamin D status (Holick, 1996).

Although there is no universal consensus on the optimal serum level of 25(OH)D, most experts define deficiency as a level below 20 ng/mL (Holick, 2007). It is estimated that vitamin D deficiency affects approximately 30%–50% of the general population worldwide, yet this widespread condition often remains undiagnosed and untreated (Lee et al., 2008). To date, there is limited data available regarding the vitamin D status of adults in Türkiye (Atli et al., 2005; Alagöl et al., 2000; Guzel et al., 2001; Erkal et al., 2006).

This study aimed to determine the prevalence of vitamin D deficiency and insufficiency among adults in Tokat Province, Türkiye, and to identify sociodemographic and lifestyle factors associated with low vitamin D status.

A total of 2,225 adults participated in the study, including 1,220 (54.8%) women and 1,005 (45.2%) men. The mean age was 47.17 ± 15.23 years. Based on serum 25(OH)D3 levels, 38.7% of participants were classified as vitamin D deficient (<10 ng/mL), 46.4% as insufficient (10−20 ng/mL), and only 14.9% had sufficient levels (>20 ng/mL) (Table 1). These findings indicate that the majority of the population has suboptimal vitamin D status.

Women were significantly more likely to be vitamin D deficient compared to men (p < 0.001). The prevalence of deficiency (<10 ng/mL) was highest among older participants: individuals with lower income and education levels tended to have lower vitamin D levels (p < 0.001). On the other hand widowed individuals had higher mean vitamin D levels than married or single individuals (p < 0.001) (Table 1).

Lifestyle factors negatively affect vitamin D levels. Participants who did not smoke or consume alcohol had lower vitamin D levels than smokers and drinkers (p < 0.001), On the other hand, there were no significant differences in mean height, weight, waist circumference, waist-to- hip ratio, or blood pressure among vitamin D groups (p > 0.05), Participants with obesity (BMI ≥30) had a higher prevalence of deficiency compared to normal-weight individuals (p < 0.001). Similarly, those with chronic diseases had significantly lower vitamin D levels than participants without chronic conditions (p < 0.001). Interestingly, although obesity is generally associated with lower vitamin D, some data suggested slightly higher levels among certain obese subgroups, highlighting possible confounding effects of age and lifestyle (Table 1).

Significant sex-based differences were observed in demographic and clinical variables. Female participants had higher rates of obesity and chronic diseases (e.g., hypertension, DM), whereas males showed higher smoking and alcohol use (p < 0.05) (Table 2).

This study comprehensively assessed the relationship between serum vitamin D levels and a broad range of sociodemographic, behavioral, and clinical factors in a large adult population in Türkiye. Our findings demonstrate that vitamin D deficiency is highly prevalent and confirm that specific subgroups—including women, older adults, and individuals with obesity—are at significantly higher risk (Tables 1, 2, 4).

More than 85% of participants had serum vitamin D levels below 20 ng/mL, consistent with global evidence indicating widespread hypovitaminosis D even in countries with abundant sunlight exposure (Qureshi et al., 2024).

Gender emerged as the strongest independent determinant: women were significantly more likely to have both severe (<10 ng/mL) and moderate (10–20 ng/mL) deficiency of vitamin D than men, possibly due to cultural clothing practices, less outdoor activity, and hormonal influences (Kader et al., 2020). Sex-based differences in lifestyle and clinical characteristics may contribute to the higher prevalence of vitamin D deficiency observed among women. This finding aligns with prior studies demonstrating higher deficiency rates in females attributable to cultural clothing patterns, lower sun exposure, and metabolic differences (Choi et al., 2019). For example: These findings align with earlier studies reporting high prevalence of deficiency across Turkey. Founded a deficiency rate of nearly 75% in adults from the Aegean region, with women at higher risk due to clothing limiting sun exposure (Hekimsoy et al., 2010)

Occupational studies by Aykal et al. (2016) and Skarphedinsdottir et al. (2014) emphasizing that indoor working conditions revealed high deficiency rates among operating room staff and anesthesia care providers in Turkey, Iceland, and the US (Du et al., 2023; Chu et al., 2021; Zhao et al., 2017). Add literature related to gender hormones.

Age showed a nuanced pattern; although mean age did not differ significantly in crude comparisons, regression analysis revealed that younger and middle-aged adults had significantly lower odds of severe deficiency than older adults, in line with literature indicating reduced cutaneous synthesis and increased chronic disease burden in older adult populations (Gallagher, 2013). Obesity was also a significant factor, with obese participants having up to 76% higher odds of deficiency—likely due to vitamin D sequestration in adipose tissue (Pereira-Santos et al., 2015) —although this association weakened but remained significant after adjustment.

Behavioral factors such as smoking and alcohol use initially appeared inversely related to deficiency but lost significance after adjustment, reflecting confounding by lifestyle and socioeconomic patterns. Likewise, lower education level was associated with greater risk in crude analyses but did not remain significant in adjusted models. Chronic diseases were related to deficiency in univariate analysis but not independently, likely due to collinearity with age and obesity.

The health consequences of vitamin D deficiency extend beyond bone metabolism. It may play a role in neurodegenerative disorders: Knekt et al. (2010) demonstrated a significant inverse association between baseline vitamin D levels and Parkinson’s disease risk in a Finnish cohort over nearly three decades. Sato et al. (1997) and Evatt et al. (2008) likewise reported lower vitamin D levels and higher prevalence of deficiency in Parkinson’s patients compared to controls. In our research, it was found that the patient subgroup, who experienced chronic disease, had a lower rates of deficiency, possibly due to smaller sample size and greater sunlight exposure in our region, but the trend remains consistent with international evidence suggesting neuroprotective effects via antioxidative mechanisms and neuronal calcium modulation. Further research with larger cohorts and cerebrospinal fluid analyses is needed to clarify this potential link.

Multiple international studies support these findings. Hovsepian et al. (2011) in Iran reported a median 25(OH)D level of 18–21 ng/mL, with deficiency prevalence exceeding 50%, higher among women and younger adults, and worse during autumn–winter. Brot et al. (2001) in Denmark showed that vitamin supplementation and intentional sun exposure raised serum levels significantly, while Heidari and Mirghassemi (2012) found a strikingly high 70% deficiency rate in Iranian adults, again worse for women. Occupational studies by Aykal et al. (2016) and Skarphedinsdottir et al. (2014) revealed high deficiency rates among operating room staff and anesthesia care providers in Türkiye, Iceland, and the US, emphasizing that indoor working conditions and lifestyle factors are key contributors.

Childhood and adolescent data echo this pattern. Aypak et al. (2013) showed nearly universal deficiency in Turkish children, with stronger associations in females and pubertal adolescents, and identified potential links to insulin resistance. Ethnic disparities have also been reported: Ford et al. (2006) highlighted higher deficiency rates among Asian and Black Afro-Caribbean groups in the UK, especially among women. Sahota et al. (2004) showed moderate seasonal variation and a clear inverse correlation between 25(OH)D and PTH levels, underlining potential effects on calcium homeostasis.

Despite abundant sunlight in Türkiye, factors such as latitude (around 39°N), cultural dress codes, modern indoor lifestyles, and limited supplementation contribute to persistent deficiency. Moreover, there is no universal consensus on optimal serum 25(OH)D thresholds; the WHO defines levels below 10 ng/mL as deficient, while other organizations consider <20 ng/mL insufficient and <10 ng/mL as severe deficiency (Prevention, 2003).

Key strengths of our study include its large representative sample and robust multivariate analysis, adding valuable national data to the global literature. However, limitations include its cross-sectional design, absence of seasonal follow-up, and lack of detailed dietary assessments. Additionally, since data collection occurred only in spring and early summer, the prevalence of vitamin D deficiency may have been underestimated due to seasonal variation. We measured vitamin D levels during summer when they are expected to peak; future research should include winter–spring sampling and more precise tracking of dietary intake, supplementation, and sun exposure habits. In addition, future studies should collect more detailed data on sunlight exposure duration, use of protective clothing, and dietary sources of vitamin D to improve exposure assessment accuracy.

In conclusion, our findings reinforce that vitamin D deficiency is highly prevalent in Türkiye and globally, with higher risk among women, older adults, obese individuals, indoor workers, and populations with cultural or lifestyle-related sun avoidance. Preventive public health strategies should prioritize awareness, routine screening, safe sun exposure, and appropriate supplementation to address this modifiable but significant health burden.

In summary, this large, population-based study confirms that vitamin D deficiency is highly prevalent in the adult population of Tokat, Türkiye, reflecting a broader national and global public health challenge. Our results highlight that women, older adults, and individuals with obesity are particularly vulnerable to severe deficiency, despite living in a region with sufficient sunlight.

Given the growing evidence linking vitamin D status not only to bone and musculoskeletal health but also to neurological, metabolic, and chronic diseases, effective public health measures are urgently needed. Regular screening, education on safe sun exposure, dietary improvements, and targeted supplementation strategies should be prioritized for high-risk groups.

Future research should be prioritized for high-risk groups. Public health authorities should also consider national vitamin D fortification policies, population-level screening programs, and awareness campaigns to address widespread deficiency.

Addressing this modifiable risk factor through coordinated prevention and awareness efforts can significantly contribute to reducing the burden of vitamin D deficiency–related morbidity and improving overall population health.