How deficient are we? A retrospective analysis of vitamin D status in 85,892 Turkish adults

Abstract

Objectives

Vitamin D deficiency remains a major global public health concern. This study aimed to assess vitamin D levels measured in individuals aged 18 years and older between 2016 and 2025 in a tertiary care hospital and to examine the association of these levels with seasonal, annual, and demographic variables.

Patients and methods

This retrospective observational study included patients who presented to Duzce University Hospital for any reason between 2016 and 2025 and had serum 25(OH)D levels measured. In total, data from 85,892 individuals were analyzed. Vitamin D status was categorized as sufficient (> 30 ng/mL), insufficient (20–30 ng/mL), or deficient (< 20 ng/mL).

Results

Among the participants, 70.2% (n = 60,309) were women and 29.8% (n = 25,583) were men. The mean age was 47.16 ± 17.62 years. The overall mean serum 25(OH)D level was 17.90 ± 12.08 ng/mL. Vitamin D deficiency (< 20 ng/mL) was identified in 68.6% of women and 61.3% of men. Overall, 66.4% (n = 57,051) of participants had Vitamin D deficiency (< 20 ng/mL), 21.7% (n = 18,635) had Vitamin D insufficiency (20–30 ng/mL), and 11.9% (n = 10,206) had sufficient Vitamin D levels (> 30 ng/mL). The mean 25(OH)D level was 19.16 ± 10.74 ng/mL in men and 17.37 ± 12.56 ng/mL in women, with significantly higher levels observed in men (p < 0.001). Participants aged 65 years and older had significantly higher vitamin D levels compared with those under 65 years (p < 0.001). Seasonally, the highest mean vitamin D levels were recorded in summer, whereas the lowest levels were found in winter.

Conclusion

Vitamin D deficiency continues to pose a substantial public health challenge in Turkey. Addressing this issue should be considered a priority, and further comprehensive studies are urgently needed to support the development of effective strategies aimed at reducing deficiency rates.

Introduction

Vitamin D, also known as the “sunshine vitamin,” is a steroid hormone that plays a key role in bone and mineral metabolism [1–3]. In addition, it has received considerable attention for its extra-skeletal pleiotropic effects, including potential roles in cardiovascular disease, metabolic syndrome, stroke, chronic lower respiratory diseases, autoimmune disorders, dementia, and cancer prevention [1, 2, 4]. Importantly, vitamin D modulates both innate and adaptive immune responses, playing a critical role in maintaining immune system homeostasis and contributing to the prevention of infections [2, 4].

Vitamin D deficiency remains a significant global public health concern and affects men and women across all age groups [4]. It is generally defined as a serum 25-hydroxyvitamin D [25(OH)D] concentration below 20 ng/mL [2]. In modern societies, the primary causes of vitamin D deficiency include insufficient sunlight exposure and limited dietary intake from natural food sources. Additional risk factors include dark skin pigmentation, pregnancy, intestinal malabsorption disorders, obesity, and advanced age [4].

Research on vitamin D levels has become an important indicator in public health assessments. In Turkey, several studies have investigated vitamin D status in specific populations [5–11]. However, many of these studies have been limited by relatively small sample sizes. Therefore, the aim of the present study is to evaluate vitamin D levels measured in individuals aged 18 years and older at a tertiary care hospital between 2016 and 2025, and to examine the association of these levels with seasonal, annual, and demographic variables.

Methods

This retrospective observational study included patients who presented to Duzce University Hospital for any reason between 2016 and 2025 and had their serum 25(OH)D levels measured. A total of 101,778 records were initially obtained through a retrospective review of the hospital automation system. To avoid duplication, repeated measurements obtained within the same year were excluded, and only the first measurement for each patient was included. Individuals with 25(OH)D levels above 100 ng/mL were excluded from the study. After these adjustments, a total of 85,892 records were included in the final analysis.

Serum 25(OH)D levels were measured using the chemiluminescence method on the Roche Cobas 6000 autoanalyzer. Based on 25(OH)D concentrations, vitamin D status was classified as sufficient (> 30 ng/mL), insufficient (20–30 ng/mL), or deficient (< 20 ng/mL).

Ethics approval and consent to participate

The study was conducted in accordance with the Helsinki declaration. Ethical approval for this study was obtained from the Duzce University Non-Interventional Health Research Ethics Committee (Date: 10 November 2025, Decision No: 2025/281). Informed consent was waived due to the retrospective nature of the study and the use of anonymized data, in accordance with national regulations and the ethics committee approval.

Statistical analysis

The distribution of numerical variables was assessed using histograms, Q–Q plots, and the Kolmogorov–Smirnov test. Given the large sample size, the Kolmogorov–Smirnov test was preferred over the Shapiro–Wilk test. The results indicated that the data did not follow a normal distribution; therefore, non-parametric statistical methods were applied. Differences between multiple groups were evaluated using the Kruskal–Wallis test. When statistically significant differences were detected, post-hoc pairwise comparisons were conducted using the Mann–Whitney U test with Bonferroni correction to account for multiple comparisons. All statistical analyses were performed using IBM SPSS Statistics for Windows, version 23.0 (IBM Corp., Armonk, NY, USA). A p-value < 0.05 was considered statistically significant.

Results

Of the participants, 70.2% (n = 60,309) were women and 29.8% (n = 25,583) were men. The mean age of the participants was 47.16 ± 17.62 years. The overall mean serum 25(OH)D level was 17.90 ± 12.08 ng/mL (Table 1). Vitamin D deficiency (< 20 ng/mL) was identified in 68.6% of women and 61.3% of men. The difference in vitamin D levels between sexes was statistically significant (p < 0.001; effect size, Cramer’s V = 0.085) (Table 1).

Table 1.

Participants’ age, vitamin D levels, and vitamin D sufficiency by gender

Age (years) (n = 85,892)		Mean ± SD	Median
		47.16 ± 17.62	47.0
25(OH)D level (µg/L) (n = 85,892)		17.90 ± 12.08	15.30
Gender	Vitamin D deficiency (< 20)	Vitamin D insufficiency (20–30)		Normal (> 30)	p
	n (%)	n (%)		n (%)
Female	41,370 (68.6)	11,728 (19.4)		7,211 (12.0)	< 0.001 a
Male	15,681 (61.3)	6,907 (27.0)		2,995 (11.7)

^aChi-Square Test, Statistically significant results are highlighted in bold (p < 0.05)

Among all participants, 66.4% (n = 57,051) had Vitamin D deficiency (< 20 ng/mL), 21.7% (n = 18,635) had Vitamin D insufficiency (20–30 ng/mL), and 11.9% (n = 10,206) had sufficient Vitamin D levels (> 30 ng/mL).

In the multinomial logistic regression analysis conducted to examine the factors influencing vitamin D sufficiency, sex was found to have a statistically significant effect on vitamin D levels (p < 0.001). Women had a significantly lower likelihood of being in the “Vitamin D insufficiency (20–30 ng/mL)” category compared with the “Vitamin D deficiency (< 20 ng/mL)” category relative to men (B = − 0.441, p < 0.001, OR = 0.64, 95% CI = 0.62–0.67). Similarly, women also had a significantly lower likelihood of being in the “Sufficient (> 30 ng/mL)” category compared with the “Vitamin D deficiency” category relative to men (B = − 0.091, p < 0.001, OR = 0.91, 95% CI = 0.87–0.96) (Table 2). These findings indicate that men exhibited higher levels of vitamin D sufficiency compared with women.

Table 2.

Multinomial logistic regression analysis of predictors of vitamin D adequacy by sex

Vitamin D Status (Ref = Vitamin D deficiency < 20)	Category (Ref)	B	SE	Exp (B) (OR)	95% CI for Exp(B)	p
Vitamin D insufficiency (20–30)	Female (Ref) → Male	–0.441	0.018	0.644	0.622–0.667	< 0.001
Normal (> 30)	Female (Ref) → Male	–0.091	0.024	0.913	0.871–0.956	< 0.001

Abbreviations: B Regression coefficient, SE Standard error, OR Odds ratio, CI Confidence interval, Statistically significant results are highlighted in bold (p < 0.05)

The mean 25(OH)D level was 19.16 ± 10.74 ng/mL in men and 17.37 ± 12.56 ng/mL in women. Vitamin D levels were significantly higher in men than in women (p < 0.001, r = 0.13). Additionally, participants aged 65 years and older had significantly higher vitamin D levels compared with those under 65 years of age (p < 0.001, r = 0.027) (Table 3).

Table 3.

Group comparisons of serum 25(OH) vitamin D levels by gender and age

25(OH) Vitamin D Levels
		n	Mean ± SD (ng/mL)	Median (ng/mL)	Mean Rank	p	r
Gender	Female	60,309	17.37 ± 12.56	14.20	40,824	< 0.001 b	0.13
	Male	25,583	19.16 ± 10.74	17.39	47,948
Age Categories	< 65	69,750	17.70 ± 11.88	15.14	42,619	< 0.001 b	0.027
	> 65	16,142	18.77 ± 12.87	16.05	44,361

^bMann–Whitney U test, Statistically significant results are highlighted in bold (p < 0.05)

A statistically significant association was found between vitamin D levels and the variables of age, season, and year (p < 0.001) (Table 4).

Table 4.

Distribution of vitamin D Deficiency, Insufficiency, and normal levels according to Age, Year, and season

		Vitamin D deficiency (< 20)		Vitamin D insufficiency (20–30)		Normal (> 30)		p
		n	%	n	%	n	%
Age Groups	18–29	12,570	72.00	3,291	18.85	1,598	9.15	< 0.001 a
	30–39	9,624	68.39	2,966	21.08	1,483	10.54
	40–49	10,628	66.87	3,496	22.00	1,770	11.14
	50–59	9,766	63.89	3,525	23.06	1,994	13.05
	60–69	8,051	61.98	3,132	24.11	1,807	13.91
	70–79	4,578	62.07	1,673	22.68	1,125	15.25
	80–89	1,649	64.77	516	20.27	381	14.96
	90+	185	68.77	36	13.38	48	17.84
Age Categories	< 65	46,960	67.33	14,966	21.46	7,824	11.22	< 0.001 a
	> 65	10,091	62.51	3,699	22.73	2,382	14.76
Periodization of Data	2016–2018	15,124	58.31	6,355	24.50	4,457	17.18	< 0.001 a
	2019–2021	11,959	77.09	2,508	16.17	1,046	6.74
	2022–2025	29,968	67.43	9,772	21.99	4,703	10.58
Season	Spring	16,668	71.87	4,066	17.53	2,457	10.59	< 0.001 a
	Summer	11,440	57.57	5,421	27.28	3,011	15.15
	Autumn	11,942	59.51	5,419	27.00	2,706	13.48
	Winter	17,001	74.69	3,729	16.38	2,032	8.93

^aChi-Square Test, Statistically significant results are highlighted in bold (p < 0.05)

The distribution of vitamin D levels by season and age group is presented in Table 5. Vitamin D levels differed significantly across seasons (p < 0.001). The highest mean vitamin D level was observed in summer, whereas the lowest level was recorded in winter.

Table 5.

Seasonal and age-related variations in vitamin D levels

		n	Mean ± SD (ng/mL)	Median (ng/mL)	Mean Rank	p (between groups)	p (intragroups)
Season	Spring (1)	23,191	16.70 ± 11.89	13.53	39,499	p < 0.001 a	1 > 4, p < 0.001 b
	Summer (2)	19,872	19.87 ± 11.91	18.03	48,472		2 > 1, 2 > 4, p < 0.001 b; 2 > 3 p = 0.002 b
	Autumn (3)	20,067	19.76 ± 12.58	17.52	47,911		3 > 1, 3 > 4 p < 0,001 b
	Winter (4)	22,762	15.77 ± 11.42	12.58	37,257
Age Groups	18–29 (2)	17,459	16.44 ± 11.22	13.83	39,593	p < 0.001 a	2 < 3, 2 < 4, 2 < 5, 2 < 6, 2 < 7, 2 < 8, 2 < 9 p < 0.001 b
	30–39 (3)	14,073	17.43 ± 11.77	14.85	41,936		3 > 9 p < 0.001 b
	40–49 (4)	15,894	17.78 ± 11.90	15.39	42,859		4 > 3 p = 0.001 b,4 > 9 p < 0.001 b
	50–59 (5)	15,285	18.70 ± 12.23	16.15	45,062		5 > 3, 5 > 4, 5 > 8, 5 > 9 p < 0.001 b
	60–69 (6)	12,990	18.95 ± 12.53	16.50	45,451		6 > 3, 6 > 4, 6 > 8, 6 > 9 p < 0.001 b
	70–79 (7)	7,376	19.05 ± 13.86	16.30	44,982		7 > 4, 7 > 8, 7 > 9 p < 0.001 b
	80–89 (8)	2,546	17.97 ± 13.37	14.69	41,399		8 > 9 p = 0.003 b
	90+(9)	269	16.81 ± 14.47	11.40	36,188
Periodization of Data	2016–2018 (1)	25,936	20.57 ± 14.15	17.40	48,051	p < 0.001 a	1 > 2, 1 > 3 p < 0.001 b
	2019–2021 (2)	15,513	15.13 ± 10.56	12.61	36,187
	2022–2025 (3)	44,443	17.31 ± 10.91	15.22	42,326		3 > 2 p < 0.001 b

^aKruskal–Wallis testi, ^bMann–Whitney U test, Statistically significant results are highlighted in bold (p < 0.05)

A significant difference was also observed across age groups (p < 0.001). Post-hoc analyses revealed that individuals in the 18–29 age group had significantly lower vitamin D levels compared with all other age groups (p < 0.001).

The proportion of individuals with vitamin D levels below 20 ng/mL varied across the years. Rates of Vitamin D deficiency were 37.4% in 2016, 61.5% in 2017, 62.6% in 2018, 76.5% in 2019, 79.5% in 2020, 68.9% in 2022, 74.4% in 2023, 61.5% in 2024, and 66.4% in 2025.

The prevalence of Vitamin D insufficiency (20–30 ng/mL) ranged between 13.4% and 27.0%, whereas the proportion of individuals with sufficient Vitamin D levels (> 30 ng/mL) ranged between 7.0% and 35.6%. The annual distribution of participants’ vitamin D levels is illustrated in Fig. 1.

Fig. 1.

Distribution of vitamin D status between 2016–2025

Discussion

This study examined serum 25(OH)D levels in a defined adult population. The overall mean 25(OH)D level among all participants was 17.90 ± 12.08 ng/mL, which is consistent with previous studies conducted in Turkey [5–10].

Vitamin D deficiency was identified in 66.4% of participants, while insufficiency was observed in 21.7%, aligning with earlier research from Turkey [5–10]. These results indicate that vitamin D deficiency is highly prevalent and affects a substantial portion of the population. Factors such as clothing habits, the absence of vitamin D fortification in foods, and dietary patterns may contribute to the high frequency of deficiency in Turkey. However, the reported prevalence of vitamin D deficiency varies across the literature. A global meta-analysis found that 47.9% of individuals worldwide had vitamin D deficiency (< 50 nmol/L) [12]. In Europe, the prevalence of vitamin D levels below 20 ng/mL was reported as 40.4% [13]. Among Danish adults, the prevalence of deficiency and insufficiency was found to be 13.8% and 52.2%, respectively [14]. In Australia, 22.7% of the population was reported to have vitamin D deficiency [15]. In the United States, data from 2001 to 2010 indicated prevalence rates of 28.9% for deficiency and 41.4% for insufficiency [16]. In Pakistan, one study reported low serum 25(OH)D levels in 90% of participants, with deficiency identified in 69.9% and insufficiency in 21.1% [17].

In the literature, 25(OH)D levels have been reported to be significantly higher in men than in women [5, 6, 12, 15, 18, 19]. The findings of our study are consistent with these results. This difference may be attributed to factors such as clothing habits, reduced sunlight exposure, and differing dietary patterns among women.

In our study, 25(OH)D levels were higher during the summer and autumn months, whereas lower levels were observed in winter and spring. This seasonal pattern is expected, as increased sunlight exposure in summer enhances vitamin D synthesis, while this effect diminishes toward winter. This finding is also consistent with results reported in previous studies [9, 12].

Changes in vitamin D levels over the years may serve as an indicator and predictor of overall public health status [20]. In our study, vitamin D deficiency was most prevalent in 2020, which may be attributed to reduced sunlight exposure during lockdowns and stay-at-home measures implemented in the COVID-19 pandemic. Additionally, when comparing the periods 2016–2019 and 2022–2025, a decrease was observed in the proportion of individuals with sufficient vitamin D levels. In the literature, some studies have reported decreasing vitamin D levels over the years [12, 21, 22], whereas others have indicated an increase [23, 24] or no significant change [25].

A meta-analysis reported the prevalence of vitamin D deficiency in older adults as 59.7% and insufficiency as 27.5% [26]. In our study, among individuals aged 65 years and older, vitamin D deficiency was found to be 62.51% and insufficiency 22.73%. Furthermore, vitamin D levels in individuals aged 65 years and older were significantly higher compared with those under 65 years of age. Our findings also showed that vitamin D deficiency was most common in the 18–29 age group, whereas sufficient vitamin D levels were most frequently observed in the 70–79 age group. Similarly, in the study by Kaytaz et al., the highest vitamin D levels were observed in individuals aged 70 years and older [11]. Although older age is generally considered a risk factor for vitamin D deficiency, increased osteoporosis risk and the presence of comorbid conditions may lead to more frequent healthcare visits and, consequently, more frequent recommendations for vitamin D supplementation. Indeed, one study reported that the average daily intake of vitamin D was significantly higher in older adults compared with younger adults [27]. The literature includes studies reporting lower vitamin D levels in younger individuals compared with older adults [7, 9, 27, 28], as well as studies indicating higher levels in younger populations [29, 30]. Additionally, a meta-analysis found no age-related differences in Europe or North America. In contrast, children and adolescents in the Asia/Pacific region had significantly lower 25(OH)D levels compared with adults and older adults, whereas in the Middle East/Africa region, children and adolescents had significantly higher levels than the other two age groups [31].

Vitamin D deficiency is a major public health concern worldwide [4, 18]. Its high prevalence contributes substantially to the global burden of disease [12]. The consumption of vitamin D–fortified foods and the use of vitamin D supplements are among the most important determinants for improving vitamin D status [20]. In this context, fortifying foods with vitamin D, increasing the intake of oily fish, and encouraging regular supplementation may help prevent vitamin D insufficiency at the population level [20]. Furthermore, governments, policymakers, healthcare professionals, and individuals should recognize the high prevalence of vitamin D deficiency and prioritize its prevention as a public health imperative [12].

Limitations

The primary limitations of our study include the inclusion of individuals with chronic illnesses in the study population and the absence of laboratory data for the year 2021. Moreover, the lack of evaluation of vitamin D or multivitamin supplementation represents an additional limitation. Finally, the lack of standardization in laboratory assays used to measure 25(OH)D may have posed a limitation when comparing our findings with those of other studies.

Conclusion

Vitamin D deficiency remains a significant public health concern in Turkey. Increasing sunlight exposure, promoting the consumption of vitamin D–fortified foods, and encouraging the use of supplements are essential for improving population health. To effectively address vitamin D deficiency, comprehensive national efforts and large-scale public health initiatives should be implemented as soon as possible.

Abbreviations

25(OH)D

25-hydroxyvitamin D

B

Regression coefficient

CI

Confidence interval

ng/mL

Nanogram per millilitre

OR

Odds ratio

p

p-value

r

Correlation coefficient

SD

Standard deviation

SPSS

Statistical Package for the Social Sciences

Authors’ contributions

The author performed all aspects of the study and approved the final manuscript.

Funding

No funding was received for this study.

Data availability

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

The study was conducted in accordance with the Helsinki declaration. Ethical approval for this study was obtained from the Duzce University Non-Interventional Health Research Ethics Committee (Date: 10 November 2025, Decision No: 2025/281). Informed consent was waived due to the retrospective nature of the study and the use of anonymized data, in accordance with national regulations and the ethics committee approval.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.