Vitamin D deficiency is a widespread problem across the globe, affecting both developing and industrialized countries. Vitamin D deficiency is often defined by low concentrations of serum 25-hydroxyvitamin D (25(OH)D) and is closely associated with negative effects on bone health, such as rickets and osteomalacia. Serum 25(OH)D is the most reliable marker for vitamin D status. How it works in detail can be read in the article "Determination and Evaluation of Vitamin D Status: Basics and Reference Values." Serum 25(OH)D levels below 30 nmol/L should be avoided through preventive measures, especially in at-risk groups.

It is challenging to obtain sufficient vitamin D from natural food sources alone to achieve a healthy vitamin D status. Some high-risk groups, however, require additional preventive measures.

The risk groups include:

• Breastfed infants usually do not receive enough vitamin D through breast milk, as its content depends on the mother's vitamin D intake. The American Academy of Pediatrics (AAP) recommends a daily supplement of 400 IU of vitamin D for infants.

• Older adults are at higher risk of vitamin D deficiency because their skin produces less vitamin D as they age, and they often spend less time outdoors.

• People with limited sun exposure, such as those wearing religious clothing or those in certain occupations, are also at higher risk for vitamin D deficiency.

• Individuals with darker skin have a reduced ability to synthesize vitamin D.

• People with fat absorption disorders, such as celiac disease or Crohn's disease, often have difficulty absorbing enough vitamin D.

• Individuals with obesity or those who have undergone gastric bypass surgery require higher vitamin D intake, either due to more vitamin D being stored in body fat or reduced absorption in the small intestine.

Regarding specific professions, Sowah et al. (2017) evaluated vitamin D levels in various occupational groups. The study aimed to investigate serum vitamin D levels and the frequency of vitamin D deficiency across different professions to identify workers at increased risk for vitamin D deficiency. The study results showed that workers primarily indoors had lower 25-hydroxyvitamin D (25(OH)D) levels than those who worked outdoors (40.6 ± 13.3 nmol/L vs. 66.7 ± 16.7 nmol/L; p < 0.0001). Shift workers, lead/smelter workers, and miners exhibited particularly low vitamin D levels, with shift workers showing a particularly high rate of vitamin D deficiency (80%). Additionally, healthcare personnel, particularly medical students and residents, had comparatively low vitamin D levels (44.0 ± 8.3 nmol/L and 45.2 ± 5.5 nmol/L, respectively), while practicing physicians had higher levels (55.0 ± 5.8 nmol/L). All occupational groups had high rates of vitamin D deficiency. The study emphasized that shift workers, healthcare workers, and people who work indoors are at high risk for vitamin D deficiency, likely due to limited sun exposure. This highlights the need for targeted health promotion measures for these groups.

Chen et al. (2021) found vitamin D deficiency in pregnant women. The study examined the vitamin D content and influencing factors in 3,080 pregnant women, as well as the effects of vitamin D deficiency on common adverse pregnancy outcomes. The prevalence of vitamin D deficiency was 83.28%, and only 1.36% of the women had sufficient vitamin D. Factors positively influencing vitamin D levels included a gestational age of 28 to 32 weeks, the summer and fall seasons, higher education (at least high school diploma), and more than 10 hours per week of outdoor time. There was a strong correlation between vitamin D levels and temperature; higher temperatures led to higher vitamin D levels. Low vitamin D levels were associated with an increased risk of miscarriage and small-for-gestational-age (SGA) newborns. Miscarriage and SGA rates were significantly higher in the vitamin D deficiency group. The vitamin D levels of pregnant women were low and influenced by several factors. Low vitamin D levels increased the risk of spontaneous abortion and SGA. Therefore, measures should be taken to improve the vitamin D status of pregnant women.

The pronounced prevalence of vitamin D deficiency is not only found in pregnant women but is also reflected worldwide. To obtain reliable and comparable estimates of vitamin D supply, current, representative population studies should be used, based on standardized measurements of serum 25(OH)D concentrations.

A comprehensive analysis of 308 studies from 81 countries and six WHO regions, with a total of 7.9 million participants, shows significant variation in the number of participants in the studies examined. The smallest study included 52 participants, while the largest study included 1,316,390 participants. Of the 308 studies, 93 came from Europe, with a total of 7,238,477 participants. In the Americas, 51 studies were conducted with 206,470 participants, in Africa 14 studies with 7,088 participants, in the Eastern Mediterranean 64 studies with 85,770 participants, in Southeast Asia 29 studies with 28,780 participants, and in the Western Pacific region 57 studies with 380,774 participants.

The geographic distribution of the studies was also broad. Most studies were conducted in countries between 20° and 60° northern latitude. In terms of income distribution, 169 studies were conducted in high-income countries, 70 in upper-middle-income countries, 64 in lower-middle-income countries, and only 5 in low-income countries

The prevalence of vitamin D deficiency, defined as serum 25(OH)D below 30 nmol/L, varies by WHO region. In the Americas region, the prevalence is 5.5% (95% CI 3.5–7.8), while in the Eastern Mediterranean region it is 35.2% (95% CI 29.6–41.0). Globally, the prevalence is 15.7% (95% CI 17.7–17.8), with a decrease from 17.6% in 2000–2010 to 14.1% in 2011–2022.

There is also a clear correlation between vitamin D deficiency and income groups. In lower-middle-income countries, the prevalence of serum 25(OH)D below 30 nmol/L is 26.7% (95% CI 19.2–34.5). Middle-income countries show a prevalence of 10.2% (95% CI 6.8–14.0), while high-income countries show a prevalence of 15.1% (95% CI 11.2–19.1).

Age and gender also play a role. Younger adults aged 18 to 44 have a prevalence of 48.5% for a vitamin D status below 50 nmol/L. Women are more affected than men, with a prevalence of 17.8% (95% CI 13.9–21.9) compared to 13.6% (95% CI 10.6–16.6) in men. Seasonality also influences vitamin D status, with values being lower in winter and spring than in summer and fall.

In the USA, the prevalence of vitamin D deficiency is 5.9%, in Canada 7.4%, and in Europe 13%, while vitamin D insufficiency (serum 25(OH)D below 50 nmol/L) is much higher: 24% in the USA, 36.8% in Canada, and 40.4% in Europe. Dark-skinned populations are disproportionately affected by low vitamin D status. In the USA, the prevalence among non-Hispanic Blacks is 24%, while it is only 2.3% among non-Hispanic Whites.

The risk of vitamin D deficiency is particularly high in lower-income countries, such as India, where an estimated 490 million people are affected. Certain patient groups, particularly those with chronic diseases such as kidney failure, have a particularly high prevalence of up to 99%. Targeted measures are needed to address this global health issue by improving vitamin D supply and minimizing the consequences of deficiency.

Vitamin D deficiency is a globally prevalent problem that affects both developing and industrialized countries, with severe health consequences, especially concerning rickets and osteomalacia. The key factors leading to deficiency are insufficient UVB radiation and inadequate dietary intake. Certain high-risk groups, such as breastfed infants and older adults, are particularly vulnerable.

Studies like those by Sowah et al. (2017) and Chen et al. (2021) highlight the significant impact of occupational exposure, lifestyle factors, and pregnancy on vitamin D status. The broad geographic analysis shows that particularly people in low-income countries and certain occupational groups have a high prevalence of vitamin D deficiency. Alarmingly, nearly 500 million people are affected in many parts of the world, such as India. The differences by age, gender, and ethnicity underscore the need for targeted preventive measures.

In summary, the global prevalence of vitamin D deficiency highlights the urgency of health interventions to improve vitamin D supply in various at-risk groups. International and national initiatives, such as promoting food fortification, could help combat vitamin D deficiency and mitigate its health consequences worldwide.

Sources: 

Amrein, K., Scherkl, M., Hoffmann, M., Neuwersch-Sommeregger, S., Köstenberger, M., Tmava Berisha, A., Martucci, G., Pilz, S., & Malle, O. (2020). Vitamin D deficiency 2.0: An update on the current status worldwide. European Journal of Clinical Nutrition, 74(11), 1498–1513. Source

Cashman, K. (2020). Vitamin D Deficiency: Defining, Prevalence, Causes, and Strategies of Addressing. Calcified Tissue International, 106. Source

Chen, B., Chen, Y., & Xu, Y. (2021). Vitamin D deficiency in pregnant women: Influenced by multiple risk factors and increase the risks of spontaneous abortion and small-for-gestational age. Medicine, 100(41), e27505. Source

Cui, A., Zhang, T., Xiao, P., Fan, Z., Wang, H., & Zhuang, Y. (2023). Global and regional prevalence of vitamin D deficiency in population-based studies from 2000 to 2022: A pooled analysis of 7.9 million participants. Frontiers in Nutrition, 10. Source

National Institute of Health. (2024). Office of Dietary Supplements—Vitamin D. Vitamin D Fact Sheet for Health Professionals. Source

Sowah, D., Fan, X., Dennett, L., Hagtvedt, R., & Straube, S. (2017). Vitamin D levels and deficiency with different occupations: A systematic review. BMC Public Health, 17(1), 519. Source