Although there is no general recommendation for the use of vitamin D supplements in Germany (Bundesinstitut für Risikobewertung 2023), half of the participants in the ninth wave of NCAM-online (51.6%) took vitamin D supplements all year round or intermittently, although only one third reported ever having been advised to do so. Vitamin D supplement use was positively associated with female sex, physical activity, and having been advised to take vitamin D supplements by a health professional, while the associations with age and tanning behavior were inconclusive. There seems to be some confusion in the German population about the potential health benefits of vitamin D supplementation and the groups at risk of vitamin D insufficiency.

Our results regarding vitamin D supplementation are difficult to compare with a previous population-based study from Germany, as we asked about supplement intake during the previous 12 months. The study by Rabenberg et al. (2015) gathered vitamin D supplementation in the past seven days and revealed a prevalence of 5.9% in women and 1.0% in men. Looking at other studies focusing on self-reported vitamin D supplement intake from other western industrialized countries shows that our findings are within the range of reported results, although direct comparisons are not possible due to methodological differences. For the UK, O’Connor and Tanna reported that 43.5% (O'Connor et al. 2018) and 42.2% (Tanna et al. 2023) of their convenience samples of predominantly female participants took vitamin D supplements over the previous 12 months. In the USA, 34.9% to 51.7% of people aged 20 years and over reported taking vitamin D supplements in the past 30 days (Orces 2019). In Scotland, 64.3% of adults surveyed took vitamin D supplements during winter, of which 37.5% took them daily during the winter months (Zaremba and Conduit-Turner 2024).

The determinants of vitamin D supplement intake identified in our study align with the findings of previous research. Tanna et al. found higher odds of vitamin D supplementation among women and older adults (Tanna et al. 2023). In the USA, the supplement intake was also higher in older participants (Orces 2019). However, it is not clear why young adults in our study (18–25 years) were less likely to take supplements all year round and more likely to report intermittent supplementation. It may depend on the individual’s motivation for taking supplements. As we did not focus on this in our study, future studies should explore this aspect in more detail.

The same is true for associations with tanning behavior. In our study, year-round vitamin D supplementation was positively associated with tanning bed use and intentional outdoor tanning. However, intermittent supplementation was negatively associated with outdoor tanning. Tanna et al. also found less frequent use of vitamin D supplements among those with high sun exposure (Tanna et al. 2023). Although vitamin D is colloquially referred to as the “sun vitamin” and vitamin D production has been shown to be a reason for outdoor tanning (Bergmann et al. 2025), the mechanisms explaining the association between tanning behavior and vitamin D supplementation should be further explored in future research.

While there are some data on population knowledge of vitamin D in European countries, our study is, to our knowledge, the first to assess this aspect in a large population-based sample in Germany. Previously, Sandmann et al. (2015) reported from their survey of 1051 people in Germany in 2010 that 45% self-assessed their vitamin D-related knowledge as poor, very poor, or extremely poor, 41% reported no knowledge of vitamin D sources for the human body, and 56% had no knowledge of health problems associated with vitamin D (Sandmann et al. 2015). As details of knowledge were not provided in this study, our study adds to the current body of research.

With regard to the health benefits of vitamin D, the most frequently items agreed upon (i.e., “strengthening the immune system” with 71.0%, “improving mood” with 48.8%, and “improving skin health” as a distractor item with 44.8%) were not related to the well-known “classical” functions of vitamin D for musculoskeletal health. Some older studies provide different results. In France and the UK, the functions of vitamin D for healthy bones, prevention of osteoporosis, and prevention of rickets were most frequently mentioned, above all other functions (Deschasaux et al. 2016; O'Connor et al. 2018). However, in a study conducted more recently, in 2021 (Tanna et al. 2023), supportive function of vitamin D for the immune system was named most frequently—similar to our results. This shift in the public awareness of vitamin D may reflect the current state of scientific debate on health benefits of vitamin D.

Notably, in our study, the functions of vitamin D related to the prevention of various diseases were more frequently agreed by those who took vitamin D supplements all year round. Although we did not ask respondents why they use supplements, it is reasonable to assume that they do so for these preventive reasons as well. Our findings show that the people who take vitamin D supplements all year round are not better informed about the associated health benefits because they were also more likely to agree with the distractor items (i.e., improvement of skin health and vision). There are currently many health claims about vitamin D in the media, but these do not distinguish between those effects for which there is scientific consensus (e.g., bone health) and those for which there is no or inconclusive evidence for supplementation in healthy individuals (e.g., cancer prevention). Our results emphasize the need for better information for the German population.

Although overdosing with vitamin D is quite rare, it cannot be ruled out due to the increasing public interest in vitamin D, and sometimes quite harmful and not serious recommendations in the media (e.g., 53,000 International Units per day for a 70 kg Person; https://www.rohe-energie.com/downloads/) (Arzneimittelkommission der deutschen Ärzteschaft 2017; Galior et al. 2018; Simon 2022). There are contraindications to vitamin D supplementation (e.g., hypercalcaemia, cardiac arrhythmias, kidney stones), which is why it is recommended in Germany to have a blood test and seek medical advice before starting supplementation (Bundesinstitut für Risikobewertung 2023).

There is also a need to improve the public knowledge about groups at risk of vitamin D deficiency and sources of vitamin D. In our study, population groups that are clearly at risk (i.e., infants and people with darker skin) were least likely to be identified as such. Regarding sources of vitamin D, while sunlight, supplement intake, and nutrition were—correctly—the most frequently named sources, about a third (31.6%) agreed that physical activity could improve the vitamin D levels.

On the basis of the results of our study, the following action points can be derived for German public health: Public health messages or advice from healthcare providers should target the true risk groups (e.g., care-dependent elderly people and those with limited daylight exposure) and encourage them to test their blood serum levels. Furthermore, it should be emphasized that supplements are only reasonable in the case of a vitamin D deficiency and should only be used long-term under medical supervision. This information should also be included on the packaging of freely available supplements to reduce the risk of sporadic overdosing.