Vitamin D (calciferol), the popular sun vitamin

from Lisa Ressi, MSc
on 27.02.2024
Obtaining vitamin D from the sun Obtaining vitamin D from the sun

Life is not possible without sunlight – this applies to animals, plants and also to us humans. We need sunlight to produce the all-important vitamin D, which is why we like to call it the sun vitamin. Find out about the sun vitamin and how it is produced in our bodies here.

What is vitamin D?

Vitamin D is the overarching term for a group of fat-soluble vitamins called calciferols. Vitamin D has a special position among vitamins, because its main requirement is not covered by the diet as in most cases, but it is produced by our body on its own. All it needs is the cholesterol precursor “7-dehydro-cholesterol” and strong enough sun exposure on the skin. Due to its far-reaching influence within our organism, many scientists no longer regard vitamin D as a vitamin, but instead classify it as a prohormone.

Effect: What is vitamin D good for – the far-reaching influence of the sun

Vitamin D is a power substance that plays a key role in the human body, because vitamin D takes on countless tasks in the body. It ensures that the intestines can absorb calcium and phosphorus from food. It is also involved in the maintenance of muscles, bones and teeth and supports the function of the immune system.

An adequate and stable vitamin D balance has a major impact on human well-being.

It is now well known that almost all the organs and tissues of the body contain vitamin D receptors to which the bioactive form of vitamin D (calcitriol or 1,25-dihydroxycholecalciferol) binds before being used for different processes in the body.

What role does vitamin D play in our organism?

Vitamin D has been undervalued for a long time. For many decades, it was thought that the vitamin, which is actually a hormone, was only relevant for our bones. However, as recent research shows, vitamin D interacts with almost all the cells of the human body. It is therefore hardly surprising that a hormone deficiency has far-reaching consequences and can be associated with cardiovascular symptoms, high blood pressure, diabetes, depression and an unhealthy aging process. 

The most well-known tasks of vitamin D:

  • Vitamin D helps maintain bones and teeth as well as normal muscle function
  • Vitamin D plays a role in the absorption of calcium and phosphorus from food
  • Vitamin D supports the function of the immune system

Vitamin D & the immune system

Vitamin D plays a key role in our immune system. The sun vitamin is not only involved in the production of immune cells and immune-controlling proteins, it also affects various immune cells (e.g. macrophages, B and T lymphocytes) and is involved in the regulation of inflammatory processes. On the one hand a weak immune system gains strength from the sun vitamins, whilst on the other hand overreactions such as allergies or autoimmune diseases can be alleviated by sufficient vitamin D.

Good to know in winter: Less vitamin D, more infections

For a long time, it was a mystery for experts to find out why colds were particularly prevalent in the cold months. It is now known that the epidemic-like vitamin D deficiency is a driving force behind colds in the population.

Because vitamin D is in short supply in our latitudes in winter. Finally, due to the low angle of the sun, no significant vitamin D formation is possible from mid-October to mid-March. Accordingly, 91% of participants in the BIOGENA Good Health Study were not optimally supplied with vitamin D during the spring blood test, 48% even demonstrated a severe vitamin D deficiency. An insufficient vitamin D status significantly increases the susceptibility of the upper respiratory tract to infections in autumn and winter and, according to an Austrian study, the frequency of illness in employees also correlates with poor or sub-optimal vitamin D supply.

In order to survive the cold and flu season well, a high-quality vitamin D preparation should therefore be part of a daily regimen once the clocks go back at the latest.


Wäger et al. 2022. BIOGENA Good Health Study: Vitamin D. unpublished
Sinnißbichler T., Viebahn I. 2011.
Vitamin D gesucht - Defizite gefunden [Vitamin D sought - deficits found]. 25-(OH)-Vitamin D Status, Supplementierung und Krankheitstage. unpublished

Vitamin D & fatigue

Fatigue can have many causes. At the end of the day, a vitamin D deficiency may very well be the cause. If you want to combat fatigue caused by a deficiency, you should increase your vitamin D supplies, according to a randomised placebo-controlled study. 120 healthy people of different ages who had vitamin D deficiency and complained of fatigue took part. The participants who – without knowing this – received vitamin D reported a decrease in their fatigue at the end of the dosing period.

Vitamin D & bones

It has long been known that vitamin D is essential for healthy bones. Vitamin D is not only necessary for the absorption of calcium from the intestine into the body, it also regulates the storage of calcium in the bones. It is no wonder that a good year-round vitamin D supply is an important step in preventing osteoporosis. But it is not only in prevention, but also in the treatment of osteoporosis that the "bone vitamin" is indispensable. Effective bone preparations provide other relevant nutrients (e.g. calcium, magnesium, vitamin K) in addition to vitamin D.

Vitamin D production: Synthesis in the skin – optimised by nature

The starting substance of vitamin D synthesis in the body is provitamin 7-dehydrocholesterol. Triggered by UV-B radiation, previtamin D3 is produced in the skin and vitamin D3, also known as cholecalciferol or commonly vitamin D, is produced from it. Vitamin D3 can be converted into its storage form calcidiol by metabolic processes and finally into its physiologically effective form, the hormone calcidiol.

In this active form, it can carry out its functions in the body. Calcitriol plays a role in the modulation of the immune response in the body and in cell division and contributes to the normal maintenance of the calcium level in the blood.

The difference between vitamins D and D3

Health articles sometimes talk about vitamin D, then about vitamin D3 – but what’s the difference? “Vitamin D” is a collective term that includes a group of fat-soluble vitamins, called calciferols. "Vitamin D3" (cholecalciferol) is the most important physiological agent for us humans within the calciferols.

Daily dosage recommendation: How much vitamin D do we need per day?

As an estimated value for an appropriate intake, the German Nutrition Society states 20 μg or 800 IU vitamin D daily for children, adolescents and adults. Due to the far-reaching insufficient supply of vitamin D in the population and the diverse benefits of higher vitamin D levels, higher doses have a clear health benefit. The safe maximum daily amount for vitamin D is 100 μg or 4,000 IU.

Vitamin D test: have your blood values measured

Vitamin D is the only nutrient that is supplied not through the diet, but primarily by the intrinsic synthesis of sunlight. However, studies such as the BIOGENA Good Health Study show repeatedly that the vitamin D stores within the population are empty. This raises the question for many people about how they achieve their own vitamin D balance. A laboratory diagnostic analysis can provide a corresponding answer to this question. In order to determine the vitamin D level, the so-called 25-Hydroxy-Vitamin D3 is measured in the blood and, depending on the laboratory, either indicated as nmol/l or as ng/ml*.

Vitamin D levels: Normal levels & deficiency

Vitamin D: What level is normal, what is optimal, and when do I have a vitamin D deficiency? The following table is provided to help with deciphering "laboratory-speak".

Evaluation of the status

25 hydroxy vitamin D serum level (nmol/l)

25 hydroxy vitamin D serum level (ng/ml) (-> equivalent to "μgl/L")

serious deficiency

< 50 nmol/l

< 20 ng/ml

slight deficiency

50–75 nmol/l

20 – 30 ng/ml

Sufficient supply

75–100 nmol/l

30 – 40 ng/ml

Optimum supply

100–150 nmol/l

40 – 60 ng/ml


* Conversion from nmol/l to ng/ml: Simply divide the value in nmol/l by 2.5.

Vitamin D deficiency

Our body is able to meet its main vitamin D requirement itself by using sunlight. that’s fine in theory, but in practice it’s far more difficult. This is because numerous disruptive factors, such as cold, mature age, dark skin type or the use of sun protection factors, can get in the way of sufficient vitamin D formation.

High demand across the board – modern lifestyle is the cause

Too little sun exposure

People with low or no sun exposure are particularly affected by poorly filled vitamin D stores, for example due to reduced mobility or because they are bedridden; but also children who have to attend school or people working in shifts should not be forgotten here – all of these people usually spend most of the day in indoor environments.

Dark skin colour

People with a dark skin colour may also be affected, as a high melanin content in the skin slows down the formation of vitamin D.


With age, human vitamin D intrinsic synthesis decreases by up to 50%, so individuals aged 65 and over should pay particular attention to stable vitamin D levels.

Pregnancy and breastfeeding

Pregnant and breastfeeding women, as well as infants in the first year of life who must be protected from direct sunlight also have an increased need.

Go to vitamin D for children

Do you want to know more about this topic? Access the article here:

Vitamin D for babies

The groups of people who should pay particular attention to their vitamin D levels include:

  • Pregnant and breastfeeding women
  • Infants, children and adolescents
  • Older people
  • Shift workers
  • People with a dark skin
  • Smokers
  • People who cover up

Vitamin D deficiency: symptoms & possible consequences 

A long-term vitamin D deficiency can have various and sometimes serious consequences. Below is an overview of the most important ones.

  • Head & nerves: fatigue, irritability, restlessness, migraine, mood swings
  • Immune system: common infections, autoimmune reactions, allergies
  • Hair: Hair loss
  • Skin: Poor wound healing
  • Bones: Impaired bone mineralisation (children: rickets, adults: osteomalacia, osteoporosis), calcium deficiency, bone pain and deformations, bone fractures
  • Muscles: Muscle weakness, muscle pain, limb pain, muscle cramps
  • Metabolism: Increased risk of diabetes, increased risk of high blood pressure

Ensure an adequate supply – here’s how

Vitamin D can be absorbed through food and also produced by the human body itself. However, standard foodstuffs can only supply a small amount of the vitamin D required. As a rule, 80-90% of the vitamin is produced in the skin by the body itself with the aid of sunlight (UV-B rays). But you need to be outdoors for this to happen. The amount the body can produce differs from person to person.

Influencing factors

Influencing factors include skin type, latitude, time of day and season, weather conditions, clothing, the amount of time spent outdoors and the use of sunscreen. In our latitudes, the optimal conditions for vitamin D production can be found between April and September, from 10 a.m. to 3 p.m. Ideally, every day during this period, a quarter of the skin (face, hands, parts of arms and legs) should be exposed to the sun without sun protection for 5 to 25 minutes (depending on the season and skin type). However, it is important to take into account the skin’s own protection time and avoid getting sunburned!

Note: Around half of the time that it would take to get sunburned without protection is sufficient. If you stay exposed to the sun for longer, you should certainly take sun protection measures.

The synthesis of vitamin D in the skin is strongly dependent on the UV-B index, which can only be achieved when the sun is in the right position or has the appropriate irradiation angle, i.e. when it reaches the required value of more than 3, which corresponds to UV-B radiation of 290–315 nm.

A simple reminder: If your shadow is longer than you yourself, sufficient vitamin D synthesis cannot be guaranteed! In the period from October to March, the sun is lower and the UV index is below 3. This is when the body consumes its vitamin D reserves, insofar as they have been sufficiently replenished during the summer.

Vitamin D in food - what foodstuffs contain vitamin D?

Vitamin D can be absorbed from food, but sufficient amounts are found in only a few foods, mainly of animal origin, such as oily fish (salmon, herring), liver, egg yolk and some mushrooms. The German Nutrition Society (DGE) gives a reference value for an adequate supply, in the event of insufficient production occurring naturally in the body, of 20 µg vitamin D per day for adults and for children from one year of age. This corresponds to 800 international units (IU). According to the DGE, the supply of vitamin D via normal food is insufficient to achieve this reference value. This difference must be compensated for via endogenous synthesis and/or supplementation with a vitamin D preparation. People who are unable to stay in the sun for long or even at all for various reasons, and/or who can only spend time outdoors with their body completely covered, as well as people with dark skin pigmentation, are recommended by the D-A-C-H nutrition societies of Germany, Austria, and Switzerland to take a vitamin D supplement.

Vitamin C intake & dosage

When ist the best time to dake vitamin D: in the morning or evening?

Regardless of whether it’s morning, lunchtime or evening: The time of day plays less of a role in vitamin D intake than the "how". Preparations that do not contain oils as an additional ingredient should best be consumed with a meal. Some experts recommend taking the sun vitamin with the richest meal of the day (usually lunch or dinner). In every case, the food should provide some fat to support the vitamin D intake.

How should I take vitamin D?

As vitamin D is fat-soluble, fatty foods (e.g. high-quality oils) improve its absorption into the body. For this reason, vitamin D should always be taken with a meal. Exceptions are preparations in which the vitamin has already been combined with a fat component (such as vitamin D drops).

Vitamin D daily dosage recommendation

The German Nutrition Society (DGE) recommends 20 μg or 800 IU vitamin D daily as an estimate for an adequate vitamin D supply when intrinsic synthesis is unable to work (e.g. in winter, when in full-time employment indoors or if you are bedridden. Due to the widespread insufficient supply of vitamin D in the population and the diverse benefits of higher vitamin D levels, higher doses have a clear health benefit. But can you take too much vitamin D? The safe maximum daily amount for vitamin D is 100 μg or 4,000 IU.

If higher doses are required, a regular blood test is recommended. The selected dosage should also be adjusted to your body weight; online vitamin D computers can help you to select the correct dosage, including the desired dosing period.

When does high-dose vitamin D3 make sense?

High-dose vitamin D3 is often used in the case of very low levels of vitamin D when it comes to correcting the deficiency as quickly as possible. Some doctors also prefer high-dose vitamin D3 in maintenance therapy, since the preparation has to be taken much less frequently and some patients feel more comfortable as a result.

For how long should you take vitamin D?

In general, it is recommended that you should take vitamin D all year round. People suffering from vitamin D deficiency should ideally consume higher amounts of vitamin D under medical supervision and laboratory diagnostic support – until they have reached their target value. However, a few weeks to months must be expected here.

Vitamin D preparations: Drops, capsules or tablets?

Those who decide to supplement the sun vitamin in daily life are spoilt for choice: Drops, capsules and tablets are probably the most common forms on the market – but which product is best?

First off: There is no ultimate vitamin D preparation for everyone. However, what should always be in the foreground when deciding to buy is the quality of the product (e.g. formulation without additives, trustworthy quality manufacturer, if possible) and the correct dosage.

Vitamin D drops are indicated by the fact that the fat-soluble vitamin is already dissolved in the oily environment and can thus be absorbed very well by the body. For the consumer, this means that they can use the drops at any time, even with or without a meal. In addition, drops are pleasant to take and can be dosed individually. Less ideal is the often lower shelf life of drops compared to capsules or tablets.

Vitamin D3 drops
Pure vitamin D3 in a practical drop form with high-quality sunflower oil for optimum absorption in the body
To the product
Vitamin D3 Tropfen

The most common pharmaceutical form on the market is vitamin D in tablet form. Here too, there are variants where oil is included equally for improved vitamin D uptake. However, many tablet products contain not only active ingredients but also unnecessary additives (such as colourings and flavourings) and are therefore less advisable.

Vitamin D capsules are often attractive due to their special purity, since they are usually without additives, they are hygienic and have a relatively long shelf life without additives. Preparations that do not automatically include oil should always be taken with a fatty meal in order to be able to absorb the vitamin well into the body.

See all vitamin D preparations

Why combine vitamin D3 with K2?

A combination of vitamin K2 and vitamin D3 is quite useful in certain situations. Especially when it comes to bone work, both vitamins pull together. Vitamin K2 has the task of depositing minerals in the bone – but vitamin D3 must activate the required proteins and also utilise calcium for this to occur. If vitamin K2 is lacking, calcium and magnesium cannot be "sucked" into the bones. If insufficient vitamin D3 is available, proteins and usable calcium are missing for bone mineralisation. Both fat-soluble vitamins must therefore be available in effective bone preparations.

Vitamin D3 and K2

Duo preparation with vitamins D3 & K2 - combined with linseed oil to improve absorption and lutein

By Vitamin D3 K2
Vitamin D3 & K2 - 120 Kapseln

Vitamin D and magnesium

Vitamin D and magnesium work together in the metabolism at different levels, which can make a combination of both under certain settings quite useful. Vitamin D, for example, supports the absorption of magnesium in the intestine, whereby an adequate level of vitamin D can generally improve magnesium absorption. Both vitamin D and magnesium contribute to the health of bones, teeth and muscles. Studies indicate that vitamin D and magnesium deficiency can promote the development of high blood pressure. People with high blood pressure can be encouraged to have the status of both nutrients checked and supplemented if necessary.

Conclusion: After all, vitamin D receptors are found in almost all organs and tissues - synonymous with and reflecting its relevance to health. Studies, such as the BIOGENA Good Health Study, show that in many cases, diet and sun alone are not sufficient for a good vitamin D level, but that the intake must also be ensured with a vitamin D supplement. Due to the health benefits, the aim should not only be to achieve an adequate supply of vitamin D, but to optimise it - under laboratory diagnostic supervision.

Frequently asked questions about vitamin D

In practice, it is not possible to meet the daily vitamin D requirement through food. By far the most vitamin D is found in fatty fish, such as salmon and herring. To a far lesser extent it is found in liver, egg yolks and some fungi, such as chanterelles and wild mushrooms.

Yes, but much less than without sunscreen. A sun protection factor of just 15 blocks 95% of the UV radiation and the body's own vitamin D synthesis is reduced accordingly. For optimal production in the skin, about a quarter of the body's surface should be uncovered to the sun in the ideal sun conditions for 10-15 minutes without sunscreen.

Vitamin D and vitamin B12 can easily be taken together. Neither vitamin interferes with absorption or further metabolism.

Vitamin D and vitamin K2 do not automatically always have to be taken together. Depending on the individual initial situation and therapeutic goal, however, taking vitamin D and vitamin K2 together can range from sensible to very advantageous. For example, vitamin K2 and vitamin D3 work together to build and maintain strong bones.

The human body forms 80-90% of vitamin D by sunlight. The UV B radiation contained in sunlight is decisive for this. This first converts a form of cholesterol in the outer skin layer into a precursor of vitamin D3 and then heat turns this into vitamin D3. This vitamin D3 is now converted by the body via metabolic pathways into its storage form calcidiol or its active form calcitriol.

How long it takes to correct a vitamin D deficiency depends on various factors. This includes, for example, body weight, the respective severity of the deficiency and the selected dosage. As a rule, however, a few weeks to months must be expected.

In Austria and Germany, vitamin D synthesis is only possible between March and October, because the position of the sun during the rest of the year does not favour sufficient UV radiation.

Based on current data (*), 40% of the population in Europe is on average under-supplied with vitamin D, but in Germany alone, the annual average is 56% (reference laboratory value of <50 nmol/l). Independent expert groups and physicians, however, recommend a higher reference laboratory value of 75 nmol/l. On the basis of this, over 85% of people are under-supplied in Germany, 75% in Switzerland, approximately 70% in the Netherlands and approximately 88% in the UK. Even southern regions of Europe are not spared from a vitamin D deficiency; data from Greece shows that up to 90% of the population is under-supplied with vitamin D (based on 75 nmol/l) and approx. 50% are below a value of 50 nmol/l.

*Cashman KD et al. 2016. Vitamin D deficiency in Europe: pandemic?. Am J Clin Nutr. 103(4): 1033–1044.
Palacios C and Gonzalez L. 2013. Is vitamin D deficiency a major global public health problem? J Steroid Biochem Mol Biol. 144PA: 138–145.

Solarium visits are not a recommended alternative to stimulate the body’s own vitamin D production. A specific wavelength in the UV-B range is required for vitamin D formation. However, most solaria mainly use UVA radiation, which achieves a good tanning effect, but can also lead to chronic skin damage, such as premature skin ageing.

Vitamin D drops are easy for everyone to take and automatically provide the fat (oil) the body needs as a medium for transport to absorb the fat-soluble vitamin well. If you prefer tablets or capsules, you should go for products that are as additive-free as possible and take the vitamin with a fatty meal.

Vitamin D is present in a small selection of foods in notable quantities. Top performers include fatty fish such as wild salmon, herring and eel, as well as liver and cod liver oil. Smaller amounts of vitamin D are provided by egg yolks, liver and some wild growing fungus such as porcini or chanterelles.


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Wäger et al. 2022. BIOGENA Good Health Study: Vitamin D

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https://flexikon.doccheck.com/de/Calcidiol Zugriff: 11.1.2024

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