What does vitamin D do

Natural medicinal active ingredient vitamin D3

In a nutshell!

Life on earth would not be possible without the light of the sun. The sun vitamin D3 is a special proof of this: we can take the vitamin with food, but we wouldn't get enough of it without direct sunlight on our skin. In order to meet the needs of our modern way of life, many people use food supplements with this vitamin.

What is so special about vitamin D2 and D3?

Vitamins are by definition vital chemical compounds that our body cannot produce itself but must be ingested with food.

The special position of vitamin D relates to this property. In contrast to the other vitamins, our body can produce this itself. The formation of vitamin D takes place using existing precursors in the skin. However, this requires the sun. Specifically, it is the UVB rays from solar radiation.

Strictly speaking, it is not a vitamin, not even a clearly defined substance. We refer to a whole group of fat-soluble vitamins as vitamin D. The best known representatives are vitamin D2 and D3. Vitamin D2 is also known as ergocalciferol. It is Z. B. contained in mushrooms. Vitamin D3 is the main active form, cholecalciferol. It is formed in the skin from the provitamin 7-dehydrocholesterol present in the body with the help of UV-B radiation. Both compounds can, however, also be taken in through food.

The hormone calciferol is then formed from vitamin D. In view of the fact that vitamin D3 is not a vitamin in the true sense of the definition, one should correctly speak of a pro-hormone, i.e. the precursor of a hormone.

Why does the body need vitamin D?

Vitamin D3 is involved in many metabolic processes. It has long been known that it promotes the absorption of calcium from the digestive tract and strengthens bones. It also regulates calcium and phosphate metabolism and influences muscle strength.
In recent years, scientists have shown in various studies that vitamin D receptors are present in 30 different types of tissue and organs. These include, for example, hormone-producing organs such as the thyroid gland. Vitamin D also plays a regulatory role in the immune system. From this it can be deduced that vitamin D3 has a wide range of tasks and is therefore involved in our health in a variety of ways.

What is the relationship between vitamin D3 and vitamin K2?

The two vitamins are very closely related to one another biochemically in the human body. This particularly affects the calcium and bone metabolism. If one of the two is deficient, the other cannot work properly. It has also been observed that the higher the vitamin D intake, the greater the likelihood of a vitamin K deficiency.

Ultimately, only laboratory tests can determine whether there is a deficiency in one of the two vitamins.

The supply of vitamin K in healthy people is unproblematic as it is well tolerated. However, it is different in patients with coagulation disorders or those who take certain medications (so-called vitamin K antagonists).

For these reasons, the calcium level and the vitamin K level should be determined in addition to the vitamin D level when taking high doses of vitamin D for a longer period of time. This means that a doctor's check is necessary in these cases.

How much vitamin D3 do people need? Where does he get it from? Can he save it?

It is estimated that people in our latitudes and when they stay outdoors regularly produce around 80 to 90% of the required vitamin D3 themselves and only get 10 to 20% through food.

The storage capacity is relatively large and contributes to the vitamin D supply in winter. The main storage locations are fat and muscle tissue, and to a lesser extent the liver.

The DGE or D-A-CH reference value is 20 micrograms of vitamin D per day if the body does not produce it naturally. The amount is often not given in micrograms (µg) but in international units (IU). 1 µg corresponds to 40 IU.

Table: Estimated values ​​for adequate vitamin D intake according to DGE in the absence of formation through the skin with the help of the sun


Vitamin D requirement
in µg / day (1 µg = 40 IU)

Estimated coverage of needs through normal diet in µg / day

Infants (0 to under 12 months)


Vitamin D tablets are recommended for rickets prophylaxis

Children (1 to under 15 years)


1 - 2

Teenagers and adults
(15 to under 65 years)


2 - 4

Adults (from 65 years)


2 - 4

Pregnant women, breastfeeding women


2 - 4

How is the vitamin D level determined and interpreted?

If you want to assess whether enough vitamin D is being absorbed or produced, you have to determine the vitamin D level in the blood. To do this, the doctor takes blood if a vitamin D deficiency is suspected and has the 25-hydroxyvitamin D value determined in the laboratory, which is a marker for the vitamin D status. The minimum blood concentration of this marker is given as 50 nanomoles per liter (nmol / l) of serum or 20 nanograms per milliliter (ng / ml). These values ​​should at least be achieved in order to maintain bone health.

25 (OH) D.
in nmol / l

25 (OH) D in ng / ml

Interpretation in relation to bone health



Inadequate care - increased risk of diseases such as rickets *, osteomalacia * and osteoporosis *



suboptimal care - possible consequences for bone health

50 -<75


adequate supply



more than adequate supply (without additional health benefits)



possible oversupply - risk of hypercalcaemia (high calcium levels in the blood), which can lead to irregular heartbeat or kidney stones

Table: Measured values ​​of 25-hydroxyvitamin-D for short 25 (OH) D, in blood serum and their interpretation in relation to bone health (source: Robert Koch Institute). However, many scientists are now of the opinion that higher blood levels must be achieved in order to ensure bone health.
* These terms are explained below under: What are the consequences of a vitamin D deficiency?

How much vitamin D is there in food?

Few foods are really good sources of vitamin D. These are predominantly of animal origin: fatty fish (e.g. salmon, herring, mackerel) and, to a much lesser extent, liver. Reasonably good sources of vitamin D are appropriately fortified margarine, egg yolks, avocado and some edible mushrooms. In Germany, we only supply 2 to 4 micrograms of vitamin D3 per day through a diet with the usual foods.


Vitamin D content in µg per 100g


3,4 - 5

Whipped cream




Beef liver




Bismark herring


Eel, smoked


Cod liver oil




Dark chocolate

2 – 5,5

Table: Vitamin D content of selected foods (according to Vitalstofflexikon.de)

How much sun do you need to produce enough vitamin D?

The question is difficult to answer as there are many factors that influence the amount of UVB radiation that is actually available.

This includes:

  • Latitude
  • Time of year and day
  • weather
  • dress
  • Duration of stay outdoors
  • Skin type
  • Use of sunscreen

The intensity of the UV radiation is important for the formation of vitamin D, because it can only be formed in the skin with the help of sunlight from a UV index of 3 or higher. It is estimated that in Germany we produce enough D3 in the summer half-year if we expose a quarter of our body surface to the sun for 5 to 25 minutes a day, depending on the skin type. In our latitudes, the intensity of the sun is usually insufficient for vitamin D formation in the winter months.

As a rule, solariums are unfortunately not suitable as an alternative, because most sunbeds only emit UVA radiation. However, we need UVB rays to produce vitamin D3.

Fortunately, we can store this vitamin. But only to a certain extent and only what is actually there. In phases in which D3 cannot be sufficiently formed in the body, our storage capacity enables us to take the vitamin in the form of so-called depot products. In practice this means that the daily 20 µg (800 IU) recommended by the German Nutrition Society can also be taken once a week in a depot form. The weekly dose is then 140 µg (5600 IU).

How is the vitamin D supply in Germany? What role do age, lifestyle and sun protection play?

Until a few years ago, it was still assumed that we were adequately supplied with vitamin D3 due to our storage capacity. In the meantime, however, various studies indicate that almost 60% of German citizens do not achieve the desired blood concentration. Older people are particularly affected. There are two main reasons for this. On the one hand, older people tend to spend less time in the fresh air and when they do, they avoid direct sun. In addition, the ability to produce vitamin D3 in the skin decreases with age.

The completely justified concerns about the development of skin cancer, but also the insight that the skin ages faster under strong UV radiation, has led to the use of sunscreens with a high sun protection factor in recent years.

It is assumed that from a factor of 10 onwards, hardly any UVB rays can reach the skin and thus stimulate vitamin D production. Most winter sports enthusiasts accordingly protect themselves intensively from the sun, while others do not go out into the fresh air enough and therefore do not get enough UVB rays.

What are the consequences of a vitamin D deficiency?

The best known and most obvious are the deficiency symptoms associated with bone formation. Vitamin D deficiency can contribute to the development of

  • Rickets: the insufficient mineralization of bones in childhood
  • Osteomalacia: the demineralization of the bone
  • Osteoporosis: the change in bone structure with age

Since vitamin D3 is involved in a large number of metabolic processes, various other diseases can also be favored in their development if there is a vitamin D deficiency. If there is a deficiency, it must be treated by a doctor with appropriate drugs.

Scientists as well as many doctors and alternative practitioners now assume that Germany and Northern Europe are regions with vitamin D deficiency. There is more and more discussion about whether there could be a connection between such phenomena as winter depression or spring fatigue and vitamin D. This would also be a plausible explanation for the fact that colds and infections increase in spring and feelings of weakness occur more frequently.

Vitamin D3 in the focus of science

There are now many studies around the world on vitamin D3 and vitamin D requirements. These initially led to the recommended values ​​being increased. Until 2012, the German Nutrition Society (DGE) considered 5 µg daily to be sufficient; today the recommended intake is 20 micrograms daily. Only a small part of this can be ingested through normal food. In children it is 1 to 2 µg and in adolescents and adults 2 to 4 µg per day.

However, current studies repeatedly point out that these values ​​apply to healthy people and only to people who already have a sufficiently high level of vitamin D in their blood. How much of the vitamin should be taken when blood levels are low is still a matter of dispute.

Growing health awareness and the large number of publications and articles have led to a veritable boom in vitamin D supplements. Vitamin D tablets are available, but now also oily drops.

Since it is a fat-soluble vitamin, the vitamin D3 drops and capsules have the advantage that they can be administered together with various oils and can therefore be better absorbed by the body.

Studies on the importance of cholecalciferol for the immune system are always up-to-date: The vitamin plays an essential role in the normal function of the human immune system. This, in turn, is the best protection against infection. That is why additional vitamin D supplements are valued in the form of dietary supplements to maintain a functioning immune system.

Can you overdose on vitamin D3?

Yes. Since vitamin D3 is a fat-soluble vitamin and can therefore be stored in the body, overdosing should be avoided. The European Food Safety Authority (EFSA) has therefore set a tolerable total daily intake of 100 micrograms of vitamin D3 for adults and children from the age of 11 and of 50 micrograms of vitamin D for children up to 10 years of age. These tolerable total intake amounts per day refer to the vitamin D intake from foods and dietary supplements. In addition, they relate to healthy people who have normal needs and whose daily needs are also met on a regular basis. For medical reasons, higher amounts of vitamin D may be indicated.

Because of the possible overdose, you should avoid taking high-dose vitamin D supplements yourself. This is why the particularly high-dose products are only available in pharmacies as prescription drugs. With the help of these prescription vitamin D supplements, an existing vitamin D deficiency can be remedied. Always discuss the intake with your doctor. Ideally, the vitamin D3 level in the blood should be determined at the same time. In contrast, dietary supplements have the task of ensuring normal requirements, especially in the seasons with little sunshine.

The sun vitamin D3 contributes to ...

  • normal absorption and utilization of calcium and phosphorus
  • normal blood calcium levels
  • the maintenance of normal bones
  • the maintenance of normal muscle function
  • the maintenance of normal teeth
  • normal function of the immune system
  • normal cell division