At a glance
A fat-soluble vitamin for calcium balance, bones, muscles and the immune system, which the body produces itself in the skin when exposed to sunlight.
In Vykea: cholecalciferol (vitamin D3), 25 to 50 µg (1,000 to 2,000 IU) per daily dose.
In Central Europe, the body's own production is limited in winter, which is why vitamin D status fluctuates over the year.

Vitamin D is a fat-soluble vitamin and holds a special position among the vitamins: the body can produce it itself in the skin when there is sufficient exposure to sunlight. Because of this production by the body and its hormone-like mode of action, the active form is also referred to as a prohormone. Vitamin D is important above all for calcium and phosphate balance, for bones and teeth, for muscle function, the immune system and cell division. In Central Europe, the body's own production is limited in the winter months, which is why vitamin D status can fluctuate over the year. In our products we use cholecalciferol (vitamin D3), the form that the body also produces itself. This entry explains the forms, food sources, functions, requirements, laboratory testing, possible signs of deficiency and interactions of vitamin D3.

Vitamin D occurs in two important basic forms. Vitamin D3 (cholecalciferol) is the form of animal origin, which humans also produce themselves in the skin. Vitamin D2 (ergocalciferol) comes from plants and fungi. Both forms are converted in the body, with vitamin D3 regarded as the more effective and more readily utilised form.

Neither basic form is active in itself. In the liver, vitamin D is first converted into calcidiol (25-hydroxyvitamin D, 25-OH-D for short). This is the storage and transport form, whose level in the blood is measured to assess vitamin D status. In the kidney, this is converted into the actual active hormone calcitriol (1,25-dihydroxyvitamin D), which acts on the vitamin D receptors in the body.

Because vitamin D is fat-soluble, it is absorbed in the gut together with dietary fats. It is therefore better utilised when taken with a meal containing fat. Our products use cholecalciferol (vitamin D3), the same form that the body produces itself when exposed to sunlight.

Only a few foods contain vitamin D in significant amounts. The most important sources are oily sea fish such as salmon, herring and mackerel, as well as cod liver oil. Egg yolk, liver and some dairy products contribute considerably smaller amounts. Plant-based foods such as certain edible mushrooms contain small amounts of vitamin D2.

Only a limited amount of vitamin D is obtained from a normal diet. The DGE puts the average intake from food in adults at about 2 to 4 µg per day, which is well below the estimated value for intake. Under suitable conditions, the larger part of the requirement is covered by the body's own production in the skin (see the section on sun and season).

Through its active hormone calcitriol, vitamin D acts at numerous sites in the body. Vitamin D receptors have been found in a great many different cell types, which explains the vitamin's wide-ranging importance:

The next section shows which of these functions are authorised as health claims.

The following health claims are authorised for vitamin D in the EU:

For children, the claim that vitamin D is needed for normal growth and development of bone in children is also authorised. This claim referring to children's development is authorised separately as an Article 14 claim (Regulation (EU) No 957/2010) and is not part of the list above under Regulation 432/2012. The claims apply to vitamin D in general and therefore also to the form used in our products, cholecalciferol (vitamin D3).

The following values are estimated values for an adequate intake in the absence of the body's own production and are taken from the D-A-CH reference values of the German Nutrition Society (DGE). They assume that the skin produces no vitamin D and are therefore higher than the share from food alone.

Amounts of vitamin D are often given both in micrograms (µg) and in International Units (IU). The conversion is: 1 µg corresponds to 40 IU and 1 IU corresponds to 0.025 µg. 20 µg therefore corresponds to 800 IU.

The nutrient reference value for labelling (NRV) is 5 µg (200 IU) per day. For adults, including pregnant and breastfeeding women, the European Food Safety Authority (EFSA) gives a tolerable upper intake level of 100 µg (4,000 IU) per day from all sources, i.e. from food and food supplements combined. EFSA confirmed this upper limit in its updated assessment of 2023.

In some situations, the risk of inadequate vitamin D status is increased: in people who rarely spend time outdoors or only do so with their skin covered, in residents of care homes, in older people, as the body's own production declines with age, and in people with darker skin living in northern latitudes. Infants are also among the groups given particular attention.

There is also a higher risk when fat absorption or absorption from the gut is impaired, for example with chronic inflammatory bowel disease, after bowel surgery or with pancreatic insufficiency, and with certain liver and kidney diseases. A purely plant-based diet can further reduce intake from food. Some medicines can also affect vitamin D metabolism (see the Interactions section).

Surveys of vitamin D status show that a considerable proportion of the population in Germany does not reach the blood levels regarded as desirable for bone health all year round, especially in winter and spring.

A special feature of vitamin D is that, under suitable conditions, the body can cover most of its requirement itself. In the skin, cholecalciferol, i.e. vitamin D3, is formed from a precursor (7-dehydrocholesterol) under the influence of UVB radiation.

How much vitamin D is produced in this way depends on several factors: the position of the sun and therefore the season, time of day and latitude, cloud cover, the length of time spent outdoors, the proportion of uncovered skin, skin colour, age and the use of sunscreen. In Central Europe, from about October to March the sun is so low in the sky that UVB radiation is usually insufficient for any significant vitamin D production. During this time, the body draws on the stores built up in summer. If this reserve is low, vitamin D status can decline over the winter.

Sunbathing itself cannot lead to a vitamin D overdose, because the skin limits production itself. The usual recommendations on protection from excessive sun and on skin cancer prevention continue to apply unchanged. What an appropriate supply over the year looks like and whether supplementation is an option is best clarified with a doctor.

Vitamin D status is determined by a doctor from a blood sample. Calcidiol (25-hydroxyvitamin D, 25-OH-D) is usually measured in serum or plasma, because it best reflects the supply over the previous weeks. Active calcitriol, by contrast, is hardly suitable for assessing general status, as its level is tightly regulated by hormones.

Different thresholds and units are used to assess 25-OH-D levels (nmol/l or ng/ml; 1 ng/ml corresponds to 2.5 nmol/l). Whether a level is classed as sufficient, borderline or deficient depends on the criteria used and the individual situation. Very low values suggest inadequate supply; very high values usually suggest a high intake from supplements or, less often, certain diseases.

Note: Interpreting the measured values and any supplementation should be left to a doctor.

A mild shortfall often causes no clear symptoms. Non-specific signs can include tiredness, increased susceptibility to infections or muscle weakness. Because vitamin D is central to bone metabolism, a longer-lasting, pronounced deficiency mainly affects the bones.

In infants and young children, severe, persistent vitamin D deficiency can lead to impaired bone mineralisation (rickets). In adults, a corresponding bone mineralisation disorder can occur (osteomalacia). Inadequate vitamin D and calcium status over a long period is also among the factors associated with reduced bone density.

Such signs are not conclusive and can have many causes.

Note: Investigation, diagnosis and deciding on any treatment should be left to a doctor.

Some medicines can affect vitamin D metabolism. Glucocorticoids (for example cortisone preparations), certain anti-epileptic drugs (such as carbamazepine) and some other active ingredients can accelerate the breakdown of vitamin D or impair its absorption. During treatment with active vitamin D analogues (for example calcitriol) or when taking calcium supplements at the same time, calcium levels should be monitored.

Note: If you take such medicines, you should check with a doctor before taking additional vitamin D.

There is a close metabolic relationship between vitamin D and calcium: vitamin D improves calcium absorption, and calcium status in turn influences vitamin D metabolism. With some conditions, such as sarcoidosis or increased calcium excretion in the urine, particular caution is needed with vitamin D intake.

Note: In such cases, intake should be managed by a doctor.

Vitamin D3 and vitamin K2 act at neighbouring points in bone and calcium metabolism and complement each other in doing so. Vitamin D3 promotes the absorption of calcium from food in the gut and thus helps to ensure that sufficient calcium is available to the body in the first place. Vitamin K2 then acts as a cofactor and activates the vitamin K-dependent proteins. Only through this step, known as carboxylation, can osteocalcin and matrix GLA protein (MGP) bind calcium. Activated osteocalcin supports the incorporation of calcium into the bone matrix, while activated MGP in the vessel walls is described in reviews as an inhibitor of unwanted vascular calcification. In their interplay, the two vitamins thus support calcium reaching the places where it is needed rather than being deposited in the vessel walls.

Reviews interpret these complementary roles as meaning that a combined intake of vitamin D and vitamin K can support bone metabolism better than an intake of either vitamin alone, for example through greater carboxylation of osteocalcin. The evidence is still limited and not consistent on all points. This is a general classification based on metabolic physiology from nutritional science, not a promise of effect for any individual product beyond the scope of the authorised claims. For the health claims authorised under EU law, the nutrient-specific wordings continue to apply.

Magnesium is also important for vitamin D metabolism. The enzymes that convert vitamin D into its storage form and its active form in the liver and kidney require magnesium as a cofactor. An adequate magnesium supply is therefore regarded as a prerequisite for the body to make proper use of vitamin D. Because vitamin D3 is fat-soluble, it is also better absorbed when taken with a meal (see the section on the forms).

You can find more about these nutrients in the Vitamin K2 and Magnesium entries.

Note: If you take coumarin-type anticoagulants (vitamin K antagonists such as phenprocoumon, e.g. Marcumar, or warfarin), you should check with a doctor before taking additional vitamin K, as vitamin K can affect how these medicines work.

Vitamin D3 (cholecalciferol) is contained in several of our products:

The % of NRV is based on the labelling value of 5 µg (200 IU). Because vitamin D is fat-soluble, we recommend taking it with a meal containing fat to support absorption. In D3K2 Instant and D3K2 Drops, vitamin D3 is combined with vitamin K2 (see the section "Why vitamin D3 and K2 belong together"). The amounts stated are below the tolerable upper intake level of 100 µg (4,000 IU) per day given by EFSA for adults; however, if you use several supplements at the same time, always take your total intake into account.

Food supplements are not a substitute for a balanced, varied diet and a healthy lifestyle.

Any further questions? Our service team will be happy to help at [email protected].