Although, strictly speaking, Vitamin D is classified as a hormone, it is usually treated as a fat-soluble vitamin. The body can produce this vital substance by itself (Vitamin D3) or assimilate it from food (D2 and D3). Since this vitamin regulates the activity of a variety of cell types, it has moved increasingly into the central focus of vitamin research in recent years, and researchers have come to the conclusion that Vitamin D cannot merely be regarded as a factor in bone health. Two renowned experts on micronutrients say:
In its active form, Vitamin D is not only essential for the metabolism of the bones, but also for the smooth functioning of practically all of the cells and organs in our bodies. The health of blood vessels, the heart, most of our organs and the intact functioning of the immune system all depend on a good supply of Vitamin D. This explains why so much emphasis is placed on the sun vitamin in preventive medicine.*
Vitamin D is often referred to as the sun vitamin, since about 90 % of the daily requirement can be produced by ultraviolet radiation from the sun striking the skin. However, over the winter months, the UVB rays (= wavelength range from 290 to 315 nanometres) cannot reach the radiation intensity required for vitamin formation at our latitude. Due to climatic conditions alone, Vitamin D levels can easily slip into the deficiency range in the darker months.
A Vitamin D3 count of 800-1000 IU (= International Units), which amounts to 20-25 micrograms, is considered sufficient to meet the average daily requirement. The situation is different when the Vitamin D level is already reduced. The requirement is then usually significantly increased, in order to achieve a balanced Vitamin D status. After examining extensive test data, the European Food Safety Authority (EFSA) has determined a daily intake of up to 4000 IU (= 100 micrograms) of Vitamin D3 to be a safe dose. If you wish to support your Vitamin D levels by taking a dietary supplement, there is a choice of Vitamin D3 preparations at a variety of dosages available nowadays, so you will always be able to find the appropriate dosage. If you are concerned about your Vitamin D levels, you can quickly get clarity by having a blood test done by your doctor.
It also makes sense to check with your doctor, because a Vitamin D3 deficiency has no clear symptoms and an increased requirement can be assumed in principle for certain groups of people, for example, people who spend most of their time indoors, or the elderly. (From the age of 30 to 65 years, the skin loses up to 80 % of its capacity to synthesise Vitamin D3!)
It is virtually impossible to cover the body's full Vitamin D requirements through daily nutrition alone. First of all, the D3 form of the vitamin, which is synthesised by the body with the aid of solar radiation, is more effective than the dietary form, D2. Furthermore, Vitamin D3 is only found in significant quantities in a few high-fat foods, such as fatty fish, cod liver oil, eggs and butter.
Note on the combination of Vitamins D3 and K2
Vitamin D3 is not dependent on the simultaneous administration of Vitamin K2 in order to become physiologically active. However, if there is a deficiency or lack of Vitamin K, and the aim is to specifically strengthen the bones, a combined dose of Vitamins K2 and D3 is recommended.
* cited from: Uwe Grober, Michael F. Holick: Vitamin D. Die Heilkraft des Sonnenvitamins (The healing power of the sun vitamin), p.17. Published by the Wissenschaftliche Verlagsgesellschaft Stuttgart, 2015.