Vitamins are essential for the human body, but only one vitamin has a very special character – vitamin D. It is the only vitamin that is produced by the human body itself, under the influence of sunlight in the skin. Vitamin D, therefore, is not really a vitamin, but a hormone.
Our body needs vitamin D, not only for protective immune response, but also for healthy bones, muscles and teeth. Like any vitamin, we can also intake vitamin D with food and convert it into the active form within the body. Oily fish (herring, eel, salmon), chicken egg yolk, butter and mushrooms are particularly rich in vitamin D3.
If you do not get enough sunshine on a daily basis, ie. by wearing too much clothing, use of sunscreen with a high sun protection factor, geographical location, season (e.g. winter months), skin type and/or you eat less vitamin D-containing foods, a vitamin D-deficiency might arise.
The current recommendation for adults is up to 20 micrograms (800 international units, IU) daily. However, this is a daily estimate based on the assumption that no vitamin D has formed in an individuals body organically due to a lack of sunlight exposure.