There are more and more scientific studies backing up the fact that vitamin D plays an important role in the human immune system. There is also plenty of evidence showing that vitamin D is a major factor in prevention of common respiratory infections. One of the largest studies (Ginde et al. 2009) made of this topic revealed that people with the lowest blood vitamin D levels had substantially more cases of common colds or the flu.
Vitamin D is in fact a group of fat-soluble prohormones, the two most relevant forms for humans being vitamins D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D2 is produced in response to UV irradiation in some phytoplankton, invertebrates and fungi. Vitamin D3 on the other hand is produced in the skin of vertebrates in response to UVB light from the sun. Vitamin D3 can also be found in fish and few other foods. The recommendable supplemental form of vitamin D is vitamin D3. This is because vitamin D3 is the same substance that is produced in human skin after sun exposure. Supplemental D3 is usually derived from cod liver oil extract or lanolin.
Vitamin D level is defined by measuring blood serum's concentration of 25-hydroxy-vitamin D. It tells us both the amount of vitamin D produced in the skin as well as the amount of vitamin D acquired through diet. The common conception has been that a concentration level above 15ng/ml (37,5 nmol/l) is adequate for those in good health. Desirable level for achieving good health has been a level above 30ng/ml (75nmol/l). However recent studies have revealed that the level which has conventionally been considered as normal is not the same as optimal. According to latest research it is very reasonable to believe that values presented below reflect more to optimal vitamin D levels:
deficient =
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