Vitamin D

If a vitamin is defined only as a required substance that is not endogenously produced, vitamin D does not meet the criteria. It is produced in the skin upon ultraviolet B (UVB) rays exposure. But as humankind becomes increasingly urban and specialized, our lifestyle is reducing the exposure of skin to sunlight and the ability of skin to synthesize vitamin D. In the near future, it could be that vitamin D will become a true vitamin and optimal health might, therefore, require an exogenous source, as there are few natural food sources, apart from fatty fish.