Even if present in small quantities, a vitamin plays an indispensable role in the normal functioning of one or more physiological processes. In general, the body is not able to synthesize these substances by itself, meaning that they have to be regularly introduced into our diets. This definition of a vitamin, in reality, largely fits the characterization of vitamin D. The heat and action of ultraviolet sun rays are in fact able to transform the 7-dehydrocholesterol present on our skin into vitamin D3. For this reason, vitamin D becomes a “true vitamin” only when humans (and any other mammals) are not adequately exposed to sunlight and therefore need to supply themselves with it through diet.
