Vitamin D is essential for the body, and its deficiency causes rickets in children and loss of bone density in adults. The enzymes that produce vitamin D (related to bone metabolism, the immune and cardiovascular systems) are activated by sun exposure.
Clouds, smog, advanced age, and dark-colored skin reduce the amount of vitamin D generated by the skin. Dermatologists advise obtaining vitamin D with a balanced diet (but there are very few foods that contain this vitamin naturally) or with supplements.