How exactly does the Sun provide us with Vitamin D?

It is a myth that sunlight provides us vitamin D or vitamin D is present in sunlight. The fact is vitamin D is synthesized in plants, animals and humans in presence of sunlight.

There are two types of vitamin D - vitamin D2 (Ergocalciferol) present in plants including ergot and mushrooms and vitamin D3 (Cholecalciferol) in animals.

Vitamin D2 (Ergocalciferol) and vitamin D3 (Cholecalciferol) are synthesised in presence of ultraviolet light (UV) of sunlight as given below.

In plants, the ergo-calciferol (vitamin D2) is derived from UV irradiation of ergosterol (a kind of sterol present in plants). In animals and humans, when skin is exposed to sunlight, chole-calciferol (vitamin D3) is produced in skin by UV irradiation of 7-dehydro-cholesterol (a kind of cholesterol present in animals and humans).

Sunlight triggers the first of three chemical reactions that converts an inactive compound in the skin into active vitamin D. Ultraviolet B rays from the sun convert a natural vitamin D precursor present in your skin, 7-dehydrocholesterol, into vitamin D3. This travels to the liver where the addition of oxygen and hydrogen to vitamin D3 changes it into 25-hydroxyvitamin D. Doctors test for this intermediate and still inactive form of vitamin D in blood to determine your vitamin D status. Final activation of 25-hydroxyvitamin D takes place in the kidneys, where more oxygen and hydrogen molecules attach to 25-hydroxyvitamin D and convert it into its active form known as 1,25 dihydroxyvitamin D, or calcitriol.