How does the skin make vitamin D using sunlight?

Answer 1

Vitamin D is required for the formation of strong bones and teeth. It is manufactured in the skin by the activation of a sterol (7-dehydrocholesterol) by ultraviolet radiation (sunlight).Vitamin D is classified as a vitamin because it is required for metabolism and deficiency can result in disease, including rickets and osteoporosis.

Vitamin D is passively produced in skin exposed to ultraviolet B radiation. The cholesterol precursor, 7-dehydrocholesterol, is unstable to UVB light, and the B-ring of the steroid structure breaks open to produce the seco-steroid, vitamin D3, more formally known as cholecalciferol. Diet is usually a minor secondary source of vitamin D, but foods such as oily fish and eggs contain only very small amounts. It is almost impossible to obtain sufficient amounts of vitamin D from diet alone, without sunlight exposure.
Hope this helps it might be in a little too much depth but it is the book answer.
Timster

Answer 2

Only a guess?
THE SUN?