Vitamin D is formed when a chemical reaction is formed on the human skin. When the skin is exposed to sun a steroid called 7-dehydrocholesterol is broken down. This leads to the formation of vitamin D.
"Sun induced vitamin D synthesis is greatly influenced by season, time of day, latitude, altitude, air pollution, skin pigmentation, sunscreen use, passing through glass and plastic, and aging. Vitamin D is metabolized sequentially in the liver and kidneys into 25-hydroxyvitamin D which is a major circulating form and 1,25-dihydroxyvitamin D which is the biologically active form respectively," says a research report.