they say that we get vitamin D from the sun . well obviously the vitamin D is not in the photon , so when the photon hits our skin does it start a bio-chemical reaction , what does I take and break it down into vitamin D.
Most people meet their vitamin D needs through exposure to sunlight [6,27]. Ultraviolet (UV) B radiation with a wavelength of 290-315 nanometers penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D3, which in turn becomes vitamin D3 [11,27-28]. Season, geographic latitude, time of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis . The UV energy above 42 degrees north latitude (a line approximately between the northern border of California and Boston) is insufficient for cutaneous vitamin D synthesis from November through February ; in far northern latitudes, this reduced intensity lasts for up to 6 months. Latitudes below 34 degrees north (a line between Los Angeles and Columbia, South Carolina) allow for cutaneous production of vitamin D throughout the year