与你所在地区纬度,紫外线辐射强度有关,看MAP阴影部分,几乎涵盖了美国2/3。。
Latitude and vitamin D production in the skin
Except during the summer months, the skin makes little if any vitamin D from the sun at latitudes above 37 degrees north (in the United States, the shaded region in the map) or below 37 degrees south of the equator. People who live in these areas are at relatively greater risk for vitamin D deficiency.