Why do people sometimes call sunlight “vitamin D”?

clipped from www.popsci.com You need activated vitamin D3, also known as calcitriol, for the maintenance of proper blood chemistry and healthy bone tissue. You can get the vitamin from supplements, but it’s way cheaper (not to mention more pleasant) to make it the old-fashioned way—by going outside and soaking up some rays. The concept is basic enough: A compound in the skin reacts with ultra-violet light from the sun to produce vitamin D precursor molecules. Two carbon atoms in the previtamin then spontaneously rearrange to create the bone-bulking vitamin D3. This all happens quickly; after just a few minutes in the sun, the body has absorbed enough light to produce several times the necessary daily intake of the vitamin. And what if you don’t get enough vitamin D? If you’re a kid, rickets. If you’re an adult, adult rickets. Time to bust out the sunglasses.