Well, I'm not buying it. The best source of Vitamin D comes from sun exposure, and nobody, especially in these latitudes where the sun's rays are strongest, should be deficient. And especially if they spend time outside. I only use sunscreen when I plan to be in the garden for any length of time, but I'm not fanatical and I spend a few minutes here and there most days in sunlight. I have (light) tan lines to prove it.
There are all sorts of factors that govern the manufacture of D in the skin, including intensity of the rays and the color of the skin (the darker, the more sun exposure required). Here, for example, is what WebMD says:
Many people living in the Southern United States can get enough vitamin D by getting about 10-15 minutes of sun exposure on their arms and face a few times a week -- as long as they don't use sunscreen, which blocks some of the UV rays necessary to make the vitamin.
That is not difficult to do in South Florida.
So, dear readers, are you Vitamin D deficient? Do you think the sunscreen explanation is credible? Do weigh in.