The medical world is now hot with the subject of Vitamin D deficiency--they now test for it in routine blood work, and believe a deficiency not only can lead to rickets, but that even higher than currently recommended amounts are needed to prevent heart disease and strokes.
Think of it, Vitamin D, is a sunshine vitamin, just having your skin exposed to the sun for reasonable periods can increase your general levels of health. What is sly if not exposed skin! We're ahead of the game guys, at least the ones who don't wear hats all the time. Sly, it's the healthy thing to do.