"It's been estimated that about 80 to 100 percent of an individual's requirement comes from exposure to sunlight. The skin absorbs the ultraviolet energy from the sun and then converts it to vitamin D."

"this vitamin is critically important for maintaining normal calcium in the blood and for bone health. The vitamin plays a crucial role in most metabolic functions and also, muscle, cardiac and neurological functions."

"Moreover, there is evidence that vitamin D may have subtle but profound effects on regulating cell growth and on our cardiovascular and immune systems. There is a strong association of sunlight exposure and increased blood levels of vitamin D with a decreased risk of many common cancers: colon, breast, prostate, ovarian."

From -  http://query.nytimes.com/gst/fullpage.html?sec=health&res=9402E2DA1239F93BA15752C0A9659C8B63