A fat-soluble vitamin (actually, it's a hormone) that may have antioxidant, anti-inflammatory, and immune-stimulating benefits for skin, among other preventive traits. Vitamin D formed in the skin by sunlight (primarily UVB radiation), or may be obtained from fortified foods and oral supplements.
A growing body of research has demonstrated vitamin D is essential for health. The media-emphasized main conflict surrounding this vitamin (which is actually a hormone, not a vitamin) revolves around sun exposure and whether or not using sunscreen hinders our bodies' ability to manufacture vitamin D. Vitamin D deficiency can be a serious health problem and sunlight can be the most abundant, natural way for our bodies to create it. But can our bodies utilize the sun to create vitamin D if we regularly use sunscreen? The answer is a resounding yes. Despite news stories to the contrary, several large, controlled studies have shown that vitamin D deficiency does not result from ongoing regular sunscreen use. Aside from sun exposure, vitamin D supplementation is still a good idea because even with sun exposure, depending on where you live and how strong the UV light is, we can still become deficient. (Sources: The Journal of the American Osteopathic Association, August 2003, pages 3-4; American Journal of Clinical Dermatology, March 2002, pages 185-191; Dermatology, January 2001, pages 27-30; and British Medical Journal, October 1999, page 1066). Skin cells, along with the cells of our other major organs, have receptor sites for vitamin D. These sites allow the conversion of vitamin D (via sun exposure) to its active form. Thus far, the most substantiated information pertaining to vitamin D and skin involves its role as a potential treatment for psoriasis and its involvement in the prevention of skin cancers. If you suffer from psoriasis, you should know that the active form of vitamin D, 1,25-dihydroxyvitamin D3 (also known as calcitriol) can help control the proliferation of skin cells that lead to psoriatic lesions and modulate the immune system's hyper-response to this disorder. Various topical prescription treatments using vitamin D derivatives are available, but making sure you consume enough vitamin D via diet or supplementation plays a role, too.
Vitamin D deficiency, which is actually quite common, can lead to problems with cell growth and differentiation throughout the body, including skin. Some researchers are working to prove that long-term vitamin D deficiency leads to skin malignancies (skin cancer). The protective effects of topically-applied vitamin D analogues (chemical compounds similar to the vitamin D our skin naturally produces in the presence of sunlight) have been demonstrated in-vivo (on a live subject), though studies have been conducted on mouse, rather than human, skin. Understandably, researchers believe that further exploration into how topically-applied vitamin D may protect skin cells from the DNA damage that leads to cancer is worthwhile. In the future, it is possible that sunscreen recommendations will be accompanied by new vitamin D dietary guidelines to ensure proper levels are achieved.
Sources for the above information: The Journal of Steroid Biochemistry and Molecular Biology, March 2010, Epublication; Journal of Bone and Mineral Metabolism, March 2010, pages 117–130; The British Journal of Dermatology, February 2010, Epublication; and October 2009, pages 732–736; Dermatologic Therapy, January 2010, pages 48–60; Journal of Immunology, November 2007, pages 6,273–6,283; The British Journal of Dermatology, October 2005, pages 706-714; Current Rheumatology Reports, October 2005, pages 356-364; Photochemistry and Photobiology, E-Publication, February 1, 2005; The Journal of Steroid Biochemistry and Molecular Biology, E-Publication, July 19, 2005 and Experimental Dermatology, December 2004, page 11).