Vitamin D in health and disease: an insight into traditional functions and new roles for the 'sunshine vitamin'.

Vitamin D is unique among the vitamins in that man can synthesise it via the action of UV radiation upon the skin. This combined with its ability to act on specific target tissues via vitamin D receptors (VDR) make its classification as a steroid hormone more appropriate. While vitamin D deficiency is a recognised problem in some northern latitude countries, recent studies have shown that even in sunny countries, such as Australia, vitamin D deficiency may be more prevalent than first thought. Vitamin D is most well known for its role in bone health; however, the discovery of VDR on a wide variety of tissue types has also opened up roles for vitamin D far beyond traditional bone health. These include possible associations with autoimmune diseases such as multiple sclerosis and inflammatory bowel diseases, cancer, CVD and muscle strength. First, this paper presents an overview of the two sources of vitamin D: exposure to UVB radiation and food sources of vitamin D, with particular focus on both Australian and international studies on dietary vitamin D intake and national fortification strategies. Second, the paper reviews recent epidemiological and experimental evidence linking vitamin D and its role in health and disease for the major conditions linked to suboptimal vitamin D, while identifying significant gaps in the research and possible future directions for research.