In the human body, the main role of vitamin D is to maintain healthy bones. Over the past few decades, it has also been gradually recognized that vitamin D has a number of other important roles in the body. Vitamin D has an effect on any cell in the body that expresses the vitamin D receptor. For most people, the primary source of vitamin D is sunlight exposure. However, in Australia, people are avoiding light exposure due to an overemphasis on skin cancer prevention. In this situation, exogenous supplements become their main source of vitamin D. Normal levels of vitamin D are ≥75 nmol/L, below 50 nmol/L is considered deficient and below 75 nmol/L is considered insufficient. Vitamin D deficiency can lead to bone diseases such as osteoporosis and osteochondrosis. Recently, vitamin D has been found to play a role in a variety of diseases such as tumors, diabetes, MS, and depression. The strongest evidence is associated with colorectal cancer, and evidence is increasing in breast cancer. It is not that low vitamin D levels cause cancer, but there is a link between them. Of interest is whether low vitamin D affects tumor prognosis. In cellular studies of cancer, vitamin D has been found to stop or slow down tumor growth, and it is thought to have an anti-proliferative effect. Moreover, vitamin D contributes to normal cell death. Cancer cell death is a problem in some forms of cancer. This may be the mechanism by which low vitamin levels are associated with cancer. It was previously speculated that most people could maintain vitamin D levels in the normal range through occasional sunlight exposure, and no specific investigation has been done. However, recent research by Kellie Bilinski has shown an alarmingly high prevalence of vitamin D deficiency. In the spring, nearly 70% of individuals had vitamin D deficiency, with the highest prevalence in women aged 20-39 years, and other risk factors coming from socioeconomic backgrounds and living in large cities. The cause of vitamin D deficiency in these individuals may be related to the use of sunscreen by women in this age group to avoid sun exposure, consistent with the Slip-Slop-Slapcampaign in Australia. Another explanation is that most of them are probably young mothers who spend a lot of time indoors. The study assumes that Australians get plenty of light, and while not the only study to find high prevalence of vitamin D deficiency, it is the only large study to look at vitamin D in Australians. The study found that vitamin D deficiency was most prominent in the spring. This is because UV levels are lowest in the spring (perhaps because people don’t go outside as much due to the cold) and because vitamin D deficiency is more prominent in the spring after the depletion of vitamin D reserves during the winter. The results of this study could add evidence to the development of vitamin D testing and supplementation guidelines. Since then, at the Western Breast Cancer Institute, KellieBilinski et al. have established a vitamin D research program and are recruiting for the VIOLET study to look at the impact of vitamin D levels on the diagnosis of prognostic markers in breast cancer. They are planning randomized controlled trials to look at the effects of vitamin D supplementation on chemotherapy-related side effects. And an online vitamin D deficiency risk assessment calculation tool is also being developed. It is hoped that such a tool will help people determine if vitamin D deficiency may be present and take appropriate steps to supplement vitamin D levels if necessary. kellie Bilinski et al. believe that guidelines on reasonable sun exposure are needed to avoid a proliferation of vitamin D deficiency due to the emphasis on skin cancer prevention.