Vitamin D is a class of compounds that are essential nutrients. Sunlight and diet are main sources of Vitamin D. Vitamin D3 is a form of vitamin D that is made in our skin upon exposure to sunlight. The inverse correlation between sun exposure and cancer mortality is the basis for the idea that vitamin D plays a protective role against cancer. Ongoing studies are examining the potential of vitamin D in reducing the risk of cancer, including a recurrence of breast cancer.
Vitamin D3
Two forms of vitamin D are significant to human health -- vitamin D2, ergocalciferol, which is made in plants, and vitamin D3, or cholecalciferol, which is synthesized in skin upon exposure to sunlight. Vitamin D3 is also found in some dietary sources, such as fatty fish. Vitamin D3 is biologically inert and is converted to an active form, called calcitriol, by modification in the liver and kidneys.
Mechanism of Action
A protein called vitamin D receptor -- or VDR -- mediates the cellular effects of vitamin D. VDR binds vitamin D and this complex and then interacts with specific regions of cellular DNA and other proteins to turn genes on and off. The altered pattern of gene function is responsible for the change in behavior of the cells. The intestine, skeleton, parathyroid glands and kidneys all contain vitamin D-responsive cells. Vitamin D regulates calcium and phosphate absorption and regulates immune function and bone mineralization. In addition, vitamin D acts on cells in other tissues, including the breast. Vitamin D also promotes cellular differentiation -- the process by which cells acquire specialized functions.
Breast Cancer Effects
Differentiation is often deregulated in cancers, and its restoration by vitamin D is one idea supporting its development as an anti-cancer agent. Studies in cultured cancer cells and rodent models of breast cancer and population-based human studies, summarized in "The Journal of Nutrition" in 2003, suggest a protective role for vitamin D in breast cancer. Vitamin D causes growth arrest and even death of breast cancer cells. Vitamin D also increases the susceptibility of cancer cells to other treatments, like chemotherapy. Rats fed a vitamin D-deficient diet are more prone to developing chemical-induced breast cancer. Observational studies in humans link states of low vitamin D and estrogen levels, such as in postmenopausal women, with increased risk of breast cancer.
Breast Cancer Recurrence
Limited information is available on the role of vitamin D in preventing breast cancer recurrence. Those with breast cancer are at a higher risk of recurrence, especially in the first few years after treatment. This risk increases with certain factors, such as a larger tumor size and younger age, less than 40, of first diagnosis. One study, published in "Journal of Clinical Oncology" in 2008, found that women with lower vitamin D levels at the time of diagnosis had significantly higher risk of cancer recurrence, spreading and poorer survival rates, compared with women with adequate levels of vitamin D. Further studies are needed to assess the benefits of vitamin D and related compounds in reducing recurrence risk.
Concerns
Vitamin D toxicity can cause symptoms like nausea and weakness. Hypercalcemia -- abnormally high calcium levels in the blood --is a serious effect of vitamin D toxicity. Hypercalcemia can cause calcification of blood vessels and tissue, damaging key organs, like the heart. Hypercalcemia also limits use of high-dose vitamin D in cancer. Scientists are developing synthetic nonhypercalcemic vitamin D-like compounds to circumvent this problem.
Although sunlight exposure simulates vitamin D synthesis, sustained heat on the skin is thought to promote degradation of vitamin D, preventing toxic accumulation. The Office of Dietary Supplements suggests a recommended daily allowance of 600 IU, or15 mcg of vitamin D for adults. Consult your physician about vitamin D-related concerns specific to your health.
References
- Office of Dietary Supplements; National Institutes of Health; Vitamin D Fact Sheet
- "Endocrine-related Cancer"; Kay W. Colston and Christine M. Hansen; March 2002
- "Brazilian Journal of Medical and Biological Research"; P. Bortman, et al.; January 2002
- "The Journal of Nutrition"; JoEllen Welsh, et al.; July 2003
- "Journal of Clinical Oncology"; Pamela J. Goodwin, et al.; May 2008
