Researchers at the University of California have shown a clear association between exposure to sunlight - specifically ultraviolet B (UVB) light - and breast cancer.
UVB exposure triggers production of vitamin D3, a form of vitamin D that's found in food and supplements.
In this new study, worldwide data on breast cancer incidence, mortality and prevalence was analyzed based on the proximity to the equator.
Breast cancer incidence was highest in people who live in the most northern and most southern parts of both hemispheres.
Even after controlling for other breast cancer risk factors - like high meat and alcohol consumption, and low vegetable intake - low vitamin D status from lack of exposure to sunlight showed a strong association with higher breast cancer incidence.
Earlier studies have shown that intake of food sources of vitamin D - like oily fish and milk - could reduce breast cancer risk by up to 35 percent.
The study is published in the May-June 2008 issue of The Breast Journal.
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