The study, published in the American Journal of Epidemiology, found women with high sun exposure had half the risk of developing advanced breast cancer as women with low sun exposure.

The study defined high sun exposure as having darker skin on the forehead -- an area usually exposed to sunlight – compared to skin color of the underarm.

In women with naturally light skin pigmentation, the group without breast cancer had significantly more sun exposure than the group with breast cancer. The difference -- seen only in women with advanced disease -- suggests vitamin D may help slow the growth of breast cancer cells.

"We believe that sunlight helps to reduce women’s risk of breast cancer because the body manufactures the active form of vitamin D from exposure to sunlight," lead study author Esther John, of the Northern California Cancer Center, in Fremont, Calif., said in a statement.

"It is possible that these effects were observed only among light-skinned women because sun exposure produces less vitamin D among women with naturally darker pigmentation."