Esther John, Ph.D., Northern California Cancer Centre, and co-researchers Gary Schwartz, Ph.D., Comprehensive Cancer Centre at Wake Forest University School of Medicine and Sue Ingles, Ph.D., University of Southern California Keck School of Medicine found that women with high sun exposure had half the risk of developing advanced breast cancer compared to women with low sun exposure.
In the study, high sun exposure was defined as having dark skin on the forehead and the findings were observed only for women with naturally light skin colour.
For the study, the researchers compared 1,788 breast cancer patients with a control group of 2,129 women who did not have breast cancer. The women who participated in the survey had a wide range of natural skin colour and a wide range of capacity to produce vitamin D in the body.
Skin colour determines how much vitamin D is produced in the body after sun exposure. Dark-skinned individuals produce up to 10 times less vitamin D than light-skinned individuals for the same amount of time spent in the sun. People with darker skin are also more likely to be vitamin D deficient than people with lighter skin.
In order to determine the actual skin colour of the participants the scientists used a portable reflectometer on the underarm, because that area is not directly exposed to sunlight. The female participants were classified as having light, medium or dark natural skin colour on the basis of the measurements.
After classification the researchers compared sun exposure between women with breast cancer and those without breast cancer. Sun exposure was measured as the difference in skin colour between the underarm and the forehead. Women with naturally light skin pigmentation who were without breast cancer had significantly more sun exposure than the group with breast cancer.
Since the difference occurred only in one group, the researchers suggested that the effect was due to differences in vitamin D production and not just because the women were sick and unable to go outdoors.
The difference was seen only in women with advanced disease, which suggested that vitamin D may be important in slowing 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," John said.
"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," he said. The researchers said that besides sunlight - multivitamins, fatty fish and fortified foods such as milk, certain cereals and fruit juices could also provide Vitamin D.
Because of the risks of sun-induced skin cancer scientists said that women should not try to reduce their risk of breast cancer by sunbathing. "If future studies continue to show reductions in breast cancer risk associated with sun exposure, increasing vitamin D intake from diet and supplements may be the safest solution to achieve adequate levels of vitamin D," Schwartz said.
"Since many risk factors for breast cancer are not modifiable, our finding that a modifiable factor, vitamin D, may reduce risk is important," Sue Ingles, Ph.D., a co-researcher from University of Southern California Keck School of Medicine said.
The study is reported online in the American Journal of Epidemiology.