A research team from the Northern California
Cancer Center, the University of Southern California, and Wake
Forest University School of Medicine has found that increased
exposure to sunlight – which increases levels of vitamin
D in the body -- may decrease the risk of advanced breast cancer.
In a study reported online this week in the American Journal
of Epidemiology, the researchers found that women with high
sun exposure had half the risk of developing advanced breast
cancer, which is cancer that has spread beyond the breast,
compared to women with low sun exposure. These findings were
observed only for women with naturally light skin color. The
study defined high sun exposure as having dark skin on the
forehead, an area that is usually exposed to sunlight.
The scientists used a portable reflectometer to measure skin
color on the underarm, an area that is usually not directly
exposed to sunlight. Based on these measurements, they classified
the women as having light, medium or dark natural skin color.
Researchers then compared sun exposure between women with
breast cancer and those without breast cancer. Sun exposure
was measured as the difference in skin color between the underarm
and the forehead.
In women with naturally light skin pigmentation, the group
without breast cancer had significantly more sun exposure
than the group with breast cancer. The fact that this difference
occurred only in one group suggests that the effect was due
to differences in vitamin D production – and wasn’t
just because the women were sick and unable to go outdoors.
In addition, the effect held true regardless of whether the
cancer was diagnosed in the summer or in the winter. The difference
was seen only in women with advanced disease, suggesting 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,” said Esther
John, Ph.D., lead researcher on the study from the Northern
California Cancer Center. “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.”
These new findings about breast cancer risk and sun exposure
based on skin color measurements are consistent with previous
research by John and colleagues that had shown that women
who reported frequent sun exposure had a lower risk of developing
breast cancer than women with infrequent sun exposure.
The researchers stressed that sunlight is not the only source
of vitamin D, which can be obtained from multivitamins, fatty
fish and fortified foods such as milk, certain cereals and
fruit juices. Women should not try to reduce their risk of
breast cancer by sunbathing because of the risks of sun-induced
skin cancer, they said.
“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,” said Gary Schwartz,
Ph.D., a co-researcher from the Comprehensive Cancer Center
at Wake Forest University School of Medicine.
“Since many risk factors for breast cancer are not modifiable,
our finding that a modifiable factor, vitamin D, may reduce
risk is important,” said Sue Ingles, Ph.D., a co-researcher
from University of Southern California Keck School of Medicine.
The researchers compared 1,788 breast cancer patients in
the San Francisco Bay area with a matched control group of
2,129 women who did not have breast cancer. They included
non-Hispanic white, Hispanic and African-American women, thus
women with a wide range of natural skin color and a wide range
of capacity to produce vitamin D in the body. Skin color is
an important factor that 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.
