The great outdoors can soothe the soul, but new research suggests that working outside might also guard against breast cancer.

The study wasn’t designed to say how working outside affects chances of developing breast cancer, but vitamin D exposure may be the driving force, the researchers suggested.

“The main hypothesis is that sun exposure through vitamin D production may decrease the risk of breast cancer after age 50,” said study author Julie Elbaek Pedersen, of the Danish Cancer Society Research Center in Copenhagen. Vitamin D deficiency has been linked to a host of diseases and conditions including breast cancer.

Vitamin D is often called the “sunshine vitamin” because your body produces it when exposed to the sun’s ultraviolet B rays. The body mainly makes vitamin D in the middle of the working day so outdoor workers are exposed to considerably higher levels than those who work indoors, Pedersen said.

“Women who work outdoors may regularly be exposed to sunlight and thereby have more sufficient long-term levels of vitamin D compared with women working indoors,” she said.

You don’t need much sun exposure to make adequate amounts of D. “Maximum daily vitamin D levels are secured after only minutes in the sun in the summertime, and more time will not increase the levels further,” Pedersen said.

In recent years, the push to wear sunscreen and avoid sun exposure to stave off skin cancer has led to concerns of widespread vitamin D deficiency. But “spending limited time outdoors in Denmark or countries with a comparable latitude may be compliant with most advice regarding sensible behaviors in the sun [use of sunscreen, seek shade particularly around midday and avoid spending prolonged time in the sun] to reduce skin cancer risk,” she explained.

When researchers compared data including work history from women younger than 70 with breast cancer from the Danish Cancer Registry to their same-aged counterparts who were cancer-free, they found that working outdoors was not associated with a lower risk of breast cancer in general. But it did lower risk among women who developed cancer after age 50.

Women who reported on-the-job exposure to sunlight for 20 or more years were 17% less likely to develop breast cancer after they turned 50, and those women with the highest level of cumulative exposure to sunlight throughout their life were 11% less likely to develop this cancer, the study authors found.

The new study did have some limitations. There was no information on vitamin D through diet or use of supplements, leisure-time sunlight exposure, or other lifestyle factors that may increase risk for breast cancer, such as the use of birth control pills, hormone replacement therapy, alcohol, obesity and lack of exercise.

The findings were published online Feb. 2 in the journal Occupational & Environmental Medicine.

Breast cancer experts urged caution in interpreting the findings.

“Looking at actual vitamin D levels in coordination of lifestyle in women might be the only way to prove the higher vitamin D levels decrease the risk of breast cancer,” said Dr. Stephanie Bernik, chief of breast service at Mount Sinai West in New York City. “Perhaps women that have occupations that place them outside are exercising more and leading healthier lifestyles,” she speculated.

If the findings are validated, this is good news, said Dr. Alice Police, the Westchester regional director of breast surgery at Northwell Health Cancer Institute in Sleepy Hollow, N.Y.

“The results are very hopeful that spending some time outside can lower your breast cancer risk,” Police said. “You don’t have to spend a lot of time outside and, in the summer, you may store up enough vitamin D to keep you safe throughout the winter,” she said. “Go outside and take a walk. This is something we all can do for our mental, physical and breast health.”

More information

Learn more about breast cancer risk at the U.S. Centers for Disease Control and Prevention.

SOURCES: Julie Elbaek Pedersen, MSc, Danish Cancer Society Research Center, Copenhagen; Stephanie Bernik, MD, chief, breast service, Mount Sinai West, and associate professor, surgery, Icahn School of Medicine, Mount Sinai, New York City; Alice Police, MD, Westchester regional director, breast surgery, Northwell Health Cancer Institute, Sleepy Hollow, N.Y.; Occupational & Environmental Medicine, Feb. 2, 2021, online