New research shows that working outdoors may provide some protection against late onset breast cancer for women older than 50. People who work outdoors are exposed to more sunlight compared with those who work indoors; therefore, they have higher levels of endogenous vitamin D. Those increased levels may offer protection against cancer, according to a group of Danish researchers.

Vitamin D is obtained from several sources as well as endogenously created within our bodies. When solar ultraviolet B radiation (UVR) interacts with 7-dehydrocholesterol in the skin, vitamin D is produced. This is the most common source of vitamin D. Another way to obtain vitamin D is from supplements and other dietary sources. Although sunlight is vital for the maintenance of normal vitamin D levels, women and men are now more restrictive about natural sunlight exposure because of an increasing awareness of skin cancer prevention. Protection from the sun has become paramount to avoid developing solar keratoses, squamous and basal cell carcinomas, melanoma, and other serious skin conditions. But while we are protecting ourselves from the sun, we are also lessening the possibility of benefiting from it.

Coincident with the increase in sun avoidance is the upsurge in computer use for work and pleasure, prompting people to stay indoors. The Danish researchers add another circumstance that has increased: the incidence of breast cancer over the past 50 years. Although many risk factors are cited, the researchers acknowledged that genetic, reproductive, medical, and lifestyle changes cannot be solely responsible for the escalation in breast cancer cases. They hypothesize that “relatively recent modern behavioural patterns of sun avoidance, contributing to vitamin D deficiency worldwide, may be associated with the increase in breast cancer incidence rates.”

To explore this hypothesis, the researchers identified 38,375 women younger than 70 years with primary breast cancer in the Danish Cancer Registry. Each woman was compared with 5 female controls who were born in the same year and selected at random from the Danish Civil Registration System. They obtained the women’s full employment history from data available through Danish pension funds. Employment data was used to evaluate each woman’s exposure to sunlight while on the job. The researchers then evaluated each woman’s risk for developing breast cancer and found no association between breast cancer risk and exposure to sunlight while working.


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However, they did find that longer duration of UVR exposure and higher cumulative exposure in those women older than 50 years resulted in an inverse association with the risk of developing breast cancer. The women whose jobs resulted in their being exposed to UVR for 20 or more years had a 17% lower risk of developing breast cancer.

The researchers call for further studies that include important lifestyle factors. This study was limited by a lack of information on obesity, alcohol consumption, amount of exercise, and oral contraceptive and/or hormone replacement therapy use among the women. These lifestyle factors might have confounded their results.

Reference

Pedersen JE, Strandberg-Larsen K, Andersson M, Hansen J.Occupational exposure to solar ultraviolet B radiation and risk of subtypes of breast cancer in Danish women. Occup Environ Med. Published online February 2, 2021. doi:10.1136/oemed-2020-107125