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Exposure To Sunlight Lowers Risks Of Four Cancers

Americans who live in the sunniest parts of the United States have higher mortality from skin cancer but significantly lower mortality from female breast and ovarian cancer, male prostate cancer and colon cancer in both men and women.

Researchers from the US National Cancer Institute, Bethesda, Maryland, used death certificates from 24 US states to glean the numbers of deaths from five cancers between 1984 and 1995. Controls, age frequency matched to a series of cases, excluded deaths from cancer and certain neurological diseases.

Dr Michael Freedman and colleagues at the NCI Radiation Epidemiology Branch point out it is well established that exposure to sunlight contributes to non-melanoma skin cancer. "By contrast, several ecological studies suggest that sunlight may protect against female breast, ovarian, prostate, and colon cancer, all diseases that contribute to a substantially higher proportion of cancer mortality in the western industrialized world."

Some studies have suggested an association between circulating vitamin D in blood, which is largely derived from sunlight, or dietary vitamin D and colorectal, prostate and female breast cancers.

The researchers used multiple logistic regression for a model that included age, sex, race, residential exposure to sunlight (based on region), and socioeconomic status, occupational exposure to sunlight, and physical activity. The last three were based on usual occupation.

As expected, there were higher numbers of deaths from skin cancer in the sunnier areas of residence, but death from the other four cancers was significantly lower in those same climates. Working outdoors in a very sunny environment was associated with fewer deaths from breast and colon cancer, and the geographical areas of greatest sunlight showed the strongest relationship. This finding was independent of the amount of physical activity required for the job.

A risk of death from all cancers, except skin cancer, grew stronger with increasing socioeconomic status.

"Recent experimental studies suggest biological plausibility of a protective effect of vitamin D on cancer, particularly for breast and colon cancer," said Dr Freeman. "Most notably, hormonal vitamin D, 1,25(OH)2 D, has been shown to promote cell differentiation and retard or terminate proliferation of human cancer cells in vitro, including breast and colon cancer cells.

"The mechanisms by which 1,25(OH)2 D may produce an anticarcinogenic, prodifferentiation effect include inhibition of growth, angiogenesis, and metastasis."

Researchers conclude that much remains to be explained about the biology of sunlight and cancer: "Our findings of significant negative associations between both residential and occupational exposure to sunlight and mortality from female breast and colon cancer warrant additional study."
Occupational and Environmental Medicine 2002; 59:257-62.

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