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Sunlight—A Cancer Drug?

2 February 2005 (All day)
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Healthy tan? Sunlight may help ward off some types of cancers.

Read the back of your tanning lotion, and the implication is clear: Excessive sun exposure can lead to skin cancer. Two new studies, however, provide evidence that sunlight may actually help fight some kinds of cancer.

Ultraviolet (UV) radiation in sunlight is known to mutate DNA and can lead to skin cancer. Yet circumstantial evidence has been building that sun exposure can also be beneficial. For example, mortality from breast, prostate, and colon cancer tends to be more severe at higher latitudes, where less UV penetrates the atmosphere. Lab studies have also shown that vitamin D--synthesized in the skin after UV exposure--has anticancer effects on many kinds of cells. "All the arrows pointed to something about past sun exposure that was protective or had a beneficial effect on [cancer] mortality," says Kathleen Egan of Vanderbilt University Medical Center in Nashville, Tennessee.

In the 2 February issue of the Journal of the National Cancer Institute, two groups present supporting evidence. A team led by epidemiologist Karin Ekström Smedby of the Karolinska Institute in Stockholm, Sweden, looked at the role of sun exposure in lymphoma, which, like skin cancer, has become more common over the past few decades. They interviewed 3740 patients diagnosed with malignant lymphomas and 3187 healthy controls. Those without the disease reported more sunbathing, vacations to sunny destinations, and even more sunburns than did patients with lymphoma. The data indicate that sun exposure can lower the risk of lymphoma by 30% to 40%. "It's not a small reduction," says Ekström Smedby.

The other study, led by epidemiologist Marianne Berwick of the University of New Mexico, examined the role of sun exposure in melanoma, the most serious kind of skin cancer. After interviewing 528 patients diagnosed with the disease and following their health for 5 years, researchers discovered that those with a history of more sun exposure were half as likely to die from melanoma than those who had had less sun exposure. One reason could be the benefits from increased vitamin D production, the team speculates, but another possible explanation is that sun exposure stimulates DNA repair, leading to less aggressive melanomas.

Both teams caution that their findings need to be replicated. Melanoma patients and people with family histories of skin cancer should remain particularly careful about sun exposure, Egan adds, because of the greater risk of further melanomas. But the two studies do support the idea that vitamin D is protective, she says. "Sunlight might not always be a bad actor in cancer."

Related sites
The Berwick et al. study
The Ekström Smedby et al. study
Editorial from JNCI
Information about melanoma

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