New studies show a cancer paradox: Sun exposure — a known cause of skin cancer — has an anti-cancer effect.

How this happens is not quite clear. But two new studies in the Feb. 2 issue of the Journal of the National Cancer Institute show that these night-and-day effects of sunshine aren't moonshine.

A large study of the fair-skinned denizens of Sweden and Denmark shows that those with the most sunburns, the most beach vacations, and/or the most sunbathing — five to 10 years ago and at age 20 — were least likely to get malignant white blood cell cancers.

And U.S. researchers report the surprising finding that people who get the most deadly kind of skin cancer — melanoma (search) — are less likely to die of the disease if, in the past, they'd spent a lot of time in the sun.

The unenviable task of making sense of all this falls to melanoma expert Kathleen M. Egan, ScD, associate professor of medicine at Vanderbilt Ingram Cancer Center in Nashville, Tenn., and colleagues. Their JNCI editorial accompanies the two studies.

"Sunlight, we know, is an established human carcinogen (search)," Egan tells WebMD. "These two investigations … report findings that sunlight is associated with a beneficial influence on cancer. That flies in the face of what we know about sunlight in terms of carcinogenic influence — but it is, in fact, in line with earlier descriptive data of a preventive effect on prostate cancer, colon cancer, and even breast cancer."

The earlier studies to which Egan refers show that U.S. cancer death rates go up the farther north a person lives.

The new studies, she says, are the first to show that these peculiar reports really are linked to sun exposure and not to racial or economic differences.

"No one is suggesting that anyone strip off whatever garments they might take off to get themselves dosed with sunlight," Egan says. "The important message is that that this anti-cancer effect is very unlikely to be sunlight itself. It is more likely to be vitamin D generated by sun exposure. We all need to stay tuned and let the research community go to work and try to tease out whether this is really a vitamin D effect."

Sunburned Scandinavians Get Fewer Lymphomas

Karin Ekström Smedby, MD, of the Karolinska Institute in Stockholm, Sweden, led a study that matched 3,740 Scandinavian patients with malignant lymphomas and leukemia with 3,187 similar people without lymphoma (search).

The lymphomas included non-Hodgkin's lymphoma (search) and Hodgkin's disease (search); the type of leukemia present was chronic lymphocytic leukemia (search).

The researchers found that those who did the most sunbathing and got the most sunburns five to 10 years ago — and those who took the most vacations in sunny places far from Scandinavia — had up to 40 percent fewer non-Hodgkin's lymphomas than those who stayed out of the sun. The findings were similar, but statistically less powerful, for other less-common lymphomas.

Ekström Smedby spoke to WebMD by telephone from Bolivia. She says the study results "definitely show some benefit for some cancers." But she urges people to remember that the link between skin cancer and sun exposure is very well established.

"It would still be wrong to be running outside and taking off your shirt and tanning and burning without limit," Ekström Smedby tells WebMD. "But it also is wrong to stay indoors all the time and not expose yourself to the sun. The best way would be the middle."

Which cancers respond to sunlight is a matter for further study. It's likely, Ekström Smedby says, that different people will respond in different ways.

"The solution is not to give general advice to all people," she says. "If these results are confirmed and we learn about the mechanisms by which sun exposure protects against cancer, the advice has to be individual-based depending on whether your skin type and family history means you're at particularly high risk of skin cancer."

Like Egan, Ekström Smedby suggests that vitamin D has a lot to do with sunlight's anti-cancer effect. Human skin makes a lot of vitamin D when exposed to sunshine.

Sun-Caused Melanoma Less Deadly?

Melanoma expert Marianne Berwick, PhD, MPH, director of cancer epidemiology and prevention at the University of New Mexico Cancer Research Facility in Albuquerque, N.M., didn't set out to measure sun exposure. She and her research team only wanted to see whether people who check their skin regularly cut their risk of dying from skin cancer.

As part of their study of 528 melanoma patients, the researchers checked the patients’ noncancerous skin for sun-induced skin changes — something doctors call solar elastosis (search).

Berwick's team did, indeed, find that "skin awareness" cuts a person's risk of dying from melanoma. To their surprise, they also found that skin cancer patients who reported the most sun exposure — and who had the skin to prove it — were least likely to die of melanoma.

Berwick is careful to note that sun exposure doesn't help cure existing melanomas. In fact, people who have one melanoma are at very high risk of getting another one if they get too much sun.

"We don't recommend people with melanoma go out into the sun," Berwick tells WebMD. "Their risk of second melanoma is pretty high. If they continue with high sun exposure, it is likely they will get a second melanoma. A little bit of sun, maybe 15 minutes a day is fine. But they would be very wise to be cautious."

As do Egan and Ekström Smedby, Berwick says that vitamin D is the real anticancer agent. She recommends that everyone be sure to get plenty of this essential vitamin — and to get a bit of sun, too.

She also suggests that everyone — especially those who get a lot of sun — spend a few minutes every month inspecting their skin for possible cancers.

"This means looking at yourself from head to toe for suspicious spots — and you'll need a partner to look at your back," Berwick says. "You should look even in places where the sun does not shine — even bottoms of feet and between your toes and under your arms — because melanomas don't always appear in obvious places. If you find it early, you could save your life."

A suspicious spot, Berwick says, usually:

—Has several different colors — black, brown, pink, red, and/or white

—Is irregular in shape

—Has an irregular border

—Is bigger than a pencil eraser

"Nice round moles are usually quite benign," Berwick says. "But if you have a funny feeling about a spot or mole, go see a dermatologist."

By Daniel J. DeNoon, reviewed by Brunilda Nazario, MD

SOURCES: Berwick, M. Journal of the National Cancer Institute, Feb. 2, 2005; vol 97: pp 195-199. Ekström Smedby, K. Journal of the National Cancer Institute,Feb. 2, 2005; vol 97: pp 199-209. Egan, K.M.Journal of the National Cancer Institute, Feb. 2, 2005; vol 97: pp 161-163. Kathleen M. Egan, ScD, associate professor of medicine, Vanderbilt Ingram Cancer Center, Nashville, Tenn. Marianne Berwick, PhD, MPH, professor of internal medicine; and chief, division of epidemiology; and director, cancer epidemiology and prevention, University of New Mexico Cancer Research Facility, Albuquerque. Karin Ekström Smedby, MD, Karolinska Institute, Stockholm, Sweden.