It puzzles even scientists, but new research suggests that the main cause of deadly skin cancer—sunlight—might also protect against the disease.

The secret, say experts, is in the skin absorbing the right amount of ultraviolet B (UVB) light—enough to stimulate a healthy. vitamin b-linked immune response but not so much that it boosts skin cancer risk.

The Study

In their study, a group led by immunologists at Stanford University worked with cells in the lab and discovered a biochemical chain of events that appears to link sunlight exposure to the skin's own immune defenses.

The researchers started from an old notion that an inactive precursor of vitamin D. called vitamin 173, is generated in the skin in response to sun exposure. Specifically, a short-wavelength form of UV light, called UVB, is responsible for D3 generation.

Through contact with various enzymes in the liver and kidneys. the body turns D3, usually inert and powerless, into an active compound called 1,25(OH)2D3. And that's where the immune-system connection kicks in, say the Stanford scientists.

The Results

They found that the new compound 1,25 (OH) 2D3 "signaled (immune) T-cells," pushing them to migrate back to specific sites in the skin's epidermis. Once there, these powerful immune system agents stand on guard against infection and even cancer, say the researchers.

Bottom line: "The same wavelengths of sunlight that are most potent in inducing skin cancer—UVB—are also the wavelengths that produce this vitamin D precursor, D3," says Dr. Martin Weinstock, chairman of the skin cancer advisory group at the American Cancer Society. And it's D3 that's the catalyst that gets the whole chain of events rolling.

Weinstock, also a professor of dermatology and community health at Brown University, says this finding is about to change the American Cancer Society's long-standing recommendation to avoid intense sunshine and always wearing sunscreen when outdoors.

Past Findings

But the Stanford study follows earlier research, published in 2005, that noted melanoma patients with higher levels of daily sun exposure actually had a better chance of survival than patients who spent less time in the sun.

"A little bit of sunlight is good for people, but I think that one of the problems that the American Cancer Society and dermatologists have is, how do you define what a little bit is?" says that study's lead, epidemiologist Marianne Berwick of the University of New Mexico's Cancer Research and Treatment Center. “How do you tell people that it's OK to have a little hit of sunlight but not too much?"

The Role Of Vitamin D

Nutritionists have known for decades that sunlight stimulates vitamin D production in the skin. In fact, this natural process is the body's major source of the nutrient. A proper amount of vitamin D is crucial to bone health, "and there's also a bunch of evidence that vitamin D may have a role in preventing colon cancer, although there's still some controversy about that," says Weinstock.

Proper Defense

So, how much sunlight is enough to get the ideal amount of vitamin D?

Katharine Tallmadge, a Washington, DC, dietitian and a spokeswoman for the American Dietetic Association, suggests that most people can probably get the US Department of Agriculture's recommended 400 daily Ills of vitamin D by spending 30 to 60 minutes outdoors each day.

Expert Agrees

Kathleen Egan, a professor of epidemiology at the H. Lee Moffitt Cancer Center and Research Institute in Tampa, Florida, agrees that it's not difficult for people to soak up the sun's goodness without boosting their cancer risk. The skin actually creates an amazing amount of vitamin I)," she says. it doesn't take much exposure to make enough of the vitamin D that certainty needed to preserve bone health, for example.

Can the Sun Stop Breast and Colon Cancer?

Sunlight may help protect against colon, breast and other cancers.

Theory: Ultraviolet (UVB) rays trigger production of vitamin D in the skin, which protects against cancer by reducing cell proliferation and angiogenesis (development of new blood vessels). Inadequate exposure to UVB rays has been blamed for about 45,000 premature cancer deaths.

Self-defense: Spend some time every day in the sun without sunscreen, but not enough time to cause reddening or burning of the skin.

Other vitamin D sources: Cold-water fish, such as sardines and salmon.

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