A Little Sun Might Shield Against Skin Cancer

In moderation, it may mobilize disease-fighting vitamin D, scientists say.

By E.J. Mundell
HealthDay Reporter
Monday, January 29, 2007; 12:00 AM

Copyright © 2007 ScoutNews, LLC. All rights reserved.

MONDAY, Jan. 29 (HealthDay News) -- It's a puzzle to scientists, but a new study suggests that the main cause of deadly skin cancer -- sunlight -- might also help protect against the disease.

The key could lie in the amount of ultraviolet B (UVB) light the skin absorbs -- enough to stimulate a healthy, vitamin D-linked immune response in the skin but not so much that it boosts skin cancer risk.

"I do think that 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?" said skin cancer researcher Marianne Berwick, chief of epidemiology at the University of New Mexico's Cancer Research and Treatment Center. "How do you tell people that it's OK to have a little bit of sunlight but not too much?"

In 2005, Berwick's team published a controversial study that found that melanoma patients with higher levels of daily sun exposure actually had better survival than patients who spent less time in the sun.

"I've been searching for an explanation for that ever since," she said.

Now, findings from a group led by immunologists at Stanford University may provide an answer. The study, led by professor of pathology Eugene Butcher, is expected to be published in the March issue of Nature Immunology.

In its study, the Stanford team 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 the notion that an inactive precursor of vitamin D, called vitamin D3, "is generated in the skin in response to sun exposure." That's been known for years. Specifically, a short-wavelength form of UV light, called UVB, is responsible for D3 generation.

D3 is inert and powerless, however. Through contact with various enzymes in the liver and kidneys, the body turns D3 into an active compound called 1,25(OH)2D3.

And that's where the immune-system connection kicks in, the Stanford authors said.