By Will Dunham
WASHINGTON, June 19 (Reuters) - This may give dedicated sun worshippers reason for pause. A new study suggests that regular tanning not only may raise the risk of skin cancer but also may be addictive.
A study published on Thursday found that chronic exposure to ultraviolet radiation triggers the release of endorphins - the so-called feel-good hormones - that function through the same biological pathway as highly addictive opiate drugs such as heroin and morphine.
The study involved laboratory mice, but the researchers said they believe the findings are applicable to people because the biological response of skin to UV radiation in mice is so similar to humans.
Regular UV radiation exposure led to physical dependence and addictive behavior in the mice, the study found. The animals even exhibited withdrawal symptoms - shaking, tremors and teeth chattering - after being treated with a drug that blocked the endorphin activity, the researchers said.
Writing in the journal Cell, they said the addictive nature of UV exposure "may contribute to the relentless rise in skin cancer incidence in humans."
"There is this dangerous addictive pathway operating," said dermatologist Dr. David Fisher of Massachusetts General Hospital and Harvard Medical School, who led the study.
Fisher said in theory sun-related skin cancer should be highly preventable merely by reducing exposure, but the addictive qualities of UV exposure may help explain the dogged "sun-seeking behavior" some people display through outdoor and indoor tanning and other pursuits.
"Behavioral exposure to the sun is being guided by influences that go past just a desire to have a nice game of Frisbee outside. There's something else motivating that behavior," Fisher added.
Exposure to ultraviolet rays through sunlight and indoor tanning equipment is considered a major risk factor for skin cancer including melanoma. The American Cancer Society said U.S. melanoma rates have been rising for at least 30 years, with about 76,000 news cases and 9,700 deaths forecast for 2014.
Cumulative damage from UV radiation also can cause premature skin aging in the form of wrinkles, lax skin and brown spots.
Exposure to UV rays stimulates production of endorphins - the same hormones stimulated by activities like vigorous exercise. They turn on opiate-related receptors via the same biological pathway triggered by prescription painkillers and other opiate drugs.
The researchers shaved the backs of the mice and gave them a daily dose of UV light - enough to induce tanning but not burning - for six weeks. Bloodstream endorphin levels rose within a week.
John Overstreet, executive director of the Indoor Tanning Association, said the study "ignores the benefits of exposure to ultraviolet light, the most obvious of which is the production of vitamin D through your skin."
"It is also important to note that there is no simple definition of addiction and the identification of addictions requires a substantial body of research. It is highly unlikely that a single study could lead to a sound conclusion on the matter. You can take anything too far, that does not mean it is an addiction," Overstreet said.
(Reporting by Will Dunham; Editing by Bernard Orr)
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