Soaking up sun a human instinct
Top dermatologist says study on mice shows conclusively ‘tanning is addictive’
Chronic sun exposure can turn people into UV-seeking addicts, according to new research into the dark side of tanning.
Experiments in mice reveal that ultraviolet radiation boosts blood levels of beta-endorphin, a natural, morphine-like painkiller released by the brain that produces feelings of mild euphoria, a “buzz” like runner’s high.
When that brain chemical is blocked using the same antioverdose drug given to heroin addicts, the sun-exposed mice go into withdrawal.
The study suggests humans have a primitive instinct to soak up the sun and may help explain the “relentless rise” in skin cancer incidence despite widespread awareness of the dangers of one of the most ubiquitous cancer-causing agents known to man, the authors say.
“It’s surprising that we’re genetically programmed to become addicted to something as dangerous as UV radiation, which is probably the most common carcinogen in the world,” senior author Dr. David Fisher of Massachusetts General Hospital and Harvard Medical School said in a statement released with the study.
UV radiation stimulates the production of vitamin D. There might have been a survival advantage to seeking the sun. But vitamin D deficiency can be easily treated today through diet or inexpensive supplements “that have absolutely no carcinogens in them like UV does,” Fisher said in an interview.
Melanoma is one of the fastestrising and most preventable forms of cancer in Canada. According to the Canadian Cancer Society, an estimated 6,500 new cases of malignant melanoma, the deadliest skin cancer, and another 76,100 cases of non-melanoma skin cancers will be diagnosed this year.
UV radiation from the sun or indoor tanning beds causes about 90 per cent of melanoma cases.
“It’s an embarrassing fact that we probably know more about what causes skin cancers than almost any other form of cancer,” Fisher said. “And yet even with the knowledge that UV radiation is the culprit,” skin cancer rates continue to climb.
“That has led me, and a number of people to suspect that perhaps there’s something else happening here.”
For their study, Fisher’s team exposed the shaved backs of mice to UV radiation five days a week for six weeks. The dose was equivalent in humans to about 20 to 30 minutes of ambient midday sun exposure during a Florida summer. The team purposely chose low doses of radiation to avoid sunburns or inflammation of the skin.
Endorphin levels in the UV-exposed mice started to rise within a week.
Next the team wondered, are there any sensory effects of having higher levels of endorphin floating around in the bloodstream?
They poked the mice with tiny, hairlike filaments into their footpads.
As the blood levels of endorphin rose, the mice became increasingly numb.
When the rodents were injected with naloxone, a drug that blocks opiate receptors — “exactly the same drug you would give a heroin overdose patient when they hit the emergency room,” Fisher said — the UV-exposed mice showed classic, “murine” or mice-like signs of opioid withdrawal. They had tremors in their paws. Their teeth chattered. “They started shaking and jumping,” Fisher said.
The withdrawal was so profound the mice learned to avoid a black box inside their cage where the injections were given, even though mice are nocturnal animals, and would normally choose the dark box over the bright one.
“(Sun) makes them feel good because ... it’s creating an addiction.” DR. IAN LANDELLS
The team also studied genetically engineered “knockout mice” missing the gene for beta-endorphin. When those mice were exposed to UV radiation, “there was no changes in pain thresholds, there was no addictive behaviour and there were no preferential choices to avoid withdrawal symptoms,” Fisher said.
The study appears in the journal Cell.
The research was done in mice and not in humans. Mice are covered with fur, they’re nocturnal and spend most of their awake hours when the sun isn’t out. But the tanning response in mice appears to be the same as it is in humans, Fisher said. As well, others have shown that chronic tanners meet the psychiatric criteria for addictive behaviour.
The new findings suggest tanning beds, “which, for the most part, are regulated as cosmetic devices, are probably producing opiate signalling and opiate-like addiction,” Fisher said.
The study “shows conclusively that there is no question that tanning is addictive,” said Dr. Ian Landells, a clinical associate professor at Memorial University in St. John’s, N.L., and a past president of the Canadian Dermatology Association.
“I’m sitting right now in the cancer clinic doing a skin cancer clinic. I saw a couple of patients this morning who have had melanomas and they came back and their skin is peeling from sunburns they got on the weekend,” he said.
“They love the sun. It makes them feel good. We need to get the message out that the reason it makes them feel good isn’t because it’s good for them. It’s because it’s creating an addiction.”
It also causes wrinkling, broken blood vessels, blotchy brown spots and other signs of accelerated aging of the skin.
“If vanity is the reason they’re going out in the sun, they should realize that they’re going to make themselves look old much faster,” Landells said. “If they want colour, use a spray tan.”
Tanned skin, he added, “is so 1980.”