Let's not go overboard when it comes to shunning the sun, William Illsey Atkinson reports. Scientists are now discovering that, with several serious afflictions, a daily dose of sunlight may be just what the doctor ordered
Saturday, Jul. 5, 2003
By William Illsey Atkinson
Globe and Mail
Just as heels and hemlines vary from season to season, so do the opinions
of health experts. The prevailing attitude toward sunlight, for example,
is slip, slop, slap: slip on clothing neck to toe, slop on sunscreen, slap
on a hat. The ideal exposure is zero.
Yes, skin cancer is a nasty animal, but it now appears the complete ban de soleil may be an overreaction.
The human body is a thing of staggering complexity -- the smallest child has more cells than the largest galaxy has stars, and supports hundreds of simultaneous feedback systems. Despite great gains in basic knowledge, medicine is still light years from understanding how these different systems operate. But new research combined with a fresh look at older work has persuaded some scientists that, in moderation, direct solar irradiation of unprotected, light-coloured skin may offer major benefits.
As the Renaissance physician Paracelsus wrote, Sola dosis facit venenum -- too much of anything will hurt you. A corollary of this law is that harmful things may help in smaller amounts.
Most people already know that low light levels may cause seasonal affective disorder, the depressive condition aptly abbreviated as SAD. For Canadians in wintertime, or cubicle-dwelling nerds year round, low light levels can lead to the "January blahs." Conversely, intense illumination can alleviate SAD -- white lightens the blues.
Neurologists have demonstrated a pathway between the human optic nerve and the brain's release of natural antidepressants. Moreover, SAD frequently recedes faster when not only the ocular retinae, but also the epidermis are illuminated. And natural light is better than artificial, even if the synthetic light source is both broad-spectrum and intense.
But the benefits of sunlight go well beyond seasonal affective disorder: a minimum amount of sunlight seems necessary to human health.
For nearly a decade, epidemiologists at UNESCO have found a strong fall-off in the rate of multiple sclerosis in populations closer to the equator. The incidence of MS is greater not only at higher latitudes, but the whiter you are.
This would be significant if it merely sprang from inflexible genetics. Molecular geneticists could then look for biochemical links between MS-related genes and the gene sequences that code for melanin, the molecule that darkens the skin in response to ultraviolet light. But a third epidemiological association has emerged, throwing the previous two correlations into stronger relief. It is this: Whether you're black, white or any shade between, your chances of developing MS are significantly lower if you've lived in a sunny climate from infancy to your mid-teens.
Conversely, dark-skinned people with low MS levels who emigrate from the tropics to higher latitudes have children whose MS risk is almost the same as that of the light-skinned natives. Conclusion A: Environmental factors here are as important as genomic ones. Conclusion B: Whatever safeguards tropical populations from MS disappears when they leave for cloudier habitats. Conclusion C: Strong sunlight may be the key.
Exactly what biochemical mechanism sunlight uses to battle MS is still unknown. MS is an autoimmune disease, one of a family of afflictions in which the body's defences mistake its own tissue for an invader and attempt to destroy it. In MS the hapless target is the sheath or sclera around nerve cells ("sclerosis" means "inflammation of the sclera"). In lupus, another autoimmune disease, the misguided immune system targets the kidneys and blood; in rheumatoid arthritis, the skeletal joints are degraded.
One thing scientists do know is that strong sunlight reduces the immune system's general efficiency. That's bad news if you're fighting an infection. But in the case of certain autoimmune diseases such as MS, it can be helpful -- reducing the body's ability to misidentify and then to attack its own tissues.
How this occurs remains poorly understood. One theory is championed by Dr. Michel Dumas of the French Institute of Epidemiology and Tropical Neurology in Limoges. He has determined that the skin of healthy humans is rich in immune-system cells. If these cells are reduced in number or effectiveness by energetic solar photons, he writes, the immune system may grow more tolerant of myelin autoantigens -- the proteins in the nerve sheath that are recognized and attacked by MS.
Oddly enough, while the ultraviolet radiation can guard against MS, it also can induce or aggravate lupus. In the small set of people who carry a defective gene called GSTM1-null, prolonged and continual sun exposure boosts the likelihood of getting lupus by a factor of three. (The healthy GSTM1 gene helps to patch up ultraviolet damage to the body's cells.)
A second theory links the MS-sunshine correlation to vitamin D. This group of organic molecules, essential to building and maintaining strong bones, has several variants. The precursor to all of them is secreted naturally by the body and migrates to the skin, where solar energy nudges it along a biochemical path toward biologically useful forms, which is why it's called "the sunshine vitamin."
It may be the anti-MS vitamin as well. Animal studies recently completed at Pennsylvania State University show that low amounts of vitamin D accelerate the onset of several autoimmune diseases, including multiple sclerosis and arthritis. By contrast, higher levels of vitamin D work to prevent them. Assuming that humans have similar biochemical machinery, vitamin D may be the missing link between lower latitudes and higher MS resistance.
With sufficient sun exposure, the body easily makes all the vitamin D it needs. That's fine for Africa, the cradle of evolution, but way up north in Canada, it's a different story. Even in Canada's southernmost latitudes -- Windsor, Ont., for example -- sunlight is too weak to catalyze vitamin D from mid-October through mid-March, even on a cloudless day. Dietary supplements are needed, in the form of enriched milk and vitamin pills.
But diet is an inefficient way to obtain vitamin D. Only a few foods such as fatty fish contain naturally high levels of the vitamin. If you're trying to get your 10-microgram minimum from beef liver, you'll need to eat 10 pounds of the stuff every day. A little sunlight is a better approach. Solar synthesis of vitamin D is self-regulating; whatever your exposure, you can't synthesize too much. But ingesting too much vitamin D via dietary supplements can be highly toxic. Sola dosis facit venenum.
Dr. Colleen E. Hayes, a professor of biochemistry at the University of Wisconsin at Madison, sums up the dangers of avoiding sun altogether for a review article in The Scientist, a biosciences journal.
"Urged by the fear of skin cancer," she says, "individuals are avoiding sun exposure and using sunscreens . . . Somewhere there is a balance between too much sun and melanoma risk, or too little sun and autoimmune disease."
North Americans have been educated to avoid the sun, and it's easy to do even if we spend a lot of time outdoors. According to a study done by the Warren Grant Clinical Center for the U.S. National Institutes of Health, a sunscreen lotion with a protection factor of only 8 prohibits all UV-based production of vitamin D in human skin.
Is this wise? Perhaps it's time for federal health authorities to look into another minimum daily requirement. In most parts of Canada, 10 to 15 minutes of sun a day should do it. A fish-belly pallor may not be the healthiest skin shade after all.
William Illsey Atkinson is a science writer in North Vancouver and the author of Nanocosm (Viking), a book on nanotechnology.
Every day we run a gamut of solar photons, including lights known as ultraviolet because on the colour spectrum they lie above violet, the liveliest photon our eyes can see unaided.
UV-A, the gentlest ultraviolet, tans our skin. UV-B burns it. Ozone absorbs UV-B, so the thinning of the atmosphere's ozone layer has made us more susceptible to nastier burning. Luckily, the atmosphere still blocks UV-C, the nastiest of the UV bunch.
It's not as harsh as UV-C, but UV-B can be ugly. The damage it causes is essentially a radiation burn. As the body rushes to repair the tissue beaten up by UV-B, blood flows to the skin and creates the angry red glow of sunburn.
Like other types of radiation, UV-B doesn't limit its damage to the here and now. It also can transform dermal DNA, making skin cells cancerous. This can be very serious: malignant melanoma is one of the more aggressive cancers.
The lighter-skinned the subject, the higher the risk that devastating cancers will occur. A single case of bad sunburn in childhood may erupt years later as a melanoma. Even if the worst-case scenario does not occur, too much sun can lower tissue elasticity and give skin a wrinkled, leathery look.
Which is why the current dogma is to avoid the sun and Victorian alabaster
has replaced gold as the upper-class badge of health
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