The health effects of sunlight: Part I

Scientific American has published an article called “Can Sunlight Cure Disease?” It’s very interesting, but also long and complicated. It’s about how ultraviolet light on skin can calm an out-of-control immune system, which is the cause of autoimmune diseases such as multiple sclerosis (MS). Auto immune diseases occur when our immune systems viciously turn against our own bodies and organs.

I’m going to try to summarize the article’s main points over two blog posts. This one focuses on the fact that many diseases are much more common at higher latitudes where there’s less sunlight and rarer near the sunny equator. This is especially notable with MS, which has prevalence rates close to zero near the equator. But the rate increases by 3.64 cases per 100,000 people for each degree of latitude. In northern Europe and North America, MS cases are well over 100 per 100,000 people and are growing stronger over time. In Australia, which has a wide range of latitudes, researchers found that MS rose from 12 per 100,000 closer to the equator to 76 per 100,000 at higher latitudes.

In general, researchers have found other signs of sunlight’s preventive effect on MS. For example, people with the most sun damage on the backs of their hands (more sun exposure) have just one-third the rate of MS than do people with less sun damage. Kids who spent less than 30 minutes a day outside had five times the risk of MS compared with kids who spent more than an hour outside.

Experiments with special UV-emitting light boxes to treat MS have begun. Patients who have used the boxes have found relief from many of their symptoms, such as fatigue. The boxes might also work for other autoimmune diseases such as type I diabetes, rheumatoid arthritis, Crohn’s disease and colitis—diseases that are more common in people who get very little sun exposure. (For years, it’s been known that exposure to sunlight or sunlamps soothes psoriasis, another autoimmune disease.)

Scientists have yet to uncover the “mysterious molecular pathway through which the skin tells the immune system to relax.” As one researcher noted, “We don’t know what the golden molecule is; we just know it’s not vitamin D.” In fact, they note that vitamin D supplementation “doesn’t help with any of these diseases.”

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